the age-dependent adjustment factors (ADAF) described in the EPA's
Supplemental Guidance for Assessing Susceptibility from Early-Life
Exposure to Carcinogens \20\ were applied. This adjustment has the
effect of increasing the estimated lifetime risks for POM by a factor
of 1.6. In addition, although only a small fraction of the total POM
emissions were not reported as individual compounds, the EPA expresses
carcinogenic potency for compounds in this group in terms of
benzo[a]pyrene equivalence, based on evidence that carcinogenic POM has
the same mutagenic mechanism of action as benzo[a]pyrene. For this
reason, the EPA's Science Policy Council \21\ recommends applying the
Supplemental Guidance to all carcinogenic polycyclic aromatic
hydrocarbons for which risk estimates are based on relative potency.
Accordingly, we have applied the ADAF to the benzo[a]pyrene equivalent
portion of all POM mixtures.
---------------------------------------------------------------------------

    \18\ U.S. EPA. Performing risk assessments that include
carcinogens described in the Supplemental Guidance as having a
mutagenic mode of action. Science Policy Council Cancer Guidelines
Implementation Work Group Communication II: Memo from W.H. Farland,
dated October 4, 2005.
    \19\ See the Risk Assessment for Source Categories document
available in the docket for a list of HAP with a mutagenic mode of
action.
    \20\ U.S. EPA. Supplemental Guidance for Assessing Early-Life
Exposure to Carcinogens. EPA/630/R-03/003F, 2005. http://www.epa.gov/ttn/atw/childrens_supplement_final.pdf.
    \21\ U.S. EPA. Science Policy Council Cancer Guidelines
Implementation Workgroup Communication II: Memo from W.H. Farland,
dated June 14, 2006.
---------------------------------------------------------------------------

    Incremental individual lifetime cancer risks associated with
emissions from the source category were estimated as the sum of the
risks for each of the carcinogenic HAP (including those classified as
carcinogenic to humans, likely to be carcinogenic to humans and
suggestive evidence of carcinogenic potential \22\) emitted by the
modeled source. Cancer incidence and the distribution of individual
cancer risks for the population within 50 km of any source were also
estimated for the source category as part of these assessments by
summing individual risks. A distance of 50 km is consistent with both
the analysis supporting the 1989 Benzene NESHAP (54 FR 38044) and the
limitations of Gaussian dispersion models, including AERMOD.
---------------------------------------------------------------------------

    \22\ These classifications also coincide with the terms ``known
carcinogen, probable carcinogen and possible carcinogen,''
respectively, which are the terms advocated in the EPA's previous
Guidelines for Carcinogen Risk Assessment, published in 1986 (51 FR
33992, September 24, 1986). Summing the risks of these individual
compounds to obtain the cumulative cancer risks is an approach that
was recommended by the EPA's SAB in their 2002 peer review of EPA's
NATA entitled, NATA--Evaluating the National-scale Air Toxics
Assessment 1996 Data--an SAB Advisory, available at: http://
yosemite.epa.gov/sab/sabproduct.nsf/
214C6E915BB04E14852570CA007A682C/$File/ecadv02001.pdf.
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    To assess risk of noncancer health effects from chronic exposures,
we summed the HQ for each of the HAP that affects a common target organ
system to obtain the HI for that target organ system (or target organ-
specific HI, TOSHI). The HQ for chronic exposures is the estimated
chronic

exposure divided by the chronic reference level, which is either the
EPA reference concentration (RfC), defined as ``an estimate (with
uncertainty spanning perhaps an order of magnitude) of a continuous
inhalation exposure to the human population (including sensitive
subgroups) that is likely to be without an appreciable risk of
deleterious effects during a lifetime,'' or, in cases where an RfC from
the EPA's IRIS database is not available, the EPA will utilize the
following prioritized sources for our chronic dose-response values: (1)
The Agency for Toxic Substances and Disease Registry Minimum Risk
Level, which is defined as ``an estimate of daily human exposure to a
substance that is likely to be without an appreciable risk of adverse
effects (other than cancer) over a specified duration of exposure'';
(2) the CalEPA Chronic Reference Exposure Level (REL), which is defined
as ``the concentration level at or below which no adverse health
effects are anticipated for a specified exposure duration''; and (3),
as noted above, in cases where scientifically credible dose-response
values have been developed in a manner consistent with the EPA
guidelines and have undergone a peer review process similar to that
used by the EPA, we may use those dose-response values in place of or
in concert with other values.
    Screening estimates of acute exposures and risks were also
evaluated for each of the HAP at the point of highest off-site exposure
for each facility (i.e., not just the census block centroids), assuming
that a person is located at this spot at a time when both the peak
(hourly) emission rate and worst-case dispersion conditions (1991
calendar year data) occur. The acute HQ is the estimated acute exposure
divided by the acute dose-response value. In each case, acute HQ values
were calculated using best available, short-term dose-response values.
These acute dose-response values, which are described below, include
the acute REL, acute exposure guideline levels (AEGL) and emergency
response planning guidelines (ERPG) for 1-hour exposure durations. As
discussed below, we used conservative assumptions for emission rates,
meteorology and exposure location for our acute analysis.
    As described in the CalEPA's Air Toxics Hot Spots Program Risk
Assessment Guidelines, Part I, The Determination of Acute Reference
Exposure Levels for Airborne Toxicants, an acute REL value (http://www.oehha.ca.gov/air/pdf/acuterel.pdf) is defined as ``the
concentration level at or below which no adverse health effects are
anticipated for a specified exposure duration.'' Acute REL values are
based on the most sensitive, relevant, adverse health effect reported
in the medical and toxicological literature. Acute REL values are
designed to protect the most sensitive individuals in the population by
the inclusion of margins of safety. Since margins of safety are
incorporated to address data gaps and uncertainties, exceeding the
acute REL does not automatically indicate an adverse health impact.
    AEGL values were derived in response to recommendations from the
National Research Council (NRC). As described in Standing Operating
Procedures (SOP) of the National Advisory Committee on Acute Exposure
Guideline Levels for Hazardous Substances (http://www.epa.gov/opptintr/aegl/pubs/sop.pdf),\23\ ``the NRC's previous name for acute exposure
levels--community emergency exposure levels--was replaced by the term
AEGL to reflect the broad application of these values to planning,
response, and prevention in the community, the workplace,
transportation, the military, and the remediation of Superfund sites.''
This document also states that AEGL values ``represent threshold
exposure limits for the general public and are applicable to emergency
exposures ranging from 10 minutes to eight hours.'' The document lays
out the purpose and objectives of AEGL by stating (page 21) that ``the
primary purpose of the AEGL program and the National Advisory Committee
for Acute Exposure Guideline Levels for Hazardous Substances is to
develop guideline levels for once-in-a-lifetime, short-term exposures
to airborne concentrations of acutely toxic, high-priority chemicals.''
In detailing the intended application of AEGL values, the document
states (page 31) that ``[i]t is anticipated that the AEGL values will
be used for regulatory and nonregulatory purposes by U.S. Federal and
state agencies and possibly the international community in conjunction
with chemical emergency response, planning, and prevention programs.
More specifically, the AEGL values will be used for conducting various
risk assessments to aid in the development of emergency preparedness
and prevention plans, as well as real-time emergency response actions,
for accidental chemical releases at fixed facilities and from transport
carriers.''
---------------------------------------------------------------------------

    \23\ NAS, 2001. Standing Operating Procedures for Developing
Acute Exposure Levels for Hazardous Chemicals, page 2.
---------------------------------------------------------------------------

    The AEGL-1 value is then specifically defined as ``the airborne
concentration of a substance above which it is predicted that the
general population, including susceptible individuals, could experience
notable discomfort, irritation, or certain asymptomatic nonsensory
effects. However, the effects are not disabling and are transient and
reversible upon cessation of exposure.'' The document also notes (page
3) that, ``Airborne concentrations below AEGL-1 represent exposure
levels that can produce mild and progressively increasing but transient
and nondisabling odor, taste, and sensory irritation or certain
asymptomatic, nonsensory effects.'' Similarly, the document defines
AEGL-2 values as ``the airborne concentration (expressed as ppm or mg/
m\3\) of a substance above which it is predicted that the general
population, including susceptible individuals, could experience
irreversible or other serious, long-lasting adverse health effects or
an impaired ability to escape.''
    ERPG values are derived for use in emergency response, as described
in the American Industrial Hygiene Association's document entitled,
Emergency Response Planning Guidelines (ERPG) Procedures and
Responsibilities (http://www.aiha.org/1documents/committees/ERPSOPs2006.pdf) which states that, ``Emergency Response Planning
Guidelines were developed for emergency planning and are intended as
health based guideline concentrations for single exposures to
chemicals.'' \24\ The ERPG-1 value is defined as ``the maximum airborne
concentration below which it is believed that nearly all individuals
could be exposed for up to 1 hour without experiencing other than mild
transient adverse health effects or without perceiving a clearly
defined, objectionable odor.'' Similarly, the ERPG-2 value is defined
as ``the maximum airborne concentration below which it is believed that
nearly all individuals could be exposed for up to 1 hour without
experiencing or developing irreversible or other serious health effects
or symptoms which could impair an individual's ability to take
protective action.''
---------------------------------------------------------------------------

    \24\ ERP Committee Procedures and Responsibilities. November 1,
2006. American Industrial Hygiene Association.
---------------------------------------------------------------------------

    As can be seen from the definitions above, the AEGL and ERPG values
include the similarly-defined severity levels 1 and 2. For many
chemicals, a severity level 1 value AEGL or ERPG has not been
developed; in these instances, higher severity level AEGL-2 or ERPG-2
values are compared to our modeled

exposure levels to screen for potential acute concerns.
    Acute REL values for 1-hour exposure durations are typically lower
than their corresponding AEGL-1 and ERPG-1 values. Even though their
definitions are slightly different, AEGL-1 values are often the same as
the corresponding ERPG-1 values, and AEGL-2 values are often equal to
ERPG-2 values. Maximum HQ values from our acute screening risk
assessments typically result when basing them on the acute REL value
for a particular pollutant. In cases where our maximum acute HQ value
exceeds 1, we also report the HQ value based on the next highest acute
dose-response value (usually the AEGL-1 and/or the ERPG-1 value).
    To develop screening estimates of acute exposures, we developed
estimates of maximum hourly emission rates by multiplying the average
actual annual hourly emission rates by a factor to cover routinely
variable emissions. We chose the factor based on process knowledge and
engineering judgment and with awareness of a Texas study of short-term
emissions variability, which showed that most peak emission events, in
a heavily-industrialized 4-county area (Harris, Galveston, Chambers and
Brazoria Counties, Texas) were less than twice the annual average
hourly emission rate. The highest peak emission event was 74 times the
annual average hourly emission rate, and the 99th percentile ratio of
peak hourly emission rate to the annual average hourly emission rate
was 9.\25\ This analysis is provided in Appendix 4 of the Draft
Residual Risk Assessment for the Oil and Gas Production and Natural Gas
Transmission and Storage Source Categories, which is available in the
docket for this action. Considering this analysis, unless specific
process knowledge or data are available to provide an alternate value,
to account for more than 99 percent of the peak hourly emissions, we
apply a conservative screening multiplication factor of 10 to the
average annual hourly emission rate in these acute exposure screening
assessments. The factor of 10 was used for both the Oil and Natural Gas
Production and the Natural Gas Transmission and Storage source
categories.
---------------------------------------------------------------------------

    \25\ See http://www.tceq.state.tx.us/compliance/field_ops/eer/index.html or docket to access the source of these data.
---------------------------------------------------------------------------

    In cases where acute HQ values from the screening step were less
than or equal to 1, acute impacts were deemed negligible and no further
analysis was performed. In cases where an acute HQ from the screening
step was greater than 1, additional site-specific data were considered
to develop a more refined estimate of the potential for acute impacts
of concern. The data refinements employed for these source categories
consisted of using the site-specific facility layout to distinguish
facility property from an area where the public could be exposed. These
refinements are discussed in the draft risk assessment document, which
is available in the docket for each of these source categories.
Ideally, we would prefer to have continuous measurements over time to
see how the emissions vary by each hour over an entire year. Having a
frequency distribution of hourly emission rates over a year would allow
us to perform a probabilistic analysis to estimate potential threshold
exceedances and their frequency of occurrence. Such an evaluation could
include a more complete statistical treatment of the key parameters and
elements adopted in this screening analysis. However, we recognize that
having this level of data is rare, hence our use of the multiplier
approach.
    To better characterize the potential health risks associated with
estimated acute exposures to HAP, and in response to a key
recommendation from the SAB's peer review of the EPA's RTR risk
assessment methodologies,\26\ we generally examine a wider range of
available acute health metrics than we do for our chronic risk
assessments. This is in response to the SAB's acknowledgement that
there are generally more data gaps and inconsistencies in acute
reference values than there are in chronic reference values.
Comparisons of the estimated maximum off-site 1-hour exposure levels
are not typically made to occupational levels for the purpose of
characterizing public health risks in RTR assessments. This is because
they are developed for working age adults and are not generally
considered protective for the general public. We note that occupational
ceiling values are, for most chemicals, set at levels higher than a 1-
hour AEGL-1.
---------------------------------------------------------------------------

    \26\ The SAB peer review of RTR Risk Assessment Methodologies is
available at: http://yosemite.epa.gov/sab/sabproduct.nsf/
4AB3966E263D943A8525771F00668381/$File/EPA-SAB-10-007-unsigned.pdf.
---------------------------------------------------------------------------

    As discussed in section VII.C.2 of this preamble, the maximum
estimated worst-case 1-hour exposure to benzene outside the facility
fence line for a facility in either source category is 12 mg/m\3\. This
estimated exposure exceeds the 6-hour REL by a factor of 9
(HQREL = 9), but is significantly below the 1-hour AEGL-1
(HQAEGL-1 = 0.07). Although this worst-case exposure
estimate does not exceed the AEGL-1, we note here that it slightly
exceeds workplace ceiling level guidelines designed to protect the
worker population for short duration (<15 minute) increases in exposure
to benzene, as discussed below. The occupational short-term exposure
limit (STEL) standard for benzene developed by the Occupational Safety
and Health Administration is 16 mg/m\3\, ``as averaged over any 15-
minute period.'' \27\ Occupational guideline STEL for exposures to
benzene have also been developed by the American Conference of
Governmental Industrial Hygienists (ACGIH) \28\ for less than 15
minutes \29\ (ACGIH threshold limit value (TLV)-STEL value of 8.0 mg/
m\3\), and by the National Institute for Occupational Safety and Health
(NIOSH) \30\ ``for any 15 minute period in a work day'' (NIOSH REL-STEL
of 3.2 mg/m\3\). These shorter duration occupational values indicate
potential concerns regarding health effects at exposure levels below
the 1-hour AEGL-1 value. We solicit comment on the use of the
occupational values described above in the interpretation of these
worst-case acute screening exposure estimates.
---------------------------------------------------------------------------

    \27\ 29 CFR 1910.1028, Benzene. Available online at http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10042.
    \28\ ACGIH (2001) Benzene. In Documentation of the TLVs[supreg]
and BEIs[supreg] with Other Worldwide Occupational Exposure Values.
ACGIH, 1300 Kemper Meadow Drive, Cincinnati, OH 45240 (ISBN: 978-1-
882417-74-2) and available online at http://www.acgih.org.
    \29\ The ACGIH definition of a TLV-STEL states that ``Exposures
above the TLV-TWA up to the TLV-STEL should be less than 15 minutes,
should occur no more than four times per day, and there should be at
least 60 minutes between successive exposures in this range.''
    \30\ NIOSH. Occupational Safety and Health Guideline for
Benzene; http://www.cdc.gov/niosh/74-137.html.
---------------------------------------------------------------------------

d. Conducting Multi-Pathway Exposure and Risk Modeling
    The potential for significant human health risks due to exposures
via routes other than inhalation (i.e., multi-pathway exposures) and
the potential for adverse environmental impacts were evaluated in a
three-step process. In the first step, we determined whether any
facilities emitted any HAP known to be PB-HAP (HAP known to be
persistent and bio-accumulative) in the environment. There are 14 PB-
HAP compounds or compound classes identified for this screening in the
EPA's Air Toxics Risk Assessment Library (available at http://www.epa.gov/ttn/fera/risk_atra_vol1.html). They are cadmium
compounds, chlordane, chlorinated dibenzodioxins and furans,

dichlorodiphenyldichloroethylene, heptachlor, hexachlorobenzene,
hexachlorocyclohexane, lead compounds, mercury compounds, methoxychlor,
polychlorinated biphenyls, POM, toxaphene and trifluralin.
    Since one or more of these PB-HAP are emitted by at least one
facility in both source categories, we proceeded to the second step of
the evaluation. In this step, we determined whether the facility-
specific emission rates of each of the emitted PB-HAP were large enough
to create the potential for significant non-inhalation human or
environmental risks under reasonable worst-case conditions. To
facilitate this step, we have developed emission rate thresholds for
each PB-HAP using a hypothetical worst-case screening exposure scenario
developed for use in conjunction with the EPA's TRIM.FaTE model. The
hypothetical screening scenario was subjected to a sensitivity analysis
to ensure that its key design parameters were established such that
environmental media concentrations were not underestimated (i.e., to
minimize the occurrence of false negatives or results that suggest that
risks might be acceptable when, in fact, actual risks are high) and to
also minimize the occurrence of false positives for human health
endpoints. We call this application of the TRIM.FaTE model TRIM-Screen.
The facility-specific emission rates of each of the PB-HAP in each
source category were compared to the TRIM-Screen emission threshold
values for each of the PB-HAP identified in the source category
datasets to assess the potential for significant human health risks or
environmental risks via non-inhalation pathways.
    There was only one facility in the Natural Gas Transmission and
Storage source category with reported emissions of PB-HAP, and the
emission rates were less than the emission threshold values. There were
29 facilities in the Oil and Natural Gas Production source category
with reported emissions of PB-HAP, and one of these had emission rates
greater than the emission threshold values. In this case, the emission
threshold value for POM was exceeded by a factor of 6. For POM, dairy,
vegetables and fruits were the three most dominant exposure pathways
driving human exposures in the hypothetical screening exposure
scenario. The single facility with emissions exceeding the emission
threshold value for POM is located in a highly industrialized area.
Therefore, since the exposure pathways which would drive high human
exposure are not locally available, multi-pathway exposures and
environmental risks were deemed negligible, and no further analysis was
performed. For further information on the multi-pathway analysis
approach, see the residual risk documentation.
e. Assessing Risks Considering Emissions Control Options
    In addition to assessing baseline inhalation risks and screening
for potential multi-pathway risks, where appropriate, we also estimated
risks considering the potential emission reductions that would be
achieved by the particular control options under consideration. In
these cases, the expected emissions reductions were applied to the
specific HAP and emissions sources in the source category dataset to
develop corresponding estimates of risk reductions.
f. Conducting Other Risk-Related Analyses: Facility-Wide Assessments
    To put the source category risks in context, we also examined the
risks from the entire ``facility,'' where the facility includes all
HAP-emitting operations within a contiguous area and under common
control. In other words, for each facility that includes one or more
sources from one of the source categories under review, we examined the
HAP emissions not only from the source category of interest, but also
from all other emission sources at the facility. The emissions data for
generating these ``facility-wide'' risks were also obtained from the
2005 NEI. For every facility included in the MACT database, we also
retrieved emissions data and release characteristics for all other
emission sources at the same facility. We estimated the risks due to
the inhalation of HAP that are emitted ``facility-wide'' for the
populations residing within 50 km of each facility, consistent with the
methods used for the source category analysis described above. For
these facility-wide risk analyses, the modeled source category risks
were compared to the facility-wide risks to determine the portion of
facility-wide risks that could be attributed to the source categories
addressed in this proposal. We specifically examined the facilities
associated with the highest estimates of risk and determined the
percentage of that risk attributable to the source category of
interest. The risk documentation available through the docket for this
action provides the methodology and the results of the facility-wide
analyses for each source category.
g. Conducting Other Analyses: Demographic Analysis
    To examine the potential for any environmental justice (EJ) issues
that might be associated with each source category, we performed a
demographic analysis of population risk. In this analysis, we evaluated
the distributions of HAP-related cancer and noncancer risks across
different social, demographic and economic groups within the
populations living near the facilities where these source categories
are located. The development of demographic analyses to inform the
consideration of EJ issues in the EPA rulemakings is an evolving
science. The EPA offers the demographic analyses in this rulemaking to
inform the consideration of potential EJ issues and invites public
comment on the approaches used and the interpretations made from the
results, with the hope that this will support the refinement and
improve the utility of such analyses for future rulemakings.
    For the demographic analyses, we focus on the populations within 50
km of any facility estimated to have exposures to HAP which result in
cancer risks of 1-in-1 million or greater, or noncancer HI of 1 or
greater (based on the emissions of the source category or the facility,
respectively). We examine the distributions of those risks across
various demographic groups, comparing the percentages of particular
demographic groups to the total number of people in those demographic
groups nationwide. The results, including other risk metrics, such as
average risks for the exposed populations, are documented in source-
category-specific technical reports in the docket for both source
categories covered in this proposal.
    The basis for the risk values used in these analyses were the
modeling results based on actual emissions levels obtained from the
HEM-3 model described above. The risk values for each census block were
linked to a database of information from the 2000 Decennial census that
includes data on race and ethnicity, age distributions, poverty status,
household incomes and education level. The Census Department
Landview[supreg] database was the source of the data on race and
ethnicity and the data on age distributions, poverty status, household
incomes and education level were obtained from the 2000 Census of
Population and Housing Summary File 3 Long Form. While race and
ethnicity census data are available at the census block level, the age
and income census data are only available at the census block group
level (which includes an

average of 26 blocks or an average of 1,350 people). Where census data
are available at the block group level, but not the block level, we
assumed that all census blocks within the block group have the same
distribution of ages and incomes as the block group.
    For each source category, we focused on those census blocks where
source category risk results show estimated lifetime inhalation cancer
risks above 1-in-1 million or chronic noncancer indices above 1 and
determined the relative percentage of different racial and ethnic
groups, different age groups, adults with and without a high school
diploma, people living in households below the national median income
and for people living below the poverty line within those census
blocks. The specific census population categories studied include:

 Total population
 White
 African American (or Black)
 Native Americans
 Other races and multiracial
 Hispanic or Latino
 Children 18 years of age and under
 Adults 19 to 64 years of age
 Adults 65 years of age and over
 Adults without a high school diploma
 Households earning under the national median income
 People living below the poverty line

    It should be noted that these categories overlap in some instances,
resulting in some populations being counted in more than one category
(e.g., other races and multiracial and Hispanic). In addition, while
not a specific census population category, we also examined risks to
``Minorities,'' a classification which is defined for these purposes as
all race population categories except white.
    For further information about risks to the populations located near
the facilities in these source categories, we also evaluated the
estimated distribution of inhalation cancer and chronic noncancer risks
associated with the HAP emissions from all the emissions sources at the
facility (i.e., facility-wide). This analysis used the facility-wide
RTR modeling results and the census data described above.
    The methodology and the results of the demographic analyses for
each source category are included in a source-category-specific
technical report for each of the categories, which are available in the
docket for this action.
h. Considering Uncertainties in Risk Assessment
    Uncertainty and the potential for bias are inherent in all risk
assessments, including those performed for the source categories
addressed in this proposal. Although uncertainty exists, we believe
that our approach, which used conservative tools and assumptions,
ensures that our decisions are health-protective. A brief discussion of
the uncertainties in the emissions datasets, dispersion modeling,
inhalation exposure estimates and dose-response relationships follows
below. A more thorough discussion of these uncertainties is included in
the risk assessment documentation (referenced earlier) available in the
docket for this action.
i. Uncertainties in the Emissions Datasets
    Although the development of the MACT dataset involved QA/quality
control processes, the accuracy of emissions values will vary depending
on the source of the data, the degree to which data are incomplete or
missing, the degree to which assumptions made to complete the datasets
are inaccurate, errors in estimating emissions values and other
factors. The emission estimates considered in this analysis generally
are annual totals for certain years that do not reflect short-term
fluctuations during the course of a year or variations from year to
year.
    The estimates of peak hourly emission rates for the acute effects
screening assessment were based on a multiplication factor of 10
applied to the average annual hourly emission rate, which is intended
to account for emission fluctuations due to normal facility operations.
Additionally, although we believe that we have data for most facilities
in these two source categories in our RTR dataset, our dataset may not
include data for all existing facilities. Moreover, there are
uncertainties with regard to the identification of sources as major or
area in the NEI for these source categories.
ii. Uncertainties in Dispersion Modeling
    While the analysis employed the EPA's recommended regulatory
dispersion model, AERMOD, we recognize that there is uncertainty in
ambient concentration estimates associated with any model, including
AERMOD. In circumstances where we had to choose between various model
options, where possible, model options (e.g., rural/urban, plume
depletion, chemistry) were selected to provide an overestimate of
ambient air concentrations of the HAP rather than underestimates.
However, because of practicality and data limitation reasons, some
factors (e.g., meteorology, building downwash) have the potential in
some situations to overestimate or underestimate ambient impacts. For
example, meteorological data were taken from a single year (1991) and
facility locations can be a significant distance from the site where
these data were taken. Despite these uncertainties, we believe that at
off-site locations and census block centroids, the approach considered
in the dispersion modeling analysis should generally yield
overestimates of ambient HAP concentrations.
iii. Uncertainties in Inhalation Exposure
    The effects of human mobility on exposures were not included in the
assessment. Specifically, short-term mobility and long-term mobility
between census blocks in the modeling domain were not considered.\31\
The assumption of not considering short or long-term population
mobility does not bias the estimate of the theoretical MIR, nor does it
affect the estimate of cancer incidence since the total population
number remains the same. It does, however, affect the shape of the
distribution of individual risks across the affected population,
shifting it toward higher estimated individual risks at the upper end
and reducing the number of people estimated to be at lower risks,
thereby increasing the estimated number of people at specific risk
levels.
---------------------------------------------------------------------------

    \31\ Short-term mobility is movement from one micro-environment
to another over the course of hours or days. Long-term mobility is
movement from one residence to another over the course of a
lifetime.
---------------------------------------------------------------------------

    In addition, the assessment predicted the chronic exposures at the
centroid of each populated census block as surrogates for the exposure
concentrations for all people living in that block. Using the census
block centroid to predict chronic exposures tends to over-predict
exposures for people in the census block who live further from the
facility, and under-predict exposures for people in the census block
who live closer to the facility. Thus, using the census block centroid
to predict chronic exposures may lead to a potential understatement or
overstatement of the true maximum impact, but is an unbiased estimate
of average risk and incidence.
    The assessments evaluate the cancer inhalation risks associated
with continuous pollutant exposures over a 70-year period, which is the
assumed lifetime of an individual. In reality, both the length of time
that modeled emissions sources at facilities actually operate (i.e.,
more or less than 70 years), and the domestic growth or decline of the
modeled industry (i.e., the increase

or decrease in the number or size of United States facilities), will
influence the risks posed by a given source category. Depending on the
characteristics of the industry, these factors will, in most cases,
result in an overestimate both in individual risk levels and in the
total estimated number of cancer cases. However, in rare cases, where a
facility maintains or increases its emission levels beyond 70 years,
residents live beyond 70 years at the same location, and the residents
spend most of their days at that location, then the risks could
potentially be underestimated. Annual cancer incidence estimates from
exposures to emissions from these sources would not be affected by
uncertainty in the length of time emissions sources operate.
    The exposure estimates used in these analyses assume chronic
exposures to ambient levels of pollutants. Because most people spend
the majority of their time indoors, actual exposures may not be as
high, depending on the characteristics of the pollutants modeled. For
many of the HAP, indoor levels are roughly equivalent to ambient
levels, but for very reactive pollutants or larger particles, these
levels are typically lower. This factor has the potential to result in
an overstatement of 25 to 30 percent of exposures.\32\
---------------------------------------------------------------------------

    \32\ U.S. EPA. National-Scale Air Toxics Assessment for 1996.
(EPA 453/R-01-003; January 2001; page 85.)
---------------------------------------------------------------------------

    In addition to the uncertainties highlighted above, there are
several factors specific to the acute exposure assessment that should
be highlighted. The accuracy of an acute inhalation exposure assessment
depends on the simultaneous occurrence of independent factors that may
vary greatly, such as hourly emissions rates, meteorology, and human
activity patterns. In this assessment, we assume that individuals
remain for 1 hour at the point of maximum ambient concentration as
determined by the co-occurrence of peak emissions and worst-case
meteorological conditions. These assumptions would tend to overestimate
actual exposures since it is unlikely that a person would be located at
the point of maximum exposure during the time of worst-case impact.
iv. Uncertainties in Dose-Response Relationships
    There are uncertainties inherent in the development of the dose-
response values used in our risk assessments for cancer effects from
chronic exposures and noncancer effects from both chronic and acute
exposures. Some uncertainties may be considered quantitatively, and
others generally are expressed in qualitative terms. We note as a
preface to this discussion a point on dose-response uncertainty that is
brought out in the EPA 2005 Cancer Guidelines; namely, that ``the
primary goal of the EPA actions is protection of human health;
accordingly, as an Agency policy, risk assessment procedures, including
default options that are used in the absence of scientific data to the
contrary, should be health protective.'' (EPA 2005 Cancer Guidelines,
pages 1-7.) This is the approach followed here as summarized in the
next several paragraphs. A complete detailed discussion of
uncertainties and variability in dose-response relationships is given
in the residual risk documentation, which is available in the docket
for this action.
    Cancer URE values used in our risk assessments are those that have
been developed to generally provide an upper bound estimate of risk.
That is, they represent a ``plausible upper limit to the true value of
a quantity'' (although this is usually not a true statistical
confidence limit).\33\ In some circumstances, the true risk could be as
low as zero; however, in other circumstances, the risk could also be
greater.\34\ When developing an upper bound estimate of risk and to
provide risk values that do not underestimate risk, health-protective
default approaches are generally used. To err on the side of ensuring
adequate health-protection, the EPA typically uses the upper bound
estimates rather than lower bound or central tendency estimates in our
risk assessments, an approach that may have limitations for other uses
(e.g., priority-setting or expected benefits analysis).
---------------------------------------------------------------------------

    \33\ IRIS glossary (http://www.epa.gov/NCEA/iris/help_gloss.htm).
    \34\ An exception to this is the URE for benzene, which is
considered to cover a range of values, each end of which is
considered to be equally plausible and which is based on maximum
likelihood estimates.
---------------------------------------------------------------------------

    Chronic noncancer reference (RfC and reference dose (RfD)) values
represent chronic exposure levels that are intended to be health-
protective levels. Specifically, these values provide an estimate (with
uncertainty spanning perhaps an order of magnitude) of daily oral
exposure (RfD) or of a continuous inhalation exposure (RfC) to the
human population (including sensitive subgroups) that is likely to be
without an appreciable risk of deleterious effects during a lifetime.
To derive values that are intended to be ``without appreciable risk,''
the methodology relies upon an uncertainty factor (UF) approach (U.S.
EPA, 1993, 1994) which includes consideration of both uncertainty and
variability. When there are gaps in the available information, UF are
applied to derive reference values that are intended to protect against
appreciable risk of deleterious effects. The UF are commonly default
values,\35\ e.g., factors of 10 or 3, used in the absence of compound-
specific data; where data are available, UF may also be developed using
compound-specific information. When data are limited, more assumptions
are needed and more UF are used. Thus, there may be a greater tendency
to overestimate risk in the sense that further study might support
development of reference values that are higher (i.e., less potent)
because fewer default assumptions are needed. However, for some
pollutants, it is possible that risks may be underestimated. While
collectively termed ``uncertainty factor,'' these factors account for a
number of different quantitative considerations when using observed
animal (usually rodent) or human toxicity data in the development of
the RfC. The UF are intended to account for: (1) Variation in
susceptibility among the members of the human population (i.e., inter-
individual variability); (2) uncertainty in extrapolating from
experimental animal data to humans (i.e., interspecies differences);
(3) uncertainty in extrapolating from data obtained in a study with
less-than-lifetime exposure (i.e., extrapolating from sub-chronic to
chronic exposure); (4) uncertainty in extrapolating the observed data
to obtain an estimate of the exposure associated with no adverse
effects; and (5) uncertainty when the database is incomplete or there
are problems with the applicability of available studies. Many of the
UF used to account for variability and uncertainty in the development
of acute reference values

are quite similar to those developed for chronic durations, but they
more often use individual UF values that may be less than 10. UF are
applied based on chemical-specific or health effect-specific
information (e.g., simple irritation effects do not vary appreciably
between human individuals, hence a value of 3 is typically used), or
based on the purpose for the reference value (see the following
paragraph). The UF applied in acute reference value derivation include:
(1) Heterogeneity among humans; (2) uncertainty in extrapolating from
animals to humans; (3) uncertainty in lowest observed adverse effect
(exposure) level to no observed adverse effect (exposure) level
adjustments; and (4) uncertainty in accounting for an incomplete
database on toxic effects of potential concern. Additional adjustments
are often applied to account for uncertainty in extrapolation from
observations at one exposure duration (e.g., 4 hours) to derive an
acute reference value at another exposure duration (e.g., 1 hour).
---------------------------------------------------------------------------

    \35\ According to the NRC report, Science and Judgment in Risk
Assessment (NRC, 1994) ``[Default] options are generic approaches,
based on general scientific knowledge and policy judgment, that are
applied to various elements of the risk assessment process when the
correct scientific model is unknown or uncertain.'' The 1983 NRC
report, Risk Assessment in the Federal Government: Managing the
Process, defined default option as ``the option chosen on the basis
of risk assessment policy that appears to be the best choice in the
absence of data to the contrary'' (NRC, 1983a, p. 63). Therefore,
default options are not rules that bind the Agency; rather, the
Agency may depart from them in evaluating the risks posed by a
specific substance when it believes this to be appropriate. In
keeping with EPA's goal of protecting public health and the
environment, default assumptions are used to ensure that risk to
chemicals is not underestimated (although defaults are not intended
to overtly overestimate risk). See EPA, 2004, An Examination of EPA
Risk Assessment Principles and Practices, EPA/100/B-04/001 available
at: http://www.epa.gov/osa/pdfs/ratf-final.pdf.
---------------------------------------------------------------------------

    Not all acute reference values are developed for the same purpose
and care must be taken when interpreting the results of an acute
assessment of human health effects relative to the reference value or
values being exceeded. Where relevant to the estimated exposures, the
lack of short-term dose-response values at different levels of severity
should be factored into the risk characterization as potential
uncertainties.
    Although every effort is made to identify peer-reviewed reference
values for cancer and noncancer effects for all pollutants emitted by
the sources included in this assessment, some HAP continue to have no
reference values for cancer or chronic noncancer or acute effects.
Since exposures to these pollutants cannot be included in a
quantitative risk estimate, an understatement of risk for these
pollutants at environmental exposure levels is possible. For a group of
compounds that are either unspeciated or do not have reference values
for every individual compound (e.g., glycol ethers), we conservatively
use the most protective reference value to estimate risk from
individual compounds in the group of compounds.
    Additionally, chronic reference values for several of the compounds
included in this assessment are currently under the EPA IRIS review and
revised assessments may determine that these pollutants are more or
less potent than the current value. We may re-evaluate residual risks
for the final rulemaking if these reviews are completed prior to our
taking final action for these source categories and a dose-response
metric changes enough to indicate that the risk assessment supporting
this notice may significantly understate human health risk.
v. Uncertainties in the Multi-Pathway and Environmental Effects
Assessment
    We generally assume that when exposure levels are not anticipated
to adversely affect human health, they also are not anticipated to
adversely affect the environment. For each source category, we
generally rely on the site-specific levels of PB-HAP emissions to
determine whether a full assessment of the multi-pathway and
environmental effects is necessary. As discussed above, we conclude
that the potential for these types of impacts is low for these source
categories.
vi. Uncertainties in the Facility-Wide Risk Assessment
    Given that the same general analytical approach and the same models
were used to generate facility-wide risk results as were used to
generate the source category risk results, the same types of
uncertainties discussed above for our source category risk assessments
apply to the facility-wide risk assessments. Additionally, the degree
of uncertainty associated with facility-wide emissions and risks is
likely greater because we generally have not conducted a thorough
engineering review of emissions data for source categories not
currently undergoing an RTR review.
vii. Uncertainties in the Demographic Analysis
    Our analysis of the distribution of risks across various
demographic groups is subject to the typical uncertainties associated
with census data (e.g., errors in filling out and transcribing census
forms), as well as the additional uncertainties associated with the
extrapolation of census-block group data (e.g., income level and
education level) down to the census block level.
2. What are the results and proposed decisions from the risk review for
the Oil and Natural Gas Production source category?
a. Results of the Risk Assessments and Analyses
    We conducted an inhalation risk assessment for the Oil and Natural
Gas Production source category. We also conducted an assessment of
facility-wide risk. Details of the risk assessments and analyses can be
found in the residual risk documentation, which is available in the
docket for this action. For informational purposes and to examine the
potential for any EJ issues that might be associated with each source
category, we performed a demographic analysis of population risks.
i. Inhalation Risk Assessment Results
    Table 2 provides an overall summary of the results of the
inhalation risk assessment.

                                       Table 2--Oil and Natural Gas Production Inhalation Risk Assessment Results
--------------------------------------------------------------------------------------------------------------------------------------------------------
                     Maximum individual cancer risk (in 1                                         Maximum chronic noncancer TOSHI \4\
                                 million) \2\                  Estimated      Estimated  annual -------------------------------------- Maximum  off-site
     Number of     ---------------------------------------   population at    cancer  incidence                                         acute  noncancer
  facilities \1\     Actual emissions       Allowable        risk >= 1-in-1    (cases per year)   Actual emissions      Allowable            HQ \5\
                           level         emissions level        million                                level         emissions level
--------------------------------------------------------------------------------------------------------------------------------------------------------
             990                  40        100-400 \3\        160,000 \3\     0.007-0.02 \3\                0.1                0.7          HQREL = 9
                                                                                (benzene)
                                                                                HQAEGL	1 = 0.07
                                                                                (benzene)
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Number of facilities evaluated in the risk analysis.
\2\ Estimated maximum individual excess lifetime cancer risk.
\3\ The EPA IRIS assessment for benzene provides a range of equally-plausible URE (2.2E-06 to 7.8E-06 per ug/m3), giving rise to ranges for the
  estimates of cancer MIR and cancer incidence. Estimated population values are not scalable with benzene URE range, but would be lower using the lower
  end of the URE range.
\4\ Maximum TOSHI. The target organ with the highest TOSHI for the Oil and Natural Gas Production source category is the respiratory system.
\5\ The maximum estimated acute exposure concentration was divided by available short-term dose-response values to develop an array of HQ values.

    As shown in Table 2, the results of the inhalation risk assessment
performed using actual emissions data indicate the maximum lifetime
individual cancer risk could be as high as 40-in-1 million, with POM
driving the highest risk, and benzene driving risks overall. The total
estimated cancer incidence from this source category is 0.02 excess
cancer cases per year (0.007 excess cancer cases per year based on the
lower end of the benzene URE range), or one case in every 50 years.
Approximately 160,000 people are estimated to have cancer risks at or
above 1-in-1 million as a result of the emissions from 89 facilities
(use of the lower end of the benzene URE range would further reduce
this population estimate). The maximum chronic non-cancer TOSHI value
for the source category could be up to 0.1 from emissions of
naphthalene, indicating no significant potential for chronic noncancer
impacts.
    As explained above, our analysis of potential differences between
actual emission levels and emissions allowable under the oil and
natural gas production MACT standard indicate that MACT-allowable
emission levels may be up to 50 times greater than actual emission
levels. Considering this difference, the risk results from the
inhalation risk assessment indicate the maximum lifetime individual
cancer risk could be as high as 400-in-1 million (100-in-1 million
based on the lower end of the benzene URE range) and the maximum
chronic noncancer TOSHI value could be as high as 0.7 at the MACT-
allowable emissions level.
ii. Facility-Wide Risk Assessment Results
    A facility-wide risk analysis was also conducted based on actual
emissions levels. Table 3 displays the results of the facility-wide
risk assessment. For detailed facility-specific results, see Table 2 of
Appendix 6 of the risk document in the docket for this rulemaking.

  Table 3--Oil and Natural Gas Production Facility-Wide Risk Assessment
                                 Results
------------------------------------------------------------------------

------------------------------------------------------------------------
Number of facilities analyzed..............................          990
Cancer Risk:
    Estimated maximum facility-wide individual cancer risk           100
     (in 1 million)........................................
    Number of facilities with estimated facility-wide                  1
     individual cancer risk of 100-in-1 million or more....
    Number of facilities at which the Oil and Natural Gas              0
     Production source category contributes 50 percent or
     more to the facility-wide individual cancer risks of
     100-in-1 million or more..............................
    Number of facilities with facility-wide individual               140
     cancer risk of 1-in-1 million or more.................
    Number of facilities at which the Oil and Natural Gas             85
     Production source category contributes 50 percent or
     more to the facility-wide individual cancer risk of 1-
     in-1 million or more..................................
Chronic Noncancer Risk:
    Maximum facility-wide chronic noncancer TOSHI..........            9
    Number of facilities with facility-wide maximum                   10
     noncancer TOSHI greater than 1........................
    Number of facilities at which the Oil and Natural Gas              0
     Production source category contributes 50 percent or
     more to the facility-wide maximum noncancer TOSHI of 1
     or more...............................................
------------------------------------------------------------------------

    The facility-wide MIR from all HAP emissions at a facility that
contains sources subject to the oil and natural gas production MACT
standards is estimated to be 100-in-1 million, based on actual
emissions. Of the 990 facilities included in this analysis, only one
has a facility-wide MIR of 100-in-1 million. At this facility, oil and
natural gas production accounts for less than 2 percent of the total
facility-wide risk. Nickel emissions from oil-fired boilers and
formaldehyde emissions from reciprocating internal combustion engines
(RICE) contribute essentially all the facility-wide risks at this
facility, with over 80 percent of the risk attributed to the nickel
emissions.\36\ There are 140 facilities with facility-wide MIR of 1-in-
1 million or greater. Of these facilities, 85 have oil and natural gas
production operations that contribute greater than 50 percent to the
facility-wide risks. As discussed above, we are proposing MACT
standards for BTEX emissions from small glycol dehydrators in this
action. These standards would reduce the risk from benzene emissions at
facilities with oil and gas production. Formaldehyde emissions will be
assessed under future RTR for RICE.
---------------------------------------------------------------------------

    \36\ We note that there is an ongoing IRIS reassessment for
formaldehyde, and that future RTR risk assessments will use the
cancer potency for formaldehyde that results from that reassessment.
As a result, the current results may not match those of future
assessments.
---------------------------------------------------------------------------

    The facility-wide maximum individual chronic noncancer TOSHI is
estimated to be 9 based on actual emissions. Of the 990 facilities
included in this analysis, 10 have facility-wide maximum chronic
noncancer TOSHI values greater than 1. Of these facilities, none had
oil and natural gas production operations that contributed greater than
50 percent to these facility-wide risks. The chronic noncancer risks at
these 10 facilities are primarily driven by acrolein emissions from
RICE.
iii. Demographic Risk Analysis Results
    The results of the demographic analyses performed to investigate
the distribution of cancer risks at or above 1-in-1 million among the
surrounding population are summarized in Table 4 below. These results,
for various demographic groups, are based on actual emissions levels
for the population living within 50 km of the facilities.

                    Table 4--Oil and Natural Gas Production Demographic Risk Analysis Results
----------------------------------------------------------------------------------------------------------------
                                                                              Population with cancer risk at or
                                                                                above 1-in-1 million due to
                                                             Nationwide    -------------------------------------
                                                                             Source category   Facility-wide HAP
                                                                              HAP emissions        emissions
----------------------------------------------------------------------------------------------------------------
Total Population.......................................        285,000,000            160,000            597,000

                                                 Race by Percent
----------------------------------------------------------------------------------------------------------------
White..................................................                 75                 62                 61
All Other Races........................................                 25                 38                 39
----------------------------------------------------------------------------------------------------------------
                                                 Race by Percent
----------------------------------------------------------------------------------------------------------------
White..................................................                 75                 62                 61
African American.......................................                 12                 12                  8
Native American........................................                0.9                0.7                1.3
Other and Multiracial..................................                 12                 25                 30
----------------------------------------------------------------------------------------------------------------
                                              Ethnicity by Percent
----------------------------------------------------------------------------------------------------------------
Hispanic...............................................                 14                 22                 34
Non-Hispanic...........................................                 86                 78                 66
----------------------------------------------------------------------------------------------------------------
                                                Income by Percent
----------------------------------------------------------------------------------------------------------------
Below Poverty Level....................................                 13                 14                 19
Above Poverty Level....................................                 87                 86                 81
----------------------------------------------------------------------------------------------------------------
                                              Education by Percent
----------------------------------------------------------------------------------------------------------------
Over 25 and without High School Diploma................                 13                 10                 16
Over 25 and with a High School Diploma.................                 87                 90                 84
----------------------------------------------------------------------------------------------------------------

    The results of the Oil and Natural Gas Production source category
demographic analysis indicate that there are approximately 160,000
people exposed to a cancer risk at or above 1-in-1 million due to
emissions from the source category, including an estimated 38 percent
that are classified as minority (listed as ``All Other Races'' in the
table above). Of the 160,000 people with estimated cancer risks at or
above 1-in-1 million from the source category, 25 percent are in the
``Other and Multiracial'' demographic group, 22 percent are in the
``Hispanic or Latino'' demographic group, and 14 percent are in the
``Below Poverty Level'' demographic group, results which are 13, 8 and
1 percentage points higher, respectively, than the respective
percentages for these demographic groups across the United States. The
percentages for the other demographic groups are lower than their
respective nationwide percentages. The table also shows that there are
approximately 597,000 people exposed to an estimated cancer risk at or
above 1-in-1 million due to facility-wide emissions, including 30
percent in the ``Other and Multiracial'' demographic group, 34 percent
in the ``Hispanic or Latino'' demographic group, 1.3 percent in the
``Native American'' demographic group and 16 percent in the ``Over 25
and without High School Diploma'' demographic group, results which are
18, 2, 0.4 and 3 percentage points higher than the percentages for
these demographic groups across the United States, respectively. The
percentages for the other demographic groups are lower than their
respective nationwide percentages.
b. What are the proposed risk decisions for the Oil and Natural Gas
Production source category?
i. Risk Acceptability
    In the risk analysis we performed for this source category,
pursuant to CAA section 112(f)(2), we considered the available health
information--the MIR; the numbers of persons in various risk ranges;
cancer incidence; the maximum noncancer HI; the maximum acute noncancer
hazard; the extent of noncancer risks; the potential for adverse
environmental effects; and distribution of risks in the exposed
population; and risk estimation uncertainty (54 FR 38044, September 14,
1989).
    For the Oil and Natural Gas Production source category, the risk
analysis we performed indicates that the cancer risks to the individual
most exposed could be as high as 40-in-1 million due to actual
emissions and as high as 400-in-1 million due to MACT-allowable
emissions (100-in-1 million, based on the lower end of the benzene URE
range). While the 40-in-1 million risk due to actual emissions is
considerably less than 100-in-1 million, which is the presumptive limit
of acceptability, the 400-in-1 million risk due to allowable emissions
is considerably higher and is considered unacceptable. We do note,
however, that the risk analysis shows low cancer incidence (1 case in
every 50 years), low potential for adverse environmental effects or
human health multi-pathway effects and that chronic noncancer health
impacts are unlikely.
    We also conclude that acute noncancer health impacts are unlikely.
As discussed above, screening estimates of acute exposures and risks
were evaluated for each of the HAP at the point of highest off-site
exposure for each facility (i.e., not just the census block centroids)
assuming that a person is located at this spot at a time when both the
peak emission rate and worst-case dispersion conditions occur. Under
these worst-case conditions, we estimate benzene acute HQ values (based
on the REL) could be as high as 9. Although the REL (which indicates
the level below which adverse effects are not anticipated) is exceeded
in this case, we believe the potential for acute effects is low for
several reasons. First, the acute modeling scenario is worst-case
because of the confluence of peak emission rates and worst-case
dispersion conditions.

Second, the benzene REL is based on a 6-hour exposure duration because
a 1-hour exposure duration value was unavailable. An REL based on a 6-
hour exposure duration is generally lower than an REL based on a 1-hour
exposure duration and, consequently, easier to exceed. Also, although
there are exceedances of the REL, the highest estimated 1-hour exposure
is less than 10 percent of the AEGL-1 value, which is a level at which
effects could be experienced. Finally, the generally sparse populations
near these facilities make it less likely that a person would be near
the plant to be exposed. For example, in the two cases where the acute
HQ value is as high as 9, there are only 30 people associated with the
census blocks within 2 miles of the two facilities.
    While our additional analysis of facility-wide risks showed that
there is one facility with maximum facility-wide cancer risk of 100-in-
1 million or greater and 10 facilities with a maximum chronic noncancer
TOSHI greater than 1, it also showed that oil and natural gas
production operations did not drive these risks.
    In determining whether risk is acceptable, we considered the
available health information, as described above. In this case,
although a number of factors we considered indicate relatively low risk
concern, we are proposing to determine that the risks are unacceptable,
in large part, because the MIR is 400-in-1 million due to MACT-
allowable emissions, which greatly exceeds the ``presumptive limit on
maximum individual lifetime risk of approximately 1-in-10 thousand
[100-in-1 million] recognized in the Benzene NESHAP (54 FR 38045).''
The MIR, based on MACT-allowable emissions, is driven by the allowable
emissions of 0.9 Mg/yr benzene under the MACT as a compliance option.
We are, therefore, proposing to eliminate the alternative compliance
option of 0.9 Mg/yr benzene from the existing glycol dehydrator MACT
requirements. With this change, the source category MIR, based on MACT-
allowable emissions, would be reduced to 40-in-1 million, which we find
acceptable in light of all the other factors considered. Thus, we are
proposing that the risks from the Oil and Natural Gas Production source
category are acceptable, with the removal of the alternative compliance
option of 0.9 Mg/yr benzene limit from the current glycol dehydrator
MACT requirements.
    Pursuant to CAA section 112(f)(4), we are proposing that this
change (i.e., removal of the 0.9 Mg/yr compliance alternative) apply 90
days after its effective date. We are requesting comment on whether or
not this is sufficient time for the large dehydrators that have been
relying on this compliance alternative to come into compliance with the
95-percent control requirement or if additional time is needed. See CAA
section 112(f)(4)(A).
    We recognize that our proposal to remove the 0.9 Mg/yr compliance
alternative for the 95-percent control glycol dehydrator MACT standard
could have negative impacts on some sources that have come to rely on
the flexibility this alternative provides. We solicit comment on any
such impacts and whether such impacts warrant adding a different
compliance alternative that would result in less risk than the 0.9 Mg/
yr benzene limit compliance option. If a commenter suggests a different
compliance alternative, the commenter should explain, in detail, what
that alternative would be, how it would work and how it would reduce
risk.
ii. Ample Margin of Safety
    We next considered whether this revised standard (existing MACT
plus removal of 0.9 Mg/yr benzene compliance option) provides an ample
margin of safety. In this analysis, we investigated available emissions
control options that might reduce the risk associated with emissions
from the source category and considered this information along with all
of the health risks and other health information considered in the risk
acceptability determination.
    For glycol dehydrators, we considered the addition of a second
control device in the same manner that was discussed in the floor
evaluation in section VII.B.1 above. The cost effectiveness associated
with that option would be $167,200/Mg, which we believe is too high to
require additional controls on glycol dehydrators.
    Similarly, we considered the addition of a second control device to
the required MACT floor control device (cost effectiveness of $18,300/
Mg). Similar to our discussion of beyond-the-MACT-floor controls for
glycol dehydrators in section VII.B.1 of this preamble, the incremental
cost to add a second control device for storage vessels would be
approximately 20 times higher than the MACT floor cost effectiveness,
or $366,000/Mg. We do not believe this cost effectiveness is
reasonable.
    For leak detection, we considered implementation of LDAR programs
that are more stringent than the current standards. An assessment
performed for various LDAR options under the NSPS in section VI.B.4.b
of this preamble yielded the lowest cost effectiveness of $5,170/Mg
($4,700/ton) for control of VOC for the options evaluated. A LDAR
program to control HAP would involve similar costs for equipment,
labor, etc., to those considered in the NSPS assessment, but since
there is approximately 20 times less HAP than VOC present in material
handled in regulated equipment, the cost effectiveness to control HAP
would be approximately 20 times greater (i.e., $100,000/Mg) for HAP,
which we believe is not reasonable.
    In accordance with the approach established in the Benzene NESHAP,
the EPA weighed all health risk measures and information considered in
the risk acceptability determination, along with the costs and economic
impacts of emissions controls, technological feasibility, uncertainties
and other relevant factors in making our ample margin of safety
determination. Considering the health risk information and the high
cost effectiveness of the options identified, we propose that the
existing MACT standards, with the removal of the 1 tpy benzene limit
compliance option from the glycol dehydrator standards, provide an
ample margin of safety to protect public health.
    While we are proposing that the oil and natural gas production MACT
standards (with the removal of the alternative compliance option of 1
tpy benzene limit) provide an ample margin of safety to protect public
health, we are concerned about the estimated facility-wide risks
identified through these screening analyses. As described previously,
the highest estimated facility-wide cancer risks are mostly due to
emissions from oil fired boilers and RICE. Both of these sources are
regulated under other source categories and we anticipate that emission
reductions from those sources will occur as standards for those source
categories are implemented.
3. What are the results and proposed decisions from the risk review for
the Natural Gas Transmission and Storage source category?
a. Results of the Risk Assessments and Analyses
    We conducted an inhalation risk assessment for the Natural Gas
Transmission and Storage source category. We also conducted an
assessment of facility-wide risk and performed a demographic analysis
of population risks. Details of the risk assessments and analyses can
be found in the residual risk documentation, which is available in the
docket for this action.

i. Inhalation Risk Assessment Results
    Table 5 provides an overall summary of the results of the
inhalation risk assessment. For informational purposes and to examine
the potential for any EJ issues that might be associated with each
source category, we performed a demographic analysis of population
risks.

                                    Table 5--Natural Gas Transmission and Storage Inhalation Risk Assessment Results
--------------------------------------------------------------------------------------------------------------------------------------------------------
                     Maximum individual cancer risk (in 1                                         Maximum chronic noncancer TOSHI \4\
                                 million) \2\                  Estimated      Estimated  annual -------------------------------------- Maximum  off-site
     Number of     ---------------------------------------   population at    cancer  incidence                                         acute  noncancer
  Facilities \1\     Actual  emissions      Allowable        risk >= 1-in-1    (cases per year)  Actual  emissions      Allowable            HQ \5\
                           level         emissions level        million                                level         emissions level
--------------------------------------------------------------------------------------------------------------------------------------------------------
             321           \3\ 30-90          \3\ 30-90          \3\ 2,500    \3\ 0.0003-0.001               0.4                0.8          HQREL = 5
                                                                                (benzene)
                                                                                HQAEGL	1 = 0.2
                                                                                (chlorobenzene)
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Number of facilities evaluated in the risk analysis.
\2\ Estimated maximum individual excess lifetime cancer risk.
\3\ The EPA IRIS assessment for benzene provides a range of equally-plausible URE (2.2E-06 to 7.8E-06 per ug/m\3\), giving rise to ranges for the
  estimates of cancer MIR and cancer incidence. Estimated population values are not scalable with benzene URE range, but would be lower using the lower
  end of the URE range.
\4\ Maximum TOSHI. The target organ with the highest TOSHI for the Natural Gas Transmission and Storage source category is the immune system.
\5\ The maximum estimated acute exposure concentration was divided by available short-term dose-response values to develop an array of HQ values.

    As shown in Table 5 above, the results of the inhalation risk
assessment performed using actual emissions data indicate the maximum
lifetime individual cancer risk could be as high as 90-in-1 million,
(30-in-1 million based on the lower end of the benzene URE range), with
benzene as the major contributor to the risk. The total estimated
cancer incidence from the source category is 0.001 excess cancer cases
per year (0.0003 excess cancer cases per year based on the lower end of
the benzene URE range), or one case in every polycyclic organic matter
1,000 years. Approximately 2,500 people are estimated to have cancer
risks at or above 1-in-1 million as a result of the emissions from 15
facilities (use of the lower end of the benzene URE range would further
reduce this population estimate). The maximum chronic noncancer TOSHI
value for the source category could be up to 0.4 from emissions of
benzene, indicating no significant potential for chronic noncancer
impacts.
    As explained above in section VII.C.1.b, our analysis of potential
differences between actual emission levels and emissions allowable
under the natural gas transmission and storage MACT standard indicate
that MACT-allowable emission levels may be up to 50 times greater than
actual emission levels at some sources. However, because some sources
are emitting at the level allowed under the current NESHAP, the risk
results from the inhalation risk assessment indicate the maximum
lifetime individual cancer risk would still be 90-in-1 million (30-in-1
million based on the lower end of the benzene URE range), based on both
actual and allowable emission levels, and the maximum chronic noncancer
TOSHI value could be as high as 0.8 at the MACT-allowable emissions
level.
ii. Facility-Wide Risk Assessment Results
    A facility-wide risk analysis was also conducted based on actual
emissions levels. Table 6 below displays the results of the facility-
wide risk assessment. For detailed facility-specific results, see Table
2 of Appendix 6 of the risk document in the docket for this rulemaking.

    Table 6--Natural Gas Transmission and Storage Facility-Wide Risk
                           Assessment Results
------------------------------------------------------------------------

------------------------------------------------------------------------
Number of Facilities Analyzed..............................          321

    Number of facilities with estimated facility-wide                  3
     individual cancer risk of 100-in-1 million or more....
    Number of facilities at which the Natural Gas                      1
     Transmission and Storage source category contributes
     50 percent or more to the facility-wide individual
     cancer risks of 100-in-1 million or more..............
    Number of facilities with facility-wide individual                74
     cancer risk of 1-in-1 million or more.................
    Number of facilities at which the Natural Gas                     10
     Transmission and Storage source category contributes
     50 percent or more to the facility-wide individual
     cancer risk of 1-in-1 million or more.................
Chronic Noncancer Risk:
    Maximum facility-wide chronic noncancer TOSHI..........           80
    Number of facilities with facility-wide maximum                   30
     noncancer TOSHI greater than 1........................
    Number of facilities at which the Natural Gas                      0
     Transmission and Storage source category contributes
     50 percent or more to the facility-wide maximum
     noncancer TOSHI of 1 or more..........................
------------------------------------------------------------------------
\1\ We note that the MIR would be 100-in-1 million if the CIIT URE for
  formaldehyde were used instead of the IRIS URE.

    The facility-wide MIR from all HAP emissions at any facility that
contains sources subject to the natural gas transmission and storage
MACT standards is estimated to be 200-in-1 million, based on actual
emissions. Of the 321 facilities included in this analysis, three have
facility-wide MIR of 100-in-1 million or greater. The facility-wide MIR
is 200-in-1 million at two of these facilities, driven by formaldehyde

from RICE.\37\ Another facility has a facility-wide risk of 100-in-1
million, with 90 percent of the risk attributed to natural gas
transmission and storage. There are 74 facilities with facility-wide
MIR of 1-in-1 million or greater. Of these facilities, 10 have natural
gas transmission and storage operations that contribute greater than 50
percent to the facility-wide risks. As discussed above, we are
proposing MACT standards for benzene emissions from small glycol
dehydrators in this action. These standards would reduce the risk from
benzene emissions at facilities with natural gas transmission and
storage operations. The facility-wide cancer risks at the facilities
with risks of 1-in-1 million or more are primarily driven by
formaldehyde emissions from RICE, which will be assessed in a future
RTR for that category.
---------------------------------------------------------------------------

    \37\ We note that there is an ongoing IRIS reassessment for
formaldehyde, and that future RTR risk assessments will use the
cancer potency for formaldehyde that results from that reassessment.
As a result, the current results may not match those of future
assessments.
---------------------------------------------------------------------------

    The facility-wide maximum individual chronic noncancer TOSHI is
estimated to be 80, based on actual emissions. Of the 321 facilities
included in this analysis, 30 have facility-wide maximum chronic
noncancer TOSHI values greater than 1. Of these facilities, none had
natural gas transmission and storage operations that contributed
greater than 50 percent to these facility-wide risks. The chronic
noncancer risks at these facilities are primarily driven by acrolein
emissions from RICE.
iii. Demographic Risk Analysis Results
    The results of the demographic analyses performed to investigate
the distribution of cancer risks at or above 1-in-1 million among the
surrounding population are summarized in Table 7 below. These results,
for various demographic groups, are based on actual emissions levels
for the population living within 50 km of the facilities.

                 Table 7--Natural Gas Transmission and Storage Demographic Risk Analysis Results
----------------------------------------------------------------------------------------------------------------
                                                                              Population with cancer risk at or
                                                                              above 1-in-1 million due to . . .
                                                             Nationwide    -------------------------------------
                                                                             Source category   Facility-wide HAP
                                                                              HAP emissions        emissions
----------------------------------------------------------------------------------------------------------------
Total Population.......................................        285,000,000              2,500             99,000
----------------------------------------------------------------------------------------------------------------
                                                 Race by Percent
----------------------------------------------------------------------------------------------------------------
White..................................................                 75                 92                 58
All Other Races........................................                 25                  8                 42
----------------------------------------------------------------------------------------------------------------
                                                 Race by Percent
----------------------------------------------------------------------------------------------------------------
White..................................................                 75                 92                 58
African American.......................................                 12                  6                 40
Native American........................................                0.9                0.1                0.2
Other and Multiracial..................................                 12                  1                  2
----------------------------------------------------------------------------------------------------------------
                                              Ethnicity by Percent
----------------------------------------------------------------------------------------------------------------
Hispanic...............................................                 14                  1                  2
Non-Hispanic...........................................                 86                 99                 98
----------------------------------------------------------------------------------------------------------------
                                                Income by Percent
----------------------------------------------------------------------------------------------------------------
Below Poverty Level....................................                 13                 17                 20
Above poverty level....................................                 87                 83                 80
----------------------------------------------------------------------------------------------------------------
                                              Education by Percent
----------------------------------------------------------------------------------------------------------------
Over 25 and without High School Diploma................                 13                 20                 15
Over 25 and with a High School Diploma.................                 87                 80                 85
----------------------------------------------------------------------------------------------------------------

    The results of the Natural Gas Transmission and Storage source
category demographic analysis indicate that there are approximately
2,500 people exposed to a cancer risk at or above 1-in-1 million due to
emissions from the source category, including an estimated 8 percent
that are classified as minority (listed as ``All Other Races'' in Table
7 above). Of the 2,500 people with estimated cancer risks at or above
1-in-1 million from the source category, 17 percent are in the ``Below
Poverty Level'' demographic group, and 20 percent are in the ``Over 25
and without High School Diploma'' demographic group, results which are
4 and 7 percentage points higher, respectively, than the percentages
for these demographic groups across the United States. The percentages
for the other demographic groups are lower than their respective
nationwide percentages. The table also shows that there are
approximately 99,000 people exposed to an estimated cancer risk at or
above 1-in-1 million due to facility-wide emissions, including an
estimated 42 percent that are classified as minority (``All Other
Races'' in Table 7 above). Of the 99,000 people with estimated cancer
risk at or above 1-in-1 million from facility-wide emissions, 40
percent are in the ``African American'' demographic group, 20 percent
are in the ``Below Poverty Level'' demographic group, and 15 percent
are in the ``Over 25 and without High School Diploma'' demographic
group, results which are 28, 7 and 2 percentage points higher,
respectively, than the percentages for these demographic groups across
the United States. The percentages for the other demographic groups are
equal to

or lower than their respective nationwide percentages.
b. What are the proposed risk decisions for the Natural Gas
Transmission and Storage source category?
i. Risk Acceptability
    In the risk analysis we performed for this source category,
pursuant to CAA section 112(f)(2), we considered the available health
information--the MIR; the numbers of persons in various risk ranges;
cancer incidence; the maximum noncancer HI; the maximum acute noncancer
hazard; the extent of noncancer risks; the potential for adverse
environmental effects; distribution of risks in the exposed population;
and risk estimation uncertainty (54 FR 38044, September 14, 1989).
    For the Natural Gas Transmission and Storage source category, the
risk analysis we performed indicates that the cancer risks to the
individual most exposed could be as high as 90-in-1 million due to
actual and allowable emissions (30-in-1 million, based on the lower end
of the benzene URE range). These risks are near 100-in-1 million, which
is the presumptive limit of acceptability. On the other hand, the risk
analysis shows low cancer incidence (1 case in every 1,000 years), low
potential for adverse environmental effects or human health multi-
pathway effects and that chronic and acute noncancer health impacts are
unlikely. We conclude that acute noncancer health impacts are unlikely
for reasons similar to those described in section VII.C.2.b.i of this
preamble.
    Our additional analysis of facility-wide risks showed that, among
three facilities with maximum facility-wide cancer risk of 100-in-1
million or greater, one facility has a facility-wide cancer risk of
100-in-1 million, with 90 percent of the risk attributed to natural gas
and transmission and storage. There are 30 facilities with a maximum
chronic noncancer TOSHI greater than 1, but natural gas transmission
and storage operations did not drive this risk.
    In determining whether risk is acceptable, we considered the
available health information, as described above. In this case, because
the MIR is approaching, but still less than 100-in-1 million risk, and
because a number of other factors indicate relatively low risk concern
(e.g., low cancer incidence, low potential for adverse environmental
effects or human health multi-pathway effects, chronic and acute
noncancer health impacts unlikely), we are proposing to determine that
the risks are acceptable.
ii. Ample Margin of Safety
    We next considered whether the existing MACT standard provides an
ample margin of safety. In this analysis, we investigated available
emissions control options that might reduce the risk associated with
emissions from the source category and considered this information,
along with all of the health risks and other health information
considered in the risk acceptability determination. The estimated MIR
of 90-in-1 million discussed above is driven by the 0.9 Mg/year benzene
limit compliance alternative for the glycol dehydrator MACT standard in
the current NESHAP. Removal of this compliance alternative would lower
the MIR for the source category to 20-in-1 million. We, therefore,
considered removing this compliance alternative as an option for
reducing risk and assessed the cost of such alternative. Without the
compliance alternative, affected glycol dehydrators (i.e., those units
with annual average benzene emissions of 0.9 Mg/yr or greater and an
annual average natural gas throughput of 283,000 scmd or greater) must
demonstrate compliance with the 95-percent control requirement, which
we believe can be shown with their existing control devices in most
cases, although, in some instances, installation of a different or an
additional control may be necessary.
    In section VII.B.1 above, we discuss the costs for requiring
controls on currently unregulated ``small glycol dehydrators,'' which
are similar, in operation and type of emission controls, to the
dehydrators subject to the current MACT (``large dehydrators''). The
HAP cost effectiveness determined for small dehydrators at the floor
level of control was $1,650/Mg. Although control methodologies are
similar for large and small dehydrators, we expect that the costs for
controls on large units could be as much as twice as high as for small
units because of the large gas flow being processed. However, we also
expect that the amount of HAP emission reduction for the large
dehydrators, in general, to be as much as, or more than, the amount
achieved by small dehydrators. In light of the above, we do not expect
the cost effectiveness of the control device needed to meet the 95-
percent control requirement for large dehydrators to exceed $3,300/Mg
(i.e., twice the cost effectiveness for small dehydrators), which we
consider to be reasonable.
    In accordance with the approach established in the Benzene NESHAP,
the EPA weighed all health risk measures and information considered in
the risk acceptability determination, along with the costs and economic
impacts of emissions controls, technological feasibility, uncertainties
and other relevant factors in making our ample margin of safety
determination. Considering the health risk information and the
reasonable cost effectiveness of the option identified, we propose that
the existing MACT standards, with the removal of the 0.9 Mg benzene
limit compliance option from the glycol dehydrator standards, provide
an ample margin of safety to protect public health.
    Pursuant to CAA section 112(f)(4), we are proposing that this
change (i.e., removal of the 0.9 Mg/yr compliance alternative) apply 90
days after its effective date. We are requesting comment on whether or
not there is sufficient time for the large dehydrators that have been
relying on this compliance alternative to come into compliance with the
95-percent control requirement or if additional time is needed. See CAA
section 112(f)(4)(A).
    We recognize that our proposal to remove the one-ton compliance
alternative for the 95-percent control glycol dehydrator MACT standard
could have negative impacts on some sources that have come to rely on
the flexibility this alternative provides. We solicit comment on any
such impacts and whether such impacts warrant adding a different
compliance alternative that would result in less risk than the 0.9 Mg/
yr benzene limit compliance option. If a commenter suggests a different
compliance alternative, the commenter should explain, in detail, what
that alternative would be, how it would work, and how it would reduce
risk.
    As described above, we are proposing that the natural gas
transmission and storage MACT standards (with the removal of the 0.9
Mg/yr benzene limit compliance option) provide an ample margin of
safety to protect public health. We recognize that one facility has a
facility-wide cancer risk of 100-in-1 million, with 90 percent of the
risk attributed to natural gas transmission and storage. This risk is
driven by benzene emissions from glycol dehydrators and is being
addressed by our proposed revision to the Natural Gas Transmission and
Storage NESHAP (removal of the 0.9 Mg/yr benzene limit compliance
option). As previously mentioned, two facilities have facility-wide MIR
of 200-in-1 million, driven by formaldehyde from RICE. Emissions from
RICE are regulated under another source category and will be assessed
under a future RTR for that category.

D. How did we perform the technology review and what are the results
and proposed decisions?

1. What was the methodology for the technology review?
    Our technology review is focused on the identification and
evaluation of ``developments in practices, processes, and control
technologies'' since the promulgation of the MACT standards for the two
oil and gas source categories. If a review of available information
identifies such developments, then we conduct an analysis of the
technical feasibility of requiring the implementation of these
developments, along with the impacts (costs, emission reductions, risk
reductions, etc.). We then make a decision on whether it is necessary
to amend the regulation to require these developments.
    Based on specific knowledge of each source category, we began by
identifying known developments in practices, processes and control
technologies. For the purpose of this exercise, we considered any of
the following to be a ``development'':
     Any add-on control technology or other equipment that was
not identified and considered during MACT development;
     Any improvements in add-on control technology or other
equipment (that was identified and considered during MACT development)
that could result in significant additional emission reduction;
     Any work practice or operational procedure that was not
identified and considered during MACT development; and
     Any process change or pollution prevention alternative
that could be broadly applied that was not identified and considered
during MACT development.
    In addition to looking back at practices, processes or control
technologies reviewed at the time we developed the MACT standards, we
reviewed a variety of sources of data to aid in our evaluation of
whether there were additional practices, processes or controls to
consider. One of these sources of data was subsequent air toxics rules.
Since the promulgation of the MACT standards for the source categories
addressed in this proposal, the EPA has developed air toxics
regulations for a number of additional source categories. We reviewed
the regulatory requirements and/or technical analyses associated with
these subsequent regulatory actions to identify any practices,
processes and control technologies considered in these efforts that
could possibly be applied to emission sources in the source categories
under this current RTR review.
    We also consulted the EPA's RBLC. The terms ``RACT,'' ``BACT,'' and
``LAER'' are acronyms for different program requirements under the CAA
provisions addressing the NAAQS. Control technologies classified as
RACT, BACT or LAER apply to stationary sources depending on whether the
source exists or is new and on the size, age and location of the
facility. The BACT and LAER (and sometimes RACT) are determined on a
case-by-case basis, usually by state or local permitting agencies. The
EPA established the RBLC to provide a central database of air pollution
technology information (including technologies required in source-
specific permits) to promote the sharing of information among
permitting agencies and to aid in identifying future possible control
technology options that might apply broadly to numerous sources within
a category or apply only on a source-by-source basis. The RBLC contains
over 5,000 air pollution control permit determinations that can help
identify appropriate technologies to mitigate many air pollutant
emission streams. We searched this database to determine whether any
practices, processes or control technologies are included for the types
of processes used for emission sources (e.g., spray booths) in the
source categories under consideration in this proposal.
    We also consulted information from the Natural Gas STAR program.
The Natural Gas STAR program is a flexible, voluntary partnership that
encourages oil and natural gas companies to adopt cost effective
technologies and practices that improve operational efficiency and
reduce pollutant emissions. The program provides the oil and gas
industry with information on new techniques and developments to reduce
pollutant emissions from the various processes.
2. What are the results and proposed decisions from the technology
review?
    There are three types of emission sources covered by the two oil
and gas NESHAP. These sources and the control technologies (including
add-on control devices and process modifications) considered during the
development of the MACT standards are: Glycol dehydrators (combustion
devices, recovery devices, process modifications), storage vessels with
the PFE (combustion devices, recovery devices) and equipment leaks
(LDAR programs, specific equipment modifications). Dehydrators are
addressed by both 40 CFR part 63, subpart HH and 40 CFR part 63,
subpart HHH, while equipment leaks and storage vessels with the PFE are
only covered by subpart HH.
    Since the promulgation of 40 CFR part 63, subpart HH, which
established MACT standards to address HAP emissions from equipment
leaks at gas processing plants, the EPA has developed LDAR programs
that are more stringent than what is required in subpart HH. The most
prevalent differences between these more stringent programs and subpart
HH relate to the frequency of monitoring and the concentration which
constitutes a ``leak.'' We do consider these programs to represent a
development in practices and evaluated whether to revise the MACT
standards for equipment leaks at natural gas processing plants under
subpart HH in light of this development.
    An analysis was performed above in section VI.B.1 to assess the VOC
reduction, costs and other impacts associated with these more stringent
LDAR program options at natural gas processing plants. One option
considered was to require compliance with 40 CFR part 60, subpart VVa
instead of 40 CFR part 60, subpart VV (the current NSPS requirement for
equipment leaks of VOC at natural gas processing plants), which changes
the leak definition (based on methane) from 10,000 ppm to 500 ppm and
requires monitoring of connectors. Because the current leak definition
under NESHAP 40 CFR part 63, subpart HH is the same as that in NSPS
subpart VV, and the ratio of VOC to HAP is approximately 20 to 1, we
expect that the HAP reduction would be 1/20th of the VOC reduction
under subpart VVa. The estimated incremental cost for that option was
determined to be $3,340 per ton of VOC. Based on the 20-to-1 ratio, we
estimate the incremental cost to control HAP at the subpart VVa level
would be approximately $66,800 per ton of HAP ($73,480/Mg). Other
options considered in section VI.B.1 of this preamble (and the
incremental cost of each option for reducing HAP) are as follows: The
use of an optical gas imaging camera monthly with an annual EPA Method
21 check ($129,000 per ton of HAP/$143,600 per Mg, if purchasing the
camera; $93,000 per ton of HAP/$103,300 per Mg, if renting the camera);
monthly optical gas imagining alone; and annual optical gas
imaging.\38\ In

light of the above, we do not believe that the additional costs of
these programs are justified.
---------------------------------------------------------------------------

    \38\ As stated above in section VI.B.1, emissions for the two
options using the optical gas imaging camera alone cannot be
quantified and, therefore, no cost effectiveness values were
determined.
---------------------------------------------------------------------------

    In addition to the plant-wide evaluations, a component analysis was
also evaluated at gas processing plants for the 40 CFR part 60, subpart
VVa-level of control (option 1 considered in section VI.B.1).\39\ That
assessment shows that the subpart VVa-level of control for connectors
has an incremental cost effectiveness of $4,360 per ton for VOC for
connectors and $144 per ton for VOC for valves. This means the
incremental cost to control HAP would be approximately $87,200 per ton
($96,900/Mg) for connectors and $2,880 per ton ($3,200/Mg) for valves.
We do not believe the additional cost for the more stringent
requirement for connectors is justified, but the additional cost for
valves is justified. Therefore, we are proposing to revise the
equipment leak requirements in 40 CFR part 63, subpart HH to lower the
leak definition for valves to an instrument reading of at least 500 ppm
as a result of our technology review.
---------------------------------------------------------------------------

    \39\ Because optical gas imaging is used to view several pieces
of equipment at a facility at once to survey for leaks, options
involving imaging are not amenable to a component by component
analysis.
---------------------------------------------------------------------------

    Some of the practices, processes or control technologies listed by
the Natural Gas STAR program applicable to the emission sources in
these categories were not identified and evaluated during the original
MACT development. While the Natural Gas STAR program does contain
information regarding new innovative techniques that are available to
reduce HAP emissions, they are not considered to have emission
reductions higher than what is set by the original MACT. One control
technology identified in the Natural Gas STAR program that would result
in no HAP emissions from glycol dehydration units would be the
replacement of a glycol dehydration unit with a desiccant dehydrator.
This technology cannot be used for natural gas operations with gas
streams having high temperature, high volume, and low pressure. Due to
the limitations posed by these conditions, we do not consider desiccant
dehydrators as MACT.
    For storage vessels, the applicable technologies identified by the
Gas STAR program, which are evaluated above for proposal under NSPS in
section VI.B.4, are similar to the cover and control technologies
currently required for storage vessels under the existing MACT.
Therefore, these technologies would not result in any further emissions
reductions than what is achieved by the original MACT.
    Our review of the RBLC did not identify any practices, processes
and control technologies applicable to the emission sources in these
categories that were not identified and evaluated during the original
MACT development. In light of the above, we are not proposing any
revisions to the existing MACT standards for storage vessels pursuant
to section 112(d)(6) of the CAA.

E. What other actions are we proposing?

1. Combustion Control Device Testing
    As explained below in section VII.E.2, under our proposal,
performance testing would be required initially and every 5 years for
non-condenser control devices. However, for certain enclosed combustion
control devices, we are proposing to allow, as an alternative to on-
site testing, a performance test conducted by a control device
manufacturer in accordance with the procedures provided in this
proposal. We propose to allow a unit whose model meets the proposed
performance criteria to claim a BTEX or HAP destruction efficiency of
98 percent at the facility. This value is lower than the 99.9-percent
destruction efficiency required in the manufacturers' test due to
variations between the test fuel specified and the gas streams
combusted at the actual facility. A source subject to the small
dehydrator BTEX limit would use the 98-percent destruction efficiency
to calculate their dehydrator's BTEX emissions for the purpose of
demonstrating compliance. For the 95-percent control MACT standard, a
control device matching the tested model would be considered to meet
that requirement. Once a device has been demonstrated to meet the
proposed performance criteria (and, therefore, is assigned a 98-percent
destruction efficiency), installation of a unit matching the tested
model at a facility would require no further performance testing (i.e.,
periodic tests would not be required every 5 years).
    We are proposing this alternative to minimize issues associated
with performance testing of certain combustion control devices. We
believe that testing units that are not configured with a distinct
combustion chamber present several technical issues that are more
optimally addressed through manufacturer testing, and once these units
are installed at a facility, through periodic inspection and
maintenance in accordance with manufacturers' recommendations. One
issue is that an extension above certain existing combustion control
device enclosures will be necessary to get adequate clearance above the
flame zone. Such extensions can more easily be configured by the
manufacturer of the control device rather than having to modify an
extension in the field to fit devices at every site. Issues related to
transporting, installing and supporting the extension in the field are
also eliminated through manufacturer testing. Another concern is that
the pitot tube used to measure flow can be altered by radiant heat from
the flame such that gas flow rates are not accurate. This issue is best
overcome by having the manufacturer select and use the pitot tube best
suited to their specific unit. For these reasons, we believe the
manufacturers' test is appropriate for these control devices with
ongoing performance ensured by periodic inspection and maintenance.
    This proposed alternative does not apply to flares, as defined in
40 CFR 63.761 and 40 CFR 63.1271, which must demonstrate compliance by
meeting the design and operation requirements in 40 CFR 63.11(b), 40
CFR 63.772(e)(2) and 40 CFR 63.1282(d)(2). It also would not apply to
thermal oxidizers having a combustion chamber/firebox where combustion
temperature and residence time can be measured during an on-site
performance test and are valid indicators of performance. These thermal
oxidizers do not present the issues described above relative to on-site
performance testing and, therefore, do not need an alternative testing
option. The proposed alternative would, therefore, apply to enclosed
combustion control devices except for these thermal oxidizers.
    In conjunction with the proposed manufacturer testing alternative,
we are proposing to add a definition for flare to clarify that flares,
as referenced in the NESHAP (and to which the proposed testing
alternative does not apply), refers to a thermal oxidation system with
an open flame (i.e., without enclosure). Accordingly, any thermal
oxidation system that does not meet the proposed flare definition would
be considered an enclosed combustion control device.
    We estimate that there are many existing facilities currently using
enclosed combustion control devices that would be required to either
conduct an on-site performance test or install and operate a control
device tested by the manufacturer under our proposal. Given the
estimated number of these combustion control devices in use, the time
required for manufacturers to test and manufacture such units, we are
proposing that existing sources have up to 3 years from the date of the
final rules' publication date to comply with

the initial performance testing requirements.
2. Monitoring, Recordkeeping and Reporting
    We are proposing to make changes to the monitoring requirements
described below to address issues we have identified through a
monitoring sufficiency review performed during the RTR process. First,
we are including calibration procedures associated with parametric
monitoring requirements in the existing NESHAP. The NESHAP require
parametric monitoring of control device parameters (e.g., temperatures
or flowrate monitoring), but did not include information on calibration
or included inadequate information on calibration of monitoring
devices. Therefore, we are specifying the calibration requirements for
temperature and flow monitors that the NESHAP currently lacks.
    In addition, under the current NESHAP, a design analysis can be
used in lieu of performance testing to demonstrate compliance and
establish operating parameter limits. We are proposing to allow the use
of the design evaluation alternative only when the control device being
used is a condenser. The design evaluation option is appropriate for
condensers because their emissions can be accurately predicted using
readily available physical property information (e.g., vapor pressure
data and condensation calculations). In those cases, one would not need
to conduct emissions testing to determine actual emissions to
demonstrate compliance with the MACT standard. For example, a
requirement that ``the temperature at the outlet of the condenser shall
be maintained at 50[deg] Fahrenheit below the condensation temperature
calculated for the compound of interest using the reference equation''
(e.g., National Institute of Standards and Technology Chemistry WebBook
at http://webbook.nist.gov/chemistry/) is adequate to assure proper
operation of the condenser and, therefore, compliance with the required
emission standard.
    For other types of control technologies, such as carbon adsorption
systems and enclosed combustion devices,\40\ the ability to predict
emissions depends on data developed by the vendor and such data may not
reliably result in an accurate prediction of emissions from a specific
facility. There are variables (e.g., air to fuel ratios and waste
constituents for combustion; varying organic concentrations,
constituents and capacity issues, including break-through for carbon
adsorption) that make theoretical predictions less reliable. The
effects of these site-specific variables on emissions are not easily
predictable and establishing monitoring conditions (e.g., combustion
temperature, vacuum regeneration) based on vendor data will likely not
account for those variables. Therefore, we propose to eliminate the
design evaluation alternative for non-condenser controls.
---------------------------------------------------------------------------

    \40\ The design analysis alternative in the existing MACT does
not apply to flares. As previously mentioned, the existing MACT
provides separate design and operation requirements for flares.
---------------------------------------------------------------------------

    For non-condenser controls (and condensers not using the design
analysis option), in addition to the initial compliance testing, we are
proposing that performance tests be conducted at least once every 5
years and whenever sources desire to establish new operating limits.
Under the current NESHAP, a performance test is only conducted in two
instances: (1) As an alternative to a design analysis for their
compliance demonstration and identification of operating parameter
ranges and (2) as a requirement to resolve a disagreement between the
EPA and the owner or operator regarding the design analysis. The
current NESHAP do not require additional performance testing beyond
these two cases (i.e., there is no periodic testing requirement). As
mentioned above, we are proposing to remove the design evaluation
option for non-condenser controls. For non-condenser controls (and
condensers not using the design analysis option), the proposed periodic
testing would ensure compliance with the emission standards by
verifying that the control device is meeting the necessary HAP
destruction efficiency determined in the initial performance test. As
discussed above in section VII.E.1, we are proposing that combustion
control devices tested under the manufacturers' procedure are not
required to conduct periodic testing. In addition, we are also
proposing that combustion control devices that can demonstrate a
uniform combustion zone temperature meeting the required control
efficiency during the initial performance test are exempt from periodic
testing. The requirement for continuous monitoring of combustion zone
temperature is an accurate indicator of control device performance and
eliminates the need for future testing.
    The current NESHAP (40 CFR 63.771(d) and 40 CFR 63.1281(d)) require
operating an enclosed combustion device at a minimum residence time of
0.5 seconds at a minimum temperature of 760 degrees Celsius. We are
proposing to remove the residence time requirement. The residence time
requirement is not needed because the compliance demonstration made
during the performance test is sufficient to ensure that the combustion
device has adequate residence time to ensure the needed destruction
efficiency. Therefore, we are proposing to remove the residence time
requirement.
    We are also clarifying at 40 CFR 63.773(d)(3)(i) and 40 CFR
63.1283(d)(3)(i) for thermal vapor incinerators, boilers and process
heaters, that the temperature sensor shall be installed at a location
representative of the combustion zone temperature. Currently, the
regulation requires that the temperature sensor be installed at a
location ``downstream of the combustion zone'' because we had thought
that the temperature downstream would be representative of combustion
zone temperature. We have now learned that may or may not be the case.
We are, therefore, proposing to amend this provision to more accurately
reflect the intended requirement.
    Next, consistent with revisions for SSM, we've revised 40 CFR
63.771(d)(4)(i) and 40 CFR 63.1281(d)(4)(i), except when maintenance or
repair on a unit cannot be completed without a shutdown of the control
device.
    Also, we've updated the criteria for prior performance test results
that can be used to demonstrate compliance in lieu of conducting a
performance test. These updates ensure that data for determining
compliance are accurate, up-to-date, and truly representative of actual
operating conditions.
    In addition, we are proposing to revise the temperature monitoring
device minimum accuracy criteria in 40 CFR 63.773(d)(3)(i) to better
reflect the level of performance that is required of the temperature
monitoring devices. We believe that temperature monitoring devices
currently used to meet the requirements of the NESHAP can meet the
proposed revised criteria without modification.
    Also, we are proposing to revise the calibration gas concentration
for the no detectable emissions procedure applicable to closed vent
systems in 40 CFR 63.772(c)(4)(ii) from 10,000 ppmv to 500 ppmv methane
to be consistent with the leak threshold of 500 ppmv in 40 CFR part 63,
subpart HH. The current calibration level is inconsistent with
achieving accurate readings at the level necessary to demonstrate there
are no detectable emissions.

    Also, we are proposing recordkeeping and reporting requirements for
carbon adsorption systems. The current NESHAP require the replacement
of all carbon in the carbon adsorption system with fresh carbon on a
regular, predetermined time interval that is no longer than the carbon
service life established for the carbon system, but provide no
recordkeeping or reporting requirement to document and assure
compliance with this standard. We believe that maintaining some sort of
log book is a reasonable alternative combined with a requirement to
report instances when specified practices are not followed. Therefore,
the proposed rule adds reporting and recordkeeping requirements for
establishing a schedule and maintaining logs of carbon replacement.
    Finally, as noted above in section VII.B.1, we are proposing a BTEX
emissions limit for small glycol dehydration unit process vents. For
the compliance demonstration, we propose that parametric monitoring of
the control device be performed. We believe that parametric monitoring
is adequate for glycol dehydrators in these two source categories
because temperature monitoring, whether it be to verify proper
condenser or combustion device operation, is a reliable indicator of
performance for reducing organic HAP emissions. We also considered the
use of a continuous emissions monitoring system (CEMS) to monitor
compliance. However, for glycol dehydrators in the oil and natural gas
sector, the necessary electricity, weather-protective enclosures and
daily staffing are not usually available. We, therefore, question the
technical feasibility of operating a CEMS correctly in this sector. We
request comment on the practicality of including provisions in the
final rule for a CEMS to monitor BTEX emissions for small glycol
dehydration units.
3. Startup, Shutdown, Malfunction
    The United States Court of Appeals for the District of Columbia
Circuit vacated portions of two provisions in the EPA's CAA section 112
regulations governing the emissions of HAP during periods of SSM.
Sierra Club v. EPA, 551 F.3d 1019 (D.C. Cir. 2008), cert. denied, 130
S. Ct. 1735 (U.S. 2010). Specifically, the Court vacated the SSM
exemption contained in 40 CFR 63.6(f)(1) and 40 CFR 63.6(h)(1), that is
part of a regulation, commonly referred to as the General Provisions
Rule, that the EPA promulgated under section 112 of the CAA. When
incorporated into CAA section 112(d) regulations for specific source
categories, these two provisions exempt sources from the requirement to
comply with the otherwise applicable CAA section 112(d) emission
standard during periods of SSM.
    We are proposing the elimination of the SSM exemption in the two
oil and gas NESHAP. Consistent with Sierra Club v. EPA, the EPA is
proposing to apply the standards in these NESHAP at all times. In
addition, we are proposing to revise 40 CFR 63.771(d)(4)(i) and 40 CFR
63.1281(d)(4)(i) to remove the provision allowing shutdown of the
control device during maintenance or repair. We are also proposing
several revisions to the General Provisions applicability table for the
MACT standard. For example, we are proposing to eliminate the
incorporation of the General Provisions' requirement that the source
develop a SSM plan. We are also proposing to eliminate or revise
certain recordkeeping and reporting requirements related to the SSM
exemption. The EPA has attempted to ensure that we have not included in
the proposed regulatory language any provisions that are inappropriate,
unnecessary or redundant in the absence of the SSM exemption. We are
specifically seeking comment on whether there are any such provisions
that we have inadvertently incorporated or overlooked.
    In proposing the MACT standards in these rules, the EPA has taken
into account startup and shutdown periods. We believe that operations
and emissions do not differ from normal operations during these periods
such that it warrants a separate standard. Therefore, we have not
proposed different standards for these periods.
    Periods of startup, normal operations and shutdown are all
predictable and routine aspects of a source's operations. However, by
contrast, malfunction is defined as a ``sudden, infrequent and not
reasonably preventable failure of air pollution control and monitoring
equipment, process equipment or a process to operate in a normal or
usual manner * * *'' (40 CFR 63.2). The EPA has determined that
malfunctions should not be viewed as a distinct operating mode and,
therefore, any emissions that occur at such times do not need to be
factored into development of CAA section 112(d) standards, which, once
promulgated, apply at all times. In Mossville Environmental Action Now
v. EPA, 370 F.3d 1232, 1242 (D.C. Cir. 2004), the Court upheld as
reasonable, standards that had factored in variability of emissions
under all operating conditions. However, nothing in CAA section 112(d)
or in case law requires that the EPA anticipate and account for the
innumerable types of potential malfunction events in setting emission
standards. See Weyerhaeuser v. Costle, 590 F.2d 1011, 1058 (D.C. Cir.
1978), (``In the nature of things, no general limit, individual permit,
or even any upset provision can anticipate all upset situations. After
a certain point, the transgression of regulatory limits caused by
``uncontrollable acts of third parties,'' such as strikes, sabotage,
operator intoxication or insanity, and a variety of other
eventualities, must be a matter for the administrative exercise of
case-by-case enforcement discretion, not for specification in advance
by regulation.'').
    Further, it is reasonable to interpret CAA section 112(d) as not
requiring the EPA to account for malfunctions in setting emissions
standards. For example, we note that CAA section 112 uses the concept
of ``best performing'' sources in defining MACT, the level of
stringency that major source standards must meet. Applying the concept
of ``best performing'' to a source that is malfunctioning presents
significant difficulties. The goal of best performing sources is to
operate in such a way as to avoid malfunctions of their units.
    Moreover, even if malfunctions were considered a distinct operating
mode, we believe it would be impracticable to take malfunctions into
account in setting CAA section 112(d) standards for oil and natural gas
production facility and natural gas transmission and storage
operations. As noted above, by definition, malfunctions are sudden and
unexpected events, and it would be difficult to set a standard that
takes into account the myriad different types of malfunctions that can
occur across all sources in each source category. Moreover,
malfunctions can also vary in frequency, degree and duration, further
complicating standard setting.
    In the event that a source fails to comply with the applicable CAA
section 112(d) standards as a result of a malfunction event, the EPA
would determine an appropriate response based on, among other things,
the good faith efforts of the source to minimize emissions during
malfunction periods, including preventative and corrective actions, as
well as root cause analyses to ascertain and rectify excess emissions.
The EPA would also consider whether the source's failure to comply with
the CAA section 112(d) standard was, in fact, ``sudden, infrequent, not
reasonably preventable'' and was not instead ``caused in part by poor
maintenance or careless operation.'' 40 CFR 63.2 (definition of
malfunction).

    Finally, the EPA recognizes that even equipment that is properly
designed and maintained can sometimes fail and that such failure can
sometimes cause or contribute to an exceedance of the relevant emission
standard. (See, e.g., State Implementation Plans: Policy Regarding
Excessive Emissions During Malfunctions, Startup, and Shutdown
(September 20, 1999); Policy on Excess Emissions During Startup,
Shutdown, Maintenance, and Malfunctions (February 15, 1983)). The EPA
is, therefore, proposing to add to the final rule an affirmative
defense to civil penalties for exceedances of emission limits that are
caused by malfunctions in both of the MACT standards addressed in this
proposal. See 40 CFR 63.761 for sources subject to the oil and natural
gas production MACT standards, or 40 CFR 63.1271 for sources subject to
the natural gas transmission and storage MACT standards (defining
``affirmative defense'' to mean, in the context of an enforcement
proceeding, a response or defense put forward by a defendant, regarding
which the defendant has the burden of proof and the merits of which are
independently and objectively evaluated in a judicial or administrative
proceeding). We also are proposing other regulatory provisions to
specify the elements that are necessary to establish this affirmative
defense; a source subject to the oil and natural gas production
facilities or natural gas transmission MACT standards must prove by a
preponderance of the evidence that it has met all of the elements set
forth in 40 CFR 63.762 and a source subject to the natural gas
transmission and storage facilities MACT standards must prove by a
preponderance of the evidence that it has met all of the elements set
forth in 40 CFR 63.1272. (See 40 CFR 22.24.) The criteria ensure that
the affirmative defense is available only where the event that causes
an exceedance of the emission limit meets the narrow definition of
malfunction in 40 CFR 63.2 (sudden, infrequent, not reasonably
preventable and not caused by poor maintenance and or careless
operation). For example, to successfully assert the affirmative
defense, the source must prove by a preponderance of evidence that
excess emissions ``[w]ere caused by a sudden, infrequent, and
unavoidable failure of air pollution control and monitoring equipment,
process equipment, or a process to operate in a normal or usual manner
* * *.'' The criteria also are designed to ensure that steps are taken
to correct the malfunction, to minimize emissions in accordance with 40
CFR 63.762 for sources subject to the oil and natural gas production