---------------------------------------------------------------------------

    Because the U.S. has a dense network of airports, many of which
have neighboring communities, we quantified the number of people living
and children attending school within 500 meters of the approximately
20,000 airports in the U.S.\190\ From this analysis, the EPA estimates
that approximately 5.2 million people live within 500 meters of an
airport runway, 363,000 of whom are children aged five and under. The
EPA also estimates that 573 schools attended by 163,000 children in
kindergarten through twelfth grade are within 500 meters of an airport
runway.\191\
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    \190\ In this analysis, we included populations living in census
blocks that intersected the 500-meter buffer around each runway in
the U.S. Potential uncertainties in this approach are described in
our report National Analysis of the Populations Residing Near or
Attending School Near U.S. Airports. EPA-420-R-20-001, available at
https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100YG4A.pdf, and in the
EPA responses to peer review comments on the report, available here:
https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100YISM.pdf.
    \191\ EPA (2020) National Analysis of the Populations Residing
Near or Attending School Near U.S. Airports. EPA-420-R-20-001.
Available at https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100YG4A.pdf.
---------------------------------------------------------------------------

    In order to identify potential disparities in the near-airport
population, we also evaluated populations at the state level. Using the
U.S. Census population data for each state in the U.S., we compared the
percent of people by age, race and indigenous peoples (i.e., children
five and under, Black, Asian, and Native American or Alaska Native)
living within 500 meters of an airport runway with the percent by age,
race, and indigenous peoples comprising the state population.\192\
Using the methodology described in Clarke (2022), the EPA identified
states in which children, Black, Asian, and Native American or Alaska
Native populations represent a greater fraction of the population
living within 500 meters of a runway compared with the percent of these
groups in the state population.\193\ Results of this analysis are
presented in the following tables.\194\ This state-level analysis
presents summary information for a subset of potentially relevant
demographic characteristics. We present data in this section regarding
a wider array of demographic characteristics when evaluating
populations living near NPIAS airports.
---------------------------------------------------------------------------

    \192\ Clarke. Memorandum to Docket EPA-HQ-OAR-2022-0389.
Estimation of Population Size and Demographic Characteristics among
People Living Near Airports by State in the United States. May 31,
2022. Docket ID EPA-HQ-2022-0389.
    \193\ Clarke. Memorandum to Docket EPA-HQ-OAR-2022-0389.
Estimation of Population Size and Demographic Characteristics among
People Living Near Airports by State in the United States. May 31,
2022. Docket ID EPA-HQ-2022-0389.
    \194\ These data are presented in tabular form for all states in
this memorandum located in the docket: Clarke. Memorandum to Docket
EPA-HQ-OAR-2022-0389. Estimation of Population Size and Demographic
Characteristics among People Living Near Airports by State in the
United States. May 31, 2022. Docket ID EPA-HQ-2022-0389.
---------------------------------------------------------------------------

    Among children five and under, there were three states (Nevada,
South Carolina, and South Dakota) in which the percent of children five
and under

living within 500 meters of a runway represents a greater fraction of
the population by a difference of one percent or greater compared with
the percent of children five and under in the state population (Table
3).

 Table 3--The Population of Children Five Years and Under Within 500 Meters of an Airport Runway Compared to the
                                State Population of Children Five Years and Under
----------------------------------------------------------------------------------------------------------------
                                          Percent of         Percent of         Number of          Number of
                                        children aged      children aged      children aged      children aged
                State                   five years and     five years and     five years and     five years and
                                      under within  500  under within  the  under within  500    under in  the
                                            meters             state              meters             state
----------------------------------------------------------------------------------------------------------------
Nevada..............................                 10                  8               1000            224,200
South Carolina......................                  9                  8                400            361,400
South Dakota........................                 11                  9              3,000             71,300
----------------------------------------------------------------------------------------------------------------

    There were nine states in which the Black population represented a
greater fraction of the population living in the near-airport
environment by a difference of one percent or greater compared with the
state as a whole. These states were California, Kansas, Kentucky,
Louisiana, Mississippi, Nevada, South Carolina, West Virginia, and
Wisconsin (Table 4).

     Table 4--The Black Population Within 500 Meters of an Airport Runway and the Black Population, by State
----------------------------------------------------------------------------------------------------------------
                                        Percent black      Percent black     Black population   Black population
                State                 within 500 meters   within the state  within 500 meters     in the state
----------------------------------------------------------------------------------------------------------------
California..........................                  8                  7             18,981          2,486,500
Kansas..............................                  8                  6              1,240            173,300
Kentucky............................                  9                  8              3,152            342,800
Louisiana...........................                 46                 32             14,669          1,463,000
Mississippi.........................                 46                 37              8,542          1,103,100
Nevada..............................                 12                  9              1,794            231,200
South Carolina......................                 31                 28             10,066          1,302,900
West Virginia.......................                 10                  3              1,452             63,900
Wisconsin...........................                  9                  6              4,869            367,000
----------------------------------------------------------------------------------------------------------------

    There were three states with a greater fraction of Asians in the
near-airport environment compared with the state as a whole by a
difference of one percent or greater: Indiana, Maine, and New Hampshire
(Table 5).

     Table 5--The Asian Population Within 500 Meters of an Airport Runway and the Asian Population, by State
----------------------------------------------------------------------------------------------------------------
                                        Percent asian      Percent asian     Asian population   Asian population
                State                 within 500 meters   within the state  within 500 meters     in the state
----------------------------------------------------------------------------------------------------------------
Indiana.............................                  4                  2              1,681            105,500
Maine...............................                  2                  1                406             13,800
New Hampshire.......................                  4                  2                339             29,000
----------------------------------------------------------------------------------------------------------------

    There were five states (Alaska, Arizona, Delaware, South Dakota,
and New Mexico) where the near-airport population had greater
representation by Native Americans and Alaska Natives compared with the
portion of the population they comprise at the state level by a
difference of one percent or greater. In Alaska, the disparity in
residential proximity to a runway was the largest: 16,020 Alaska
Natives were estimated to live within 500 meters of a runway,
representing 48 percent of the population within 500 meters of an
airport runway. In contrast, Alaska Natives comprise 15 percent of the
Alaska state population (Table 6).

 Table 6--The Native American and Alaska Native Population Within 500 Meters of an Airport Runway and the Native
                                 American and Alaska Native Population, by State
----------------------------------------------------------------------------------------------------------------
                                                                             Native American
                                        Percent Native     Percent Native       and Alaska      Native American
                State                    American and       American and    Native population      and Alaska
                                        Alaska Native      Alaska Native        within 500     Native population
                                      within 500 meters   within the state        meters          in the state
----------------------------------------------------------------------------------------------------------------
Alaska..............................                 48                 15             16,020            106,300
Arizona.............................                 18                  5              5,017            335,300
Delaware............................                  2                  1                112              5,900
New Mexico..........................                 21                 10              2,265            208,900
South Dakota........................                 22                  9              1,606             72,800
----------------------------------------------------------------------------------------------------------------

    In a separate analysis, the EPA focused on evaluating the potential
for disparities in populations residing near the NPIAS airports. The
EPA compared the demographic composition of people living within one
kilometer of runways at 2,022 of the approximately 3,300 NPIAS airports
with the demographic composition of people living at a distance of one
to five kilometers from the same airports.195 196 In this
analysis, over one-fourth of airports (i.e., 515) were identified at
which children under five were more highly represented in the zero to
one kilometer distance compared with the percent of children under five
living one to five kilometers away (Table 7). There were 666 airports
where people of color had a greater presence in the zero to one
kilometer area closest to airport runways than in populations farther
away. There were 761 airports where people living at less than two-
times the Federal Poverty Level represented a higher proportion of the
overall population within one kilometer of airport runways compared
with the proportion of people living at less than two times the Federal
Poverty Level among people living one to five kilometers away.
---------------------------------------------------------------------------

    \195\ For this analysis, we evaluated the 2,022 airports with a
population of greater than 100 people inside the zero to one
kilometer distance to avoid low population counts distorting the
assessment of percent contributions of each group to the total
population within the zero to one kilometer distance.
    \196\ Kamal et al., Memorandum to Docket EPA-HQ-OAR-2022-0389.
Analysis of Potential Disparity in Residential Proximity to Airports
in the Conterminous United States. May 24, 2022. Docket ID EPA-HQ-
2022-0389. Methods used are described in this memo and include the
use of block group resolution data to evaluate the representation of
different demographic groups near-airport and for those living one
to five kilometers away.

     Table 7--Number of Airports (Among the 2,022 Airports Evaluated) With Disparity for Certain Demographic
 Populations Within One Kilometer of an Airport Runway in Relation to the Comparison Population Between One and
                                     Five Kilometers From an Airport Runway
----------------------------------------------------------------------------------------------------------------
                                                        Number of airports with disparity
                                --------------------------------------------------------------------------------
       Demographic group          Total airports                   Disparity 5-    Disparity 10-
                                  with disparity  Disparity 1-5%        10%             20%       Disparity 20%+
----------------------------------------------------------------------------------------------------------------
Children under five years of                 515             507               7               1               0
 age...........................
People with income less than                 761             307             223             180              51
 twice the Federal Poverty
 Level.........................
People of Color (all non-White               666             377             126             123              40
 races, ethnicities and
 indigenous peoples)...........
Non-Hispanic Black.............              405             240              77              67              21
Hispanic.......................              551             402              85              47              17
Non-Hispanic Asian.............              268             243              18               4               3
Non-Hispanic Native American or              144             130               6               7               1
 Alaska Native \197\...........
Non-Hispanic Hawaiian or                      18              17               1               0               0
 Pacific Islander..............
Non-Hispanic Other Race........               11              11               0               0               0
Non-Hispanic Two or More Races.              226             226               0               0               0
----------------------------------------------------------------------------------------------------------------

    To understand the extent of the potential disparity among the 2,022
NPIAS airports, Table 7 provides information about the distribution in
the percent differences in the proportion of children, individuals with
incomes below two times the Federal Poverty Level, and people of color
living within one kilometer of a runway compared with those living one
to five kilometers away. For children, Table 7 indicates that for the
vast majority of these airports where there is a higher percentage of
children represented in the near-airport population, differences are
relatively small (e.g., less than five percent). For the airports where
disparity is evident on the basis of poverty, race and ethnicity, the
disparities are potentially large, ranging up to 42 percent for those
with incomes below two times the Federal Poverty Level, and up to 45
percent for people of color.\198\
---------------------------------------------------------------------------

    \197\ This analysis of 2,022 NPIAS airports did not include
airports in Alaska.
    \198\ Kamal et al., Memorandum to Docket EPA-HQ-OAR-2022-0389.
Analysis of Potential Disparity in Residential Proximity to Airports
in the Conterminous United States. May 24, 2022. Docket ID EPA-HQ-
2022-0389.
---------------------------------------------------------------------------

    There are uncertainties in the results provided here inherent to
the proximity-based approach used. These uncertainties include the use
of block group data to provide population numbers for each demographic
group analyzed, and uncertainties in the Census data, including from
the use of data from different analysis years (e.g., 2010 Census Data
and 2018 income data). These uncertainties are described

and their implications discussed in Kamal et al. (2022).\199\
---------------------------------------------------------------------------

    \199\ Kamal et al., Memorandum to Docket EPA-HQ-OAR-2022-0389.
Analysis of Potential Disparity in Residential Proximity to Airports
in the Conterminous United States. May 24, 2022. Docket ID EPA-HQ-
2022-0389.
---------------------------------------------------------------------------

    The data summarized in this section indicate that there is a
greater prevalence of children under five years of age, an at-risk
population for lead effects, within 500 meters or one kilometer of some
airports compared to more distant locations. This information also
indicates that there is a greater prevalence of people of color and of
low-income populations within 500 meters or one kilometer of some
airports compared with people living more distant. If such differences
were to contribute to disproportionate and adverse impacts on
particular communities, they could indicate an EJ concern. Given the
number of children in close proximity to runways, including those in
communities with EJ concerns, there is a potential for substantial
implications for children's health, depending on lead exposure levels
and associated risk.
    Some commenters on the proposed findings expressed concern that
communities in close proximity to general aviation airports are often
low-income communities and communities of color who are
disproportionately burdened by lead exposure.\200\ Some commenters also
noted that children who attend school near airports may experience
higher levels of exposure compared with children who attend school more
distant from an airport, and they cite recent research reporting higher
blood lead levels in children who attend school near one highly active
general aviation airport.\201\ The EPA responds to these comments in
the Response to Comments document for this action.
---------------------------------------------------------------------------

    \200\ During the public comment period on the proposed findings
for this action, commenters provided an additional evaluation of
populations living near airports that they conclude to indicate that
disparity by race and income is larger and occurs more frequently at
airports that have the highest lead emissions and the highest
residential population density compared with airports where less
lead is emitted and population density is lower. This comment is
available in the docket at regulations.gov: EPA-HQ-OAR-2022-0389-
0238.
    \201\ Zahran et al., 2022. Leaded Aviation Gasoline Exposure
Risk and Child Blood Lead Levels. Proceedings of the National
Academy of Sciences Nexus. 2:1-11.
---------------------------------------------------------------------------

B. Federal Actions To Reduce Lead Exposure

    The Federal Government has a longstanding commitment to programs to
reduce exposure to lead, particularly for children. In December 2018,
the President's Task Force on Environmental Health Risks and Safety
Risks to Children released the Federal Action Plan to Reduce Childhood
Lead Exposures and Associated Health Impacts (Federal Lead Action
Plan), detailing the Federal Government's commitments and actions to
reduce lead exposure in children, some of which are described in this
section.\202\ Building on the 2018 Federal Lead Action Plan, in October
2022, the EPA finalized its Strategy to Reduce Lead Exposures and
Disparities in U.S. Communities (Lead Strategy).\203\ The Lead Strategy
describes the EPA-wide and government-wide approaches to strengthen
public health protections, address legacy lead contamination for
communities with the greatest exposures, and promote environmental
justice. In this section, we describe some of the EPA's actions to
reduce lead exposures from air, water, lead-based paint, and
contaminated sites.
---------------------------------------------------------------------------

    \202\ Federal Lead Action Plan to Reduce Childhood Lead
Exposures and Associated Health Impacts. (2018) President's Task
Force on Environmental Health Risks and Safety Risks to Children.
Available at https://www.epa.gov/sites/default/files/2018-12/documents/fedactionplan_lead_final.pdf.
    \203\ EPA (2022) EPA Strategy to Reduce Lead Exposures and
Disparities in U.S. Communities. EPA 540R22006. Available at https://www.epa.gov/system/files/documents/2022-11/Lead%20Strategy_1.pdf.
---------------------------------------------------------------------------

    In 1976, the EPA listed lead under CAA section 108, making it what
is called a ``criteria air pollutant.'' \204\ Once lead was listed, the
EPA issued primary and secondary NAAQS under sections 109(b)(1) and
(2), respectively. The EPA issued the first NAAQS for lead in 1978 and
revised the lead NAAQS in 2008 by reducing the level of the standard
from 1.5 micrograms per cubic meter to 0.15 micrograms per cubic meter
and revising the averaging time and form to an average over a
consecutive three-month period, as described in 40 CFR 50.16.\205\ The
EPA's 2016 Federal Register document describes the Agency's decision to
retain the existing Lead NAAQS.\206\ The Lead NAAQS is currently
undergoing review.\207\ \208\
---------------------------------------------------------------------------

    \204\ 41 FR 14921 (April 8, 1976). See also, e.g., 81 FR 71910
(Oct. 18, 2016) for a description of the history of the listing
decision for lead under CAA section 108.
    \205\ 73 FR 66965 (Nov. 12, 2008).
    \206\ 81 FR 71912-71913 (Oct. 18, 2016).
    \207\ Documents pertaining to the current review of the NAAQS
for Lead can be found here: https://www.epa.gov/naaqs/lead-pb-air-quality-standards.
    \208\ The EPA released the ISA for Lead, External Review Draft,
as part of the Agency's current review of the science regarding
health and welfare effects of lead. EPA/600/R-23/061. This draft
assessment is undergoing peer review by the Clean Air Scientific
Advisory Committee (CASAC) and public comment, and is available at:
https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=357282.
---------------------------------------------------------------------------

    States are primarily responsible for ensuring attainment and
maintenance of the NAAQS. Under section 110 of the Act and related
provisions, states are to submit, for the EPA's review and, if
appropriate, approval, state implementation plans that provide for the
attainment and maintenance of such standards through control programs
directed to sources of the pollutants involved.
    Additional EPA programs to address lead in the environment include
the prohibition on gasoline containing lead or lead additives for
highway use under section 211 of the Act; the new source performance
standards under section 111 of the Act; and emissions standards for
solid waste incineration units and the national emission standards for
hazardous air pollutants (NESHAP) under sections 129 and 112 of the
Act, respectively.
    The EPA has taken a number of actions associated with these air
pollution control programs, including completion of several regulations
requiring reductions in lead emissions from stationary sources
regulated under the CAA sections 111, 112 and 129. For example, in
January 2012, the EPA updated the NESHAP for the secondary lead
smelting source category.\209\ These amendments to the original maximum
achievable control technology standards apply to facilities nationwide
that use furnaces to recover lead from lead-bearing scrap, mainly from
automobile batteries. Regulations completed in 2013 for commercial and
industrial solid waste incineration units also require reductions in
lead emissions.\210\ In February 2023, the EPA finalized amendments to
the NSPS (as a new subpart) and the Area Source NESHAP for the Lead
Acid Battery Manufacturing source category.\211\ The amendments to the
standards for affected processes including grid casting, lead
reclamation, and paste mixing operations at lead acid battery
facilities will result in reductions in lead emissions and improvements
in compliance assurance measures.
---------------------------------------------------------------------------

    \209\ 77 FR 555 (Jan. 5, 2012).
    \210\ 78 FR 9112 (Feb. 7, 2013).
    \211\ 88 FR 11556 (Feb. 23, 2023).
---------------------------------------------------------------------------

    A broad range of Federal programs beyond those that focus on air
pollution control provide for nationwide reductions in environmental
releases and human exposures to lead. For example, pursuant to section
1417 of the Safe Drinking Water Act (SDWA), any pipe, pipe or plumbing
fitting or fixture, solder, or flux may not be used in new
installations or repairs of any public water system or plumbing in a

residential or non-residential facility providing water for human
consumption or introduced into commerce (except uses for manufacturing
or industrial purposes) unless it is considered ``lead free'' as
defined by that Act.\212\ The EPA's Lead and Copper Rule,\213\ first
promulgated in 1991, regulates lead in public drinking water systems
through a treatment technique that requires water systems to monitor
drinking water at customer taps and, if an action level is exceeded,
undertake a number of actions including those to control corrosion to
minimize lead exposure.\214\ On January 15, 2021, the agency published
the most recent revisions, the Lead and Copper Rule Revisions
(LCRR),\215\ and subsequently reviewed the rule in accordance with
Executive Order 13990.\216\ While the LCRR took effect in December
2021, the agency concluded that there are significant opportunities to
improve the LCRR.\217\ The EPA is developing a new proposed rule, the
Lead and Copper Rule Improvements (LCRI),\218\ to further strengthen
the lead drinking water regulations. The EPA identified priority
improvements for the LCRI: proactive and equitable lead service line
replacement (LSLR), strengthening compliance tap sampling to better
identify communities most at risk of lead in drinking water and to
compel lead reduction actions, and reducing the complexity of the
regulation through improvement of the action and trigger level
construct.\219\ The EPA intends to propose and promulgate the LCRI
prior to October 16, 2024.
---------------------------------------------------------------------------

    \212\ Effective in Jan. 2014, the amount of lead permitted in
pipes, fittings, and fixtures was lowered. See, section 1417 of the
Safe Drinking Water Act: Prohibition on Use of Lead Pipes, Solder,
and Flux at https://www.epa.gov/sdwa/use-lead-free-pipes-fittings-fixtures-solder-and-flux-drinking-water.
    \213\ 40 CFR part 141, subpart I (June 7, 1991).
    \214\ 40 CFR part 141, subpart I (June 7, 1991).
    \215\ 86 FR 4198. (Jan. 15, 2021).
    \216\ E.O. 13990. Protecting Public Health and the Environment
and Restoring Science to Tackle the Climate Crisis. 86 FR 7037 (Jan.
20, 2021).
    \217\ 86 FR 31939 (Dec. 17, 2021).
    \218\ See https://www.epa.gov/ground-water-and-drinking-water/review-national-primary-drinking-water-regulation-lead-and-copper.
Accessed on Nov. 30, 2021.
    \219\ 86 FR 31939 (Dec. 17, 2021).
---------------------------------------------------------------------------

    While the EPA continues to improve regulatory actions to reduce
lead exposure in drinking water, the EPA recognizes that directly
assisting states and communities and providing dedicated funding
provided in the Bipartisan Infrastructure Law for lead service line
identification and replacement of full lead service lines (LSLs) is
also important in safeguarding public health. The EPA is providing $15
billion through the Drinking Water State Revolving Fund (DWSRF)
dedicated exclusively to lead service line identification and
replacement. In addition, $11.7 billion in DWSRF general supplemental
funding, provided by the Bipartisan Infrastructure Law, is going to
projects to improve drinking water quality, including those to reduce
lead in drinking water. For this funding, states are required to
provide 49% as additional subsidization in the form of principal
forgiveness and/or grants. States must provide additional subsidization
to water systems that meet the state's disadvantaged community criteria
as described in section 1452(d) of SDWA, furthering the objectives of
the Justice40 Initiative. In October 2022, the EPA announced projects
selected to receive over $30 million in grant funding that will help
communities and schools address lead in drinking water and remove lead
pipes across the country in underserved and other disadvantaged
communities through the Water Infrastructure Improvements for the
Nation Act's Reducing Lead in Drinking Water grant program. The EPA
recently announced the Lead Service Line Replacement Accelerators
initiative which will provide targeted technical assistance to
communities in Connecticut, Pennsylvania, New Jersey, and Wisconsin to
support expanded access to funding and to accelerate lead pipe
replacement. While the EPA is focusing initial efforts in four states,
the Agency anticipates this work will serve as a roadmap for additional
lead service line replacement efforts across the nation in the future.
    Federal programs to reduce exposure to lead in paint, dust, and
soil are specified under the comprehensive Federal regulatory framework
developed under the Residential Lead-Based Paint Hazard Reduction Act
(Title X). Under Title X (codified, in part, as Title IV of the Toxic
Substances Control Act [TSCA]), the EPA has established regulations and
associated programs in six categories: (1) Training, certification and
work practice requirements for persons engaged in lead-based paint
activities (abatement, inspection and risk assessment); accreditation
of training providers; and authorization of state and Tribal lead-based
paint programs; (2) training, certification, and work practice
requirements for persons engaged in home renovation, repair and
painting (RRP) activities; accreditation of RRP training providers; and
authorization of state and Tribal RRP programs; (3) ensuring that, for
most housing constructed before 1978, information about lead-based
paint and lead-based paint hazards flows from sellers to purchasers,
from landlords to tenants, and from renovators to owners and occupants;
(4) establishing standards for identifying dangerous levels of lead in
paint, dust and soil; (5) providing grant funding to establish and
maintain state and Tribal lead-based paint programs; and (6) providing
information on lead hazards to the public, including steps that people
can take to protect themselves and their families from lead-based paint
hazards.
    The most recent rules issued under Title IV of TSCA revised the
dust-lead hazard standards (DLHS) and dust-lead clearance levels (DLCL)
which were established in a 2001 final rule entitled ``Identification
of Dangerous Levels of Lead.'' \220\ The DLHS are incorporated into the
requirements and risk assessment work practice standards in the EPA's
Lead-Based Paint Activities Rule, codified at 40 CFR part 745, subpart
L. They provide the basis for risk assessors to determine whether dust-
lead hazards are present in target housing (i.e., most pre-1978
housing) and child-occupied facilities (pre-1978 nonresidential
properties where children 6 years of age or under spend a significant
amount of time such as daycare centers and kindergartens). If dust-lead
hazards are present, the risk assessor will identify acceptable options
for controlling the hazards in the respective property, which may
include abatements and/or interim controls. In July 2019, the EPA
published a final rule revising the DLHS from 40 micrograms per square
foot and 250 micrograms per square foot to 10 micrograms per square
foot and 100 micrograms per square foot of lead in dust on floors and
windowsills, respectively.\221\ The DLCL are used to evaluate the
effectiveness of a cleaning following an abatement. If the dust-lead
levels are not below the clearance levels, the components (i.e.,
floors, windowsills, troughs) represented by the failed sample(s) shall
be recleaned and retested. In January 2021, the EPA published a final
rule revising the DLCL to match the DLHS, lowering them from 40
micrograms per square foot and 250 micrograms per square foot to 10
micrograms per square foot and 100 micrograms per square foot on floors
and windowsills, respectively.\222\ The EPA is now reconsidering the
2019 and 2021 rules in accordance with Executive Order 13990 \223\ and
in response to a

May 2021 decision by U.S. Court of Appeals for the Ninth Circuit. In
August 2023, EPA proposed updating the DLHS and DLCL again.\224\ If
finalized as proposed, the DLHS for floors and window sills would be
any reportable level greater than zero, as analyzed by any laboratory
recognized by EPA's National Lead Laboratory Accreditation Program. The
new DLCL would be 3 micrograms per square foot ([mu]g/ft\2\) for
floors, 20 [mu]g/ft\2\ for window sills and 25 [mu]g/ft\2\ for window
troughs.
---------------------------------------------------------------------------

    \220\ 66 FR 1206 (Jan. 5, 2001).
    \221\ 84 FR 32632 (July 9, 2019).
    \222\ 86 FR 983 (Jan. 7, 2021).
    \223\ 86 FR 7037 (Jan. 20, 2021).
    \224\ 88 FR 50444 (August 1, 2023).
---------------------------------------------------------------------------

    Programs associated with the Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA or Superfund) \225\ and
Resource Conservation Recovery Act (RCRA) \226\ also implement removal
and remedial response programs that reduce or abate exposures to
releases or threatened releases of lead and other hazardous substances.
Furthermore, CERCLA section 104(a)(1) authorizes the EPA and other
Federal agencies to respond to releases or threatened releases of
pollutants or contaminants when the release, or potential release, may
present an imminent and substantial danger to the public health or
welfare. In addition, CERCLA section 104(a)(1) and the National Oil and
Hazardous Substances Pollution Contingency Plan (NCP) authorize
remedial investigations (e.g., monitoring, testing, information
collection) and removal actions for hazardous substances, pollutants,
or contaminants.
---------------------------------------------------------------------------

    \225\ For more information about the EPA's CERCLA program, see
https://www.epa.gov/superfund.
    \226\ For more information about the EPA's RCRA program, see
https://www.epa.gov/rcra.
---------------------------------------------------------------------------

    The EPA develops and implements protective levels for lead in soil
(and other media when appropriate) at Superfund sites and, together
with states, at RCRA corrective action facilities. The Office of Land
and Emergency Management develops policy and guidance for addressing
multimedia lead contamination and determining appropriate response
actions at lead-contaminated sites. Federal programs, including those
implementing RCRA, provide for management of hazardous substances such
as lead in hazardous and municipal solid waste (e.g., 50 FR 28702, July
15, 1985; 52 FR 45788, December 1, 1987).

C. Lead Endangerment Petitions for Rulemaking and the EPA Responses

    The Administrator's final findings further respond to several
citizen petitions on this subject, including the following: petition
for rulemaking submitted by Friends of the Earth in 2006, petition for
rulemaking submitted by Friends of the Earth, Oregon Aviation Watch and
Physicians for Social Responsibility in 2012, petition for
reconsideration submitted by Friends of the Earth, Oregon Aviation
Watch, and Physicians for Social Responsibility in 2014, and petition
for rulemaking from Alaska Community Action on Toxics, Center for
Environmental Health, Friends of the Earth, Montgomery-Gibbs
Environmental Coalition, Oregon Aviation Watch, the County of Santa
Clara, CA, and the Town of Middleton, WI, in 2021. These petitions and
the EPA's responses are described more fully in the proposal for this
action.227 228
---------------------------------------------------------------------------

    \227\ See https://www.epa.gov/regulations-emissions-vehicles-and-engines/petitions-and-epa-response-memorandums-related-lead.
    \228\ 87 FR 62772 (Oct. 17, 2022).
---------------------------------------------------------------------------

    In the most recent of these petitions, submitted in 2021, Alaska
Community Action on Toxics, Center for Environmental Health, Friends of
the Earth, Montgomery-Gibbs Environmental Coalition, Oregon Aviation
Watch, the County of Santa Clara, CA, and the Town of Middleton, WI,
again petitioned the EPA to conduct a proceeding under CAA section 231
regarding whether lead emissions from piston-engine aircraft cause or
contribute to air pollution that may reasonably be anticipated to
endanger public health or welfare.\229\ The EPA responded in 2022
noting our intent to develop a proposal under CAA section 231(a)(2)(A)
regarding whether lead emissions from piston-engine aircraft cause or
contribute to air pollution that may reasonably be anticipated to
endanger public health or welfare, and, after evaluating public
comments on the proposal, issue any final determination in 2023, as the
Agency is doing in this action.\230\
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    \229\ The 2021 petition is available at https://www.epa.gov/system/files/documents/2022-01/aviation-leaded-avgas-petition-exhibits-final-2021-10-12.pdf.
    \230\ EPA's response to the 2021 petition is available at
https://www.epa.gov/system/files/documents/2022-01/ltr-response-aircraft-lead-petitions-aug-oct-2022-01-12.pdf.
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III. Legal Framework for This Action

    In this action, the EPA is finalizing two separate determinations--
an endangerment finding and a cause or contribute finding--under
section 231(a)(2)(A) of the Clean Air Act. The EPA has, most recently,
finalized such findings under CAA section 231 for greenhouse gases
(GHGs) in 2016 (2016 Findings), and in that action the EPA provided a
detailed explanation of the legal framework for making such findings
and the statutory interpretations and caselaw supporting its
approach.\231\ In this final action, the Administrator used the same
approach of applying a two-part test under section 231(a)(2)(A) as
described in the 2016 Findings and relied on the same interpretations
supporting that approach, which are briefly described in this section,
and set forth in greater detail in the 2016 Findings.\232\ This is also
the same approach that the EPA used in making endangerment and cause or
contribute findings for GHGs under section 202(a) of the CAA in 2009
(2009 Findings),\233\ which was affirmed by the U.S. Court of Appeals
for the D.C. Circuit in 2012.\234\ As explained further in the 2016
Findings, the text of the CAA section 231(a)(2)(A), which concerns
aircraft emissions, mirrors the text of CAA section 202(a), which
concerns motor vehicle emissions and which was the basis for the 2009
Findings.235 236 Accordingly, for the same reasons as
discussed in the 2016 Findings, the EPA believes it is reasonable to
use the same approach under section 231(a)(2)(A)'s similar text as was
used under section 202(a) for the 2009 Findings, and it is acting
consistently with that framework for purposes of these final findings
under section 231.\237\ As this approach has

been previously discussed at length in the 2016 Findings, as well as in
the 2009 Findings, the EPA provides only a brief description in this
final action.
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    \231\ 81 FR 54422-54475 (Aug. 15, 2016).
    \232\ See e.g., 81 FR 55434-54440 (Aug. 15, 2016).
    \233\ 74 FR 66505-66510 (Dec. 15, 2009).
    \234\ Coalition for Responsible Regulation, Inc. v. EPA, 684
F.3d 102 (D.C. Cir. 2012) (CRR) (rev'd in part on other grounds sub
nom. Utility Air Regulatory Group v. EPA, 573 U.S. 302 (2014)). As
discussed in greater detail in the 2016 Findings, the Supreme Court
granted some of the petitions for certiorari that were filed on CRR,
while denying others, but agreed to decide only the question:
``Whether EPA permissibly determined that its regulation of
greenhouse gas emissions from new motor vehicles triggered
permitting requirements under the Clean Air Act for stationary
sources that emit greenhouse gases.'' 81 FR 54422, 54442 (Aug. 15,
2016). Thus, the Supreme Court did not disturb the D.C. Circuit's
holding in CRR that affirmed the 2009 Endangerment Finding.
    \235\ For example, the text in CAA section 202(a) that was the
basis for the 2009 Findings addresses ``the emission of any air
pollutant from any class or classes of new motor vehicles or new
motor vehicle engines, which in [the Administrator's] judgment
cause, or contribute to, air pollution which may reasonably be
anticipated to endanger public health or welfare.'' Similarly,
section 231(a)(2)(A) concerns ``the emission of any air pollutant
from any class or classes of aircraft engines which in [the
Administrator's] judgment causes, or contributes to, air pollution
which may reasonably be anticipated to endanger public health or
welfare.'' Additional discussion of the parallels in the statutory
text and legislative history between CAA section 202(a) and
231(a)(2)(A) can be found in the 2016 Findings. See 81 FR 55434--
55437 (Aug. 15, 2016).
    \236\ 81 FR 55434 (Aug. 15, 2016).
    \237\ 81 FR 55434 (Aug. 15, 2016).
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A. Statutory Text and Basis for This Action

    Section 231(a)(2)(A) of the CAA provides that the ``Administrator
shall, from time to time, issue proposed emission standards applicable
to the emission of any air pollutant from any class or classes of
aircraft engines which in his judgment causes, or contributes to, air
pollution which may reasonably be anticipated to endanger public health
or welfare.'' \238\ In this action, the EPA is addressing the predicate
for regulatory action under CAA section 231 through a two-part test,
which as noted previously, is the same as the test used in the 2016
Findings under section 231 and in the 2009 Findings under section 202
of the CAA.
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    \238\ Regarding ``welfare,'' the CAA states that ``[a]ll
language referring to effects on welfare includes, but is not
limited to, effects on soils, water, crops, vegetation, manmade
materials, animals, wildlife, weather, visibility, and climate,
damage to and deterioration of property, and hazards to
transportation, as well as effects on economic values and on
personal comfort and well-being, whether caused by transformation,
conversion, or combination with other air pollutants.'' CAA section
302(h). Regarding ``public health,'' there is no definition of
``public health'' in the Clean Air Act. The Supreme Court has
discussed the concept of ``public health'' in the context of whether
costs can be considered when setting NAAQS. Whitman v. American
Trucking Ass'n, 531 U.S. 457 (2001). In Whitman, the Court imbued
the term with its most natural meaning: ``the health of the
public.'' Id. at 466.
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    As the first step of the two-part test, the Administrator must
decide whether, in his judgment, the air pollution under consideration
may reasonably be anticipated to endanger public health or welfare. As
the second step, the Administrator must decide whether, in his
judgment, emissions of an air pollutant from certain classes of
aircraft engines cause or contribute to this air pollution. If the
Administrator answers both questions in the affirmative, he will issue
standards under section 231.\239\
---------------------------------------------------------------------------

    \239\ See Massachusetts v. EPA, 549 U.S. 497, 533 (2007)
(interpreting an analogous provision in CAA section 202).
---------------------------------------------------------------------------

    In accordance with the EPA's interpretation of the text of section
231(a)(2)(A), as described in the 2016 Findings, the phrase ``may
reasonably be anticipated'' and the term ``endanger'' in section
231(a)(2)(A) authorize, if not require, the Administrator to act to
prevent harm and to act in conditions of uncertainty.\240\ They do not
limit him to merely reacting to harm or to acting only when certainty
has been achieved; indeed, the references to anticipation and to
endangerment imply that the failure to look to the future or to less
than certain risks would be to abjure the Administrator's statutory
responsibilities. As the D.C. Circuit explained, the language ``may
reasonably be anticipated to endanger public health or welfare'' in CAA
section 202(a) requires a ``precautionary, forward-looking scientific
judgment about the risks of a particular air pollutant, consistent with
the CAA's precautionary and preventive orientation.'' \241\ The court
determined that ``[r]equiring that the EPA find `certain' endangerment
of public health or welfare before regulating greenhouse gases would
effectively prevent the EPA from doing the job that Congress gave it in
[section] 202(a)--utilizing emission standards to prevent reasonably
anticipated endangerment from maturing into concrete harm.'' \242\ The
same language appears in section 231(a)(2)(A), and the same
interpretation applies in that context.
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    \240\ See 81 FR 54435 (Aug. 15, 2016).
    \241\ CRR, 684 F.3d at 122 (internal citations omitted) (June
26, 2012).
    \242\ CRR, 684 F.3d at 122 (internal citations omitted) (June
26, 2012).
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    Moreover, by instructing the Administrator to consider whether
emissions of an air pollutant cause or contribute to air pollution in
the second part of the two-part test, the Act makes clear that he need
not find that emissions from any one sector or class of sources are the
sole or even the major part of the air pollution considered. This is
clearly indicated by the use of the term ``contribute.'' Further, the
phrase ``in his judgment'' authorizes the Administrator to weigh risks
and to consider projections of future possibilities, while also
recognizing uncertainties and extrapolating from existing data.
    Finally, when exercising his judgment in making both the
endangerment and cause-or-contribute findings, the Administrator
balances the likelihood and severity of effects. Notably, the phrase
``in his judgment'' modifies both ``may reasonably be anticipated'' and
``cause or contribute.''
    Often, past endangerment and cause or contribute findings have been
proposed concurrently with proposed standards under various sections of
the CAA, including section 231.\243\ Comment has been taken on these
proposed findings as part of the notice and comment process for the
emission standards.\244\ However, there is no requirement that the
Administrator propose or finalize the endangerment and cause or
contribute findings concurrently with proposed standards and, most
recently under section 231, the EPA made endangerment and cause or
contribute findings for GHGs separate from, and prior to, proceeding to
set standards.
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    \243\ 81 FR 54425 (Aug. 15, 2016).
    \244\ See, e.g., Rulemaking for non-road compression-ignition
engines under section 213(a)(4) of the CAA, Proposed Rule at 58 FR
28809, 28813-14 (May 17, 1993), Final Rule at 59 FR 31306, 31318
(June 17, 1994); Rulemaking for highway heavy-duty diesel engines
and diesel sulfur fuel under sections 202(a) and 211(c) of the CAA,
Proposed Rule at 65 FR 35430 (June 2, 2000), and Final Rule at 66 FR
5002 (Jan. 18, 2001).
---------------------------------------------------------------------------

    As noted in the proposal,\245\ the Administrator is applying the
procedural provisions of CAA section 307(d) to this action, pursuant to
CAA section 307(d)(1)(V), which provides that the provisions of 307(d)
apply to ``such other actions as the Administrator may determine.''
\246\ Any subsequent standard-setting rulemaking under CAA section 231
would also be subject to the procedures under CAA section 307(d), as
provided in CAA section 307(d)(1)(F) (applying the provisions of CAA
section 307(d) to the promulgation or revision of any aircraft emission
standard under CAA section 231). Thus, these final findings are subject
to the same procedural requirements that would apply if the final
findings were part of a standard-setting rulemaking.
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    \245\ 87 FR 62773-62774 (Oct. 17, 2022).
    \246\ As the Administrator is applying the provisions of CAA
section 307(d) to this action under section 307(d)(1)(V), we need
not determine whether those provisions would apply to this action
under section 307(d)(1)(F).
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B. Considerations for the Endangerment and Cause or Contribute Analyses
Under Section 231(a)(2)(A)

    In the context of this final action, the EPA understands section
231(a)(2)(A) of the CAA to call for the Administrator to exercise his
judgment and make two separate determinations: first, whether the
relevant kind of air pollution (here, lead air pollution) may
reasonably be anticipated to endanger public health or welfare, and
second, whether emissions of any air pollutant from classes of the
sources in question (here, any aircraft engine that is capable of using
leaded aviation gasoline), cause or contribute to this air
pollution.\247\
---------------------------------------------------------------------------

    \247\ See CRR, 684 F.3d at 117 (explaining two-part analysis
under section 202(a)) (June 26, 2012).
---------------------------------------------------------------------------

    This analysis entails a scientific judgment by the Administrator
about the potential risks posed by lead emissions to public health and
welfare. In this final action, the EPA used the same approach in making
scientific judgments regarding endangerment as it has previously
described in the 2016

Findings, and its analysis was guided by the same five principles that
guided the Administrator's analysis in those Findings.\248\
---------------------------------------------------------------------------

    \248\ See, e.g., 81 FR 54434-55435 (Aug. 15, 2016).
---------------------------------------------------------------------------

    Similarly, the EPA took the same approach to the cause or
contribute analysis as was previously explained in the 2016
Findings.\249\ For example, as previously noted, section 231(a)(2)(A)'s
instruction to consider whether emissions of an air pollutant cause or
contribute to air pollution makes clear that the Administrator need not
find that emissions from any one sector or class of sources are the
sole or even the major part of an air pollution problem.\250\ Moreover,
like the language in CAA section 202(a) that governed the 2009
Findings, the statutory language in section 231(a)(2)(A) does not
contain a modifier on its use of the term ``contribute.'' \251\ Unlike
other CAA provisions, it does not require ``significant'' contribution.
Compare, e.g., CAA sections 111(b); 213(a)(2), (4). Congress made it
clear that the Administrator is to exercise his judgment in determining
contribution, and authorized regulatory controls to address air
pollution even if the air pollution problem results from a wide variety
of sources.\252\ While the endangerment test looks at the air pollution
being considered as a whole and the risks it poses, the cause or
contribute test is designed to authorize the EPA to identify and then
address what may well be many different sectors, classes, or groups of
sources that are each part of the problem.\253\
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    \249\ See, e.g., 81 FR 54437-54438 (Aug. 15, 2016).
    \250\ See, e.g., 81 FR 54437-54438 (Aug. 15, 2016).
    \251\ See, e.g., 81 FR 54437-54438 (Aug. 15, 2016).
    \252\ See 81 FR 54437-54438 (Aug. 15, 2016).
    \253\ See 81 FR 54437-54438 (Aug. 15, 2016).
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    Moreover, as the EPA has previously explained, the Administrator
has ample discretion in exercising his reasonable judgment and
determining whether, under the circumstances presented, the cause or
contribute criterion has been met.\254\ As noted in the 2016 Findings,
in addressing provisions in section 202(a), the D.C. Circuit has
explained that the Act at the endangerment finding step did not require
the EPA to identify a precise numerical value or ``a minimum threshold
of risk or harm before determining whether an air pollutant
endangers.'' \255\ Accordingly, the EPA ``may base an endangerment
finding on `a lesser risk of greater harm . . . or a greater risk of
lesser harm' or any combination in between.'' \256\ As the language in
section 231(a)(2)(A) is analogous to that in section 202(a), it is
reasonable to apply this interpretation to the endangerment
determination under section 231(a)(2)(A).\257\ Moreover, the logic
underlying this interpretation supports the general principle that
under CAA section 231 the EPA is not required to identify a specific
minimum threshold of contribution from potentially subject source
categories in determining whether their emissions ``cause or
contribute'' to the endangering air pollution.\258\ The reasonableness
of this principle is further supported by the fact that section 231
does not impose on the EPA a requirement to find that such contribution
is ``significant,'' let alone the sole or major cause of the
endangering air pollution.\259\
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    \254\ See 81 FR 54437-54438 (Aug. 15, 2016).
    \255\ CRR, 684 F.3d at 122-123 (June 26, 2012).
    \256\ CRR, 684 F.3d at 122-123. (quoting Ethyl Corp., 541 F.2d
at 18) (June 26, 2012).
    \257\ 81 FR 54438 (Aug. 15, 2016).
    \258\ 81 FR 54438 (Aug. 15, 2016).
    \259\ 81 FR 54438 (Aug. 15, 2016).
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    Finally, as also described in the 2016 Findings, there are a number
of possible ways of assessing whether air pollutants cause or
contribute to the air pollution which may reasonably be anticipated to
endanger public health and welfare, and no single approach is required
or has been used exclusively in previous cause or contribute
determinations under title II of the CAA.\260\
---------------------------------------------------------------------------

    \260\ See 81 FR 54462 (Aug. 15, 2016).
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C. Regulatory Authority for Emission Standards

    Though the EPA is not proposing standards in this final action, in
issuing these final findings, the EPA becomes subject to a duty under
CAA section 231 regarding emission standards applicable to emissions of
lead from aircraft engines. As noted in section III.A. of this
document, section 231(a)(2)(A) of the CAA directs the Administrator of
the EPA to propose and promulgate emission standards applicable to the
emission of any air pollutant from classes of aircraft engines which in
his or her judgment causes or contributes to air pollution that may
reasonably be anticipated to endanger public health or welfare.
    CAA section 231(a)(2)(B) further directs the EPA to consult with
the Administrator of the FAA on such standards, and it prohibits the
EPA from changing aircraft emission standards if such a change would
significantly increase noise and adversely affect safety. CAA section
231(a)(3) provides that after we provide an opportunity for a public
hearing on standards, the Administrator shall issue standards ``with
such modifications as he deems appropriate.'' In addition, under CAA
section 231(b), the effective date of any standards shall provide the
necessary time to permit the development and application of the
requisite technology, giving appropriate consideration to the cost of
compliance, as determined by the EPA in consultation with the U.S.
Department of Transportation (DOT).
    Once the EPA adopts standards, CAA section 232 then directs the
Secretary of DOT to prescribe regulations to ensure compliance with the
EPA's standards. Finally, section 233 of the CAA vests the authority to
promulgate emission standards for aircraft or aircraft engines only in
the Federal Government. States are preempted from adopting or enforcing
any standard respecting aircraft or aircraft engine emissions unless
such standard is identical to the EPA's standards.\261\
---------------------------------------------------------------------------

    \261\ CAA section 233.
---------------------------------------------------------------------------

D. Response to Certain Comments on the Legal Framework for This Action

    In commenting on the legal framework for this action, some
commenters assert that the EPA does have authority under CAA section
231(a)(2)(A) to both find that lead air pollution may reasonably be
anticipated to endanger the public health and welfare and to find that
engine emissions of lead from certain aircraft cause or contribute to
the lead air pollution that may reasonably be anticipated to endanger
the public health and welfare. We agree with these comments.
    Other commenters assert that the EPA does not have the legal
authority to proceed with this proposal or regulate aviation fuel.
These commenters state that Congress excluded aircraft from the CAA of
1970, that the EPA does not have authority to regulate aircraft fuel
(citing a regulatory definition of ``transportation fuel'' in 40 CFR
80.1401) and that aircraft are not motor vehicles (citing a regulatory
definition of ``motor vehicles'' in 40 CFR 85.1703). These commenters
say that the definitions of transportation fuel and motor vehicles were
not changed through 1977 or 1990 amendments to the CAA. Additionally,
commenters assert that the ``EPA points to findings for Green House
Gases (GHGs) under section 202(a) supportive of its proposed
authority,'' quoting that section and emphasizing the terms ``new motor
vehicles'' and ``new motor vehicle engines'' which are used in it.
    In response, the EPA notes that these commenters have fundamentally
misunderstood the nature of this action and the legal authority upon
which it relies. These final findings do not

establish regulatory standards for leaded avgas, nor are they related
in any way to the regulatory definitions of transportation fuels in 40
CFR 80.1401 or of motor vehicles in 40 CFR 85.1703, which implement EPA
programs under Part A of Title II of the CAA and do not apply to
aircraft that are governed by Part B of Title II. EPA's regulatory
provisions implementing Title II Part B and related to air pollution
from aircraft are found in 40 CFR parts 87, 1030, and 1031. The EPA's
authority for this action is not based on its authority to regulate
fuels under CAA section 211 or its authority to regulate motor vehicles
or motor vehicle engines under CAA section 202(a). Rather, the EPA's
authority for this action comes from CAA section 231(a)(2). Further,
this action is focused on the threshold endangerment and cause or
contribute criteria, which are being undertaken in proceedings that are
separate and distinct from any follow-on regulatory action; no
regulatory provisions were proposed and none are being finalized in
this action.
    In response to the claims that aircraft are excluded from the CAA
and that the EPA does not have authority to conduct this endangerment
and cause or contribute finding, we disagree. As described in the
proposal, the EPA is acting under the express authority prescribed by
Congress in section 231(a)(2)(A) of the CAA, which, as amended,
provides that the Administrator ``shall, from time to time, issue
proposed emission standards applicable to the emission of any air
pollutant from any class or classes of aircraft engines which in his
judgment causes, or contributes to, air pollution which may reasonably
be anticipated to endanger public health or welfare.'' The D.C. Circuit
recognized EPA's authority to promulgate emission standards applicable
to air pollutants from aircraft engines under CAA section 231 in
National Association of Clean Air Agencies v. EPA, 489 F.3d 1221 (D.C.
Cir. 2007) (``NACAA''). Similarly, in the 1970 amendments to the CAA,
section 231(a)(2) provided that the Administrator ``shall issue
proposed emission standards applicable to emissions of any air
pollutant from any class or classes of aircraft or aircraft engines
which in his judgment cause or contribute to or are likely to cause or
contribute to air pollution which endangers the public health or
welfare.'' Public Law 91-604. Thus, the statement in the comment that
the 1970 CAA excluded aircraft is incorrect.\262\ Further, the EPA has
previously made endangerment and cause or contribute findings related
to emissions from aircraft engines under section 231 of the CAA.
---------------------------------------------------------------------------

    \262\ The change to the current language in section 231(a)(2)
occurred in 1977, see Clean Air Act Amendments of 1977, Public Law
95-95, 91 Stat. 685, 791 (1977).
---------------------------------------------------------------------------

    As explained in the proposal, and in section III. above, in this
action the Administrator is using the same approach of applying a two-
part test under section 231(a)(2)(A) as described in the finalized
endangerment and cause or contribute findings under CAA section 231 for
greenhouse gases (GHGs) emissions from aircraft in 2016.\263\ We
further explained that this approach is the same approach that the EPA
used in making endangerment and cause and contribute findings for GHGs
under section 202(a) of the CAA in 2009 (2009 Findings), which is
reasonable in light of the parallels of the language and structure
between sections 231(a)(2)(A) and 202(a)(1) of the CAA.\264\ Some
comments misconstrued EPA's discussion of section 202(a) in the
proposal to infer that EPA was relying on its authority under section
202(a) in this action. That is not the case. While using the same
approach as in the 2009 Findings, the EPA is not acting under the
authority of section 202(a) in making these final findings, but rather,
is relying on the authority under section 231(a)(2)(A) as described
herein, which expressly authorizes regulation of emissions of air
pollutants from aircraft engines which the Administrator judges to
cause or contribute to air pollution which may reasonably be
anticipated to endanger public health or welfare.
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    \263\ See e.g., 81 FR 55434-54440 (Aug. 15, 2016).
    \264\ 74 FR 66496, 66505-10 (Dec. 15, 2009); see also Coalition
for Responsible Regulation, Inc. v. EPA, 684 F.3d 102 (D.C. Cir.
2012) (CRR) (subsequent history omitted) (affirming EPA's approach
in the 2009 Findings).
---------------------------------------------------------------------------

    Additional commenters state that they are opposed to any rulemaking
that could lead to the elimination of leaded avgas before a
comparatively priced substitute fuel is available for widespread use.
As an initial matter, the EPA notes that, as described in section
III.A. of this document, in this action, the EPA is addressing the
predicate to regulatory action under CAA section 231 through a two-part
test. In the first step of the two-part test, the Administrator must
decide whether, in his judgment, the air pollution under consideration
may reasonably be anticipated to endanger public health or welfare. As
the second step, the Administrator must decide whether, in his
judgment, emissions of an air pollutant from certain classes of
aircraft engines cause or contribute to this air pollution. If the
Administrator answers both questions in the affirmative, as he is doing
here, the EPA becomes subject to a duty to propose and promulgate
standards under section 231, but the EPA is not proposing or
promulgating any standards in this action. These commenters have
concerns regarding the cost and availability of unleaded fuels that
might be required to meet a future emission standard for lead. To
reiterate, the EPA is not proposing or promulgating any standards in
this action, nor is the EPA reaching any conclusions about the possible
elimination of leaded avgas or the cost or availability of
comparatively priced substitute fuels; those issues will be addressed,
if at all, only in a future standard-setting rulemaking. As for future
standards, the delegation of authority in CAA section 231 to the EPA
``is both explicit and extraordinarily broad,'' NACAA, 489 F.3d at
1229, and ``confer[s] broad discretion to the [EPA] Administrator to
weigh various factors in arriving at appropriate standards,'' id. at
1230. However, as described in section III.C. of this document, CAA
section 231(a)(2)(B) directs the EPA to consult with the Administrator
of the FAA on such standards, and it prohibits the EPA from changing
aircraft emission standards if such a change would significantly
increase noise and adversely affect safety. Further, under CAA section
231(b), the effective date of any standards shall provide the necessary
time to permit the development and application of the requisite
technology, giving appropriate consideration to the cost of compliance,
as determined by the EPA in consultation with the U.S. Department of
Transportation (DOT).

IV. The Final Endangerment Finding Under CAA Section 231

    In this action, the Administrator finds that lead air pollution may
reasonably be anticipated to endanger the public health and welfare
within the meaning of CAA section 231(a)(2)(A). This section discusses
both the public health and welfare aspects of the endangerment finding
and describes the scientific evidence that informs the Administrator's
final determination. The vast majority of comments supported the EPA's
proposal and agreed with the EPA's description of the health and
welfare effects of lead air pollution. The Agency's responses to public
comments on the proposed endangerment finding, including those opposing
finalizing the finding, can be

found in the Response to Comments document for this action. After
consideration of the comments on this topic, the EPA concludes that the
scientific evidence supports finalizing the finding as proposed.

A. Scientific Basis of the Endangerment Finding

1. Lead Air Pollution
    Lead is emitted and exists in the atmosphere in a variety of forms
and compounds and is emitted by a wide range of sources.\265\ Lead is
persistent in the environment. Atmospheric transport distances of
airborne lead vary depending on its form and particle size, as
discussed in section II.A. of this document, with coarse lead-bearing
particles deposited to a greater extent near the source, while fine
lead-bearing particles can be transported long distances before being
deposited. Through atmospheric deposition, lead is distributed to other
environmental media, including soils and surface water bodies.\266\
Lead is retained in soils and sediments, where it provides a historical
record and, depending on several factors, can remain available in some
areas for extended periods for environmental or human exposure, with
any associated potential public health and public welfare impacts.
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    \265\ EPA (2013) ISA for Lead. Section 2.2. ``Sources of
Atmospheric Pb.'' p. 2-1. EPA, Washington, DC, EPA/600/R-10/075F,
2013.
    \266\ EPA (2013) ISA for Lead. Executive Summary. ``Sources,
Fate and Transport of Lead in the Environment, and the Resulting
Human Exposure and Dose.'' pp. lxxviii-lxxix. EPA, Washington, DC,
EPA/600/R-10/075F, 2013.
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    For purposes of this action, the EPA defines the ``air pollution''
referred to in section 231(a)(2)(A) of the CAA as lead, which we also
refer to as lead air pollution in this document.\267\
---------------------------------------------------------------------------

    \267\ The lead air pollution can occur as elemental lead or in
lead-containing compounds, and this definition of the air pollution
recognizes that lead in air (whatever form it is found in, including
in inorganic and organic compounds containing lead) has the
potential to elicit public health and welfare effects. We note, for
example, that the 2013 Lead ISA and 2008 AQCD described the
toxicokinetics of inorganic and organic forms of lead and studies
evaluating lead-related health effects commonly measure total lead
level (i.e., all forms of lead in various biomarker tissues such as
blood).
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2. Health Effects and Lead Air Pollution
    In 2013, the EPA completed the Integrated Science Assessment for
Lead which built on the findings of previous AQCDs for Lead. These
documents critically assess and integrate relevant scientific
information regarding the health and welfare effects of lead and have
undergone extensive critical review by the EPA, the Clean Air
Scientific Advisory Committee (CASAC), and the public. As such, these
assessments provide the primary scientific and technical basis for the
Administrator's finding that lead air pollution is reasonably
anticipated to endanger public health and welfare.268 269
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    \268\ EPA (2013) ISA for Lead. EPA, Washington, DC, EPA/600/R-
10/075F, 2013.
    \269\ EPA (2006) Air Quality Criteria for Lead. EPA, Washington,
DC, EPA/600/R-5/144aF, 2006.
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    As summarized in section II.A. of this document, human exposure to
lead that is emitted into the air can occur by multiple pathways.
Inhalation pathways include both ambient air outdoors and ambient air
that has infiltrated into indoor environments. Additional exposure
pathways may involve media other than air, including indoor and outdoor
dust, soil, surface water and sediments, vegetation and biota. The
bioavailability of air-related lead is modified by several factors in
the environment (e.g., the chemical form of lead, environmental fate of
lead emitted to air). That notwithstanding, as described in section
II.A. of this document, it is well-documented that exposures to lead
emitted into the air can result in increased blood lead levels,
particularly for children living near air lead sources, due to their
proximity to these sources of exposure.\270\
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    \270\ EPA (2013) ISA for Lead. Section 5.4. ``Summary.'' p. 5-
40. EPA, Washington, DC, EPA/600/R-10/075F, 2013.
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    As described in the EPA's 2013 Lead ISA and in prior AQCDs, lead
has been demonstrated to exert a broad array of deleterious effects on
multiple organ systems. The 2013 Lead ISA characterizes the causal
nature of relationships between lead exposure and health effects using
a weight-of-evidence approach.\271\ We summarize here those health
effects for which the EPA in the 2013 Lead ISA has concluded that the
evidence supports a determination of either a ``causal relationship,''
``likely to be causal relationship,'' or ``suggestive of a causal
relationship'' between lead exposure and a health effect.\272\ In the
discussion that follows, we summarize findings regarding effects
observed in children, effects observed in adults, and additional
effects observed that are not specific to an age group.
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    \271\ The causal framework draws upon the assessment and
integration of evidence from across scientific disciplines, spanning
atmospheric chemistry, exposure, dosimetry and health effects
studies (i.e., epidemiologic, controlled human exposure, and animal
toxicological studies), and assessment of the related uncertainties
and limitations that ultimately influence our understanding of the
evidence. This framework employs a five-level hierarchy that
classifies the overall weight of evidence with respect to the causal
nature of relationships between criteria pollutant exposures and
health and welfare effects using the following categorizations:
causal relationship; likely to be causal relationship; suggestive
of, but not sufficient to infer, a causal relationship; inadequate
to infer the presence or absence of a causal relationship; and not
likely to be a causal relationship. EPA (2013) ISA for Lead.
Preamble section. p. xliv. EPA, Washington, DC, EPA/600/R-10/075F,
2013.
    \272\ EPA (2013) ISA for Lead. Table ES-1. ``Summary of causal
determinations for the relationship between exposure to Pb and
health effects.'' pp. lxxxiii-lxxxvii. EPA, Washington, DC, EPA/600/
R-10/075F, 2013.
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    The EPA has concluded that there is a ``causal relationship''
between lead exposure during childhood (pre and postnatal) and a range
of health effects in children, including the following: cognitive
function decrements; the group of externalizing behaviors comprising
attention, increased impulsivity, and hyperactivity; and developmental
effects (i.e., delayed pubertal onset).\273\ In addition, the EPA has
concluded that the evidence supports a conclusion that there is a
``likely to be causal relationship'' between lead exposure and conduct
disorders in children and young adults, internalizing behaviors such as
depression, anxiety and withdrawn behavior, auditory function
decrements, and fine and gross motor function decrements.\274\
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    \273\ EPA (2013) ISA for Lead. Table ES-1. ``Summary of causal
determinations for the relationship between exposure to Pb and
health effects.'' p. lxxxiii and p. lxxxvi. EPA, Washington, DC,
EPA/600/R-10/075F, 2013.
    \274\ EPA (2013) ISA for Lead. Table ES-1. ``Summary of causal
determinations for the relationship between exposure to Pb and
health effects.'' pp. lxxxiii-lxxxiv. EPA, Washington, DC, EPA/600/
R-10/075F, 2013.
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    Multiple epidemiologic studies conducted in diverse populations of
children consistently demonstrate the harmful effects of lead exposure
on cognitive function (as measured by decrements in intelligence
quotient [IQ], decreased academic performance, and poorer performance
on tests of executive function). These findings are supported by
extensively documented toxicological evidence substantiating the
plausibility of these findings in the epidemiological literature and
provide information on the likely mechanisms underlying these
neurotoxic effects.\275\
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    \275\ EPA (2013) ISA for Lead. Executive Summary. ``Effects of
Pb Exposure in Children.'' pp. lxxxvii-lxxxviii. EPA, Washington,
DC, EPA/600/R-10/075F, 2013.
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    Intelligence quotient is a well-established, and among the most
rigorously standardized, cognitive function measure that has been used
extensively as a measure of the negative

effects of exposure to lead.276 277 Examples of other
measures of cognitive function negatively associated with lead exposure
include measures of intelligence and cognitive development and
cognitive abilities, such as learning, memory, and executive functions,
as well as academic performance and achievement.\278\
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    \276\ EPA (2013) ISA for Lead. Section 4.3.2. ``Cognitive
Function.'' p. 4-59. EPA, Washington, DC, EPA/600/R-10/075F, 2013.
    \277\ EPA (2006) Air Quality Criteria for Lead. Sections 6.2.2
and 8.4.2. EPA, Washington, DC, EPA/600/R-5/144aF, 2006.
    \278\ EPA (2013) ISA for Lead. Section 4.3.2. ``Cognitive
Function.'' p. 4-59. EPA, Washington, DC, EPA/600/R-10/075F, 2013.
---------------------------------------------------------------------------

    In summarizing the evidence relating neurocognitive effects to lead
exposure metrics, the 2013 Lead ISA notes that ``in individual studies,
postnatal (early childhood and concurrent) blood [lead] levels are also
consistently associated with cognitive function decrements in children
and adolescents.'' 279 280 The 2013 Lead ISA additionally
notes that the findings from experimental animal studies indicate that
lead exposures during multiple early lifestages and periods are
observed to induce impairments in learning, and that these findings
``are consistent with the understanding that the nervous system
continues to develop (i.e., synaptogenesis and synaptic pruning remains
active) throughout childhood and into adolescence.'' \281\ The 2013
Lead ISA further notes that ``it is clear that [lead] exposure in
childhood presents a risk; further, there is no evidence of a threshold
below which there are no harmful effects on cognition from [lead]
exposure,'' and additionally recognizes uncertainty about the patterns
of [lead] exposure that contribute to the blood [lead] levels analyzed
in epidemiologic studies (uncertainties which are greater in studies of
older children and adults than in studies of younger children who do
not have lengthy exposure histories).\282\ Evidence suggests that while
some neurocognitive effects of lead in children may be transient, some
lead-related cognitive effects may be irreversible and persist into
adulthood,\283\ potentially affecting lower educational attainment and
financial well-being.\284\
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    \279\ In this statement, the term ``concurrent'' is referring to
blood lead measurements that were taken concurrently with the
neurocognitive testing.
    \280\ EPA (2013) ISA for Lead. Section 1.9.4. ``Pb Exposure and
Neurodevelopmental Deficits in Children.'' p. 1-76. EPA, Washington,
DC, EPA/600/R-10/075F, 2013.
    \281\ EPA (2013) ISA for Lead. Section 1.9.4. ``Pb Exposure and
Neurodevelopmental Deficits in Children.'' p. 1-76. EPA/600/R-10/
075F, 2013.
    \282\ EPA (2013) ISA for Lead. Executive Summary. ``Effects of
Pb Exposure in Children.'' pp. lxxxvii-lxxxviii. EPA, Washington,
DC, EPA/600/R-10/075F, 2013.
    \283\ EPA (2013) ISA for Lead. Section 1.9.5. ``Reversibility
and Persistence of Neurotoxic Effects of Pb.'' p. 1-76. EPA,
Washington, DC, EPA/600/R-10/075F, 2013.
    \284\ EPA (2013) ISA for Lead. Section 4.3.14. ``Public Health
Significance of Associations between Pb Biomarkers and
Neurodevelopmental Effects.'' p. 4-279. EPA, Washington, DC, EPA/
600/R-10/075F, 2013.
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    The 2013 Lead ISA concluded that neurodevelopmental effects in
children were among the effects best substantiated as occurring at the
lowest blood lead levels, and that these categories of effects were
clearly of the greatest concern with regard to potential public health
impact.\285\ For example, in considering population risk, the 2013 Lead
ISA notes that ``[s]mall shifts in the population mean IQ can be highly
significant from a public health perspective.'' \286\ Specifically, if
lead-related decrements are manifested uniformly across the range of IQ
scores in a population, ``a small shift in the population mean IQ may
be significant from a public health perspective because such a shift
could yield a larger proportion of individuals functioning in the low
range of the IQ distribution, which is associated with increased risk
of educational, vocational, and social failure'' as well as a decrease
in the proportion with high IQ scores.\287\
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    \285\ EPA (2013) ISA for Lead. Section 1.9.1. ``Public Health
Significance.'' p. 1-68. EPA, Washington, DC, EPA/600/R-10/075F,
2013.
    \286\ EPA (2013) ISA for Lead. Executive Summary. ``Public
Health Significance.'' p. xciii. EPA, Washington, DC, EPA/600/R-10/
075F, 2013.
    \287\ EPA (2013) ISA for Lead. Section 1.9.1. ``Public Health
Significance.'' p. 1-68. EPA, Washington, DC, EPA/600/R-10/075F,
2013.
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    With regard to lead effects identified for the adult population,
the 2013 Lead ISA concluded that there is a ``causal relationship''
between lead exposure and hypertension and coronary heart disease in
adults. The 2013 Lead ISA concluded that cardiovascular effects in
adults were those of greatest public health concern for adults because
the evidence indicated that these effects occurred at the lowest blood
lead levels, compared to other health effects, although it further
noted that the role of past versus recent exposures to lead is
unclear.\288\
---------------------------------------------------------------------------

    \288\ EPA (2013) ISA for Lead. Section 1.9.1. ``Public Health
Significance.'' p. 1-68. EPA, Washington, DC, EPA/600/R-10/075F,
2013.
---------------------------------------------------------------------------

    With regard to evidence of cardiovascular effects and other effects
of lead on adults, the 2013 Lead ISA notes that ``[a] large body of
evidence from both epidemiologic studies of adults and experimental
studies in animals demonstrates the effect of long-term [lead] exposure
on increased blood pressure and hypertension.'' \289\ In addition to
its effect on blood pressure, ``[lead] exposure can also lead to
coronary heart disease and death from cardiovascular causes and is
associated with cognitive function decrements, symptoms of depression
and anxiety, and immune effects in adult humans.'' \290\ The extent to
which the effects of lead on the cardiovascular system are reversible
is not well characterized. Additionally, the frequency, timing, level,
and duration of lead exposure causing the effects observed in adults
has not been pinpointed, and higher exposures earlier in life may play
a role in the development of health effects measured later in
life.\291\ The 2013 Lead ISA states that ``[i]t is clear however, that
[lead] exposure can result in harm to the cardiovascular system that is
evident in adulthood and may also affect a broad array of organ
systems.'' \292\ In summarizing the public health significance of lead
on the adult population, the 2013 Lead ISA notes that ``small [lead]-
associated increases in the population mean blood pressure could result
in an increase in the proportion of the population with hypertension
that is significant from a public health perspective.'' \293\
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    \289\ EPA (2013) ISA for Lead. Executive Summary. ``Effects of
Pb Exposure in Adults.'' p. lxxxviii. EPA/600/R-10/075F, 2013.
    \290\ EPA (2013) ISA for Lead. Executive Summary. ``Effects of
Pb Exposure in Adults.'' p. lxxxviii. EPA/600/R-10/075F, 2013.
    \291\ EPA (2013) ISA for Lead. Executive Summary. ``Effects of
Pb Exposure in Adults.'' p. lxxxviii. EPA/600/R-10/075F, 2013.
    \292\ EPA (2013) ISA for Lead. Executive Summary. ``Effects of
Pb Exposure in Adults.'' p. lxxxviii. EPA/600/R-10/075F, 2013.
    \293\ EPA (2013) ISA for Lead. Executive Summary. ``Public
Health Significance.'' p. xciii. EPA, Washington, DC, EPA/600/R-10/
075F, 2013.
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    In addition to the effects summarized here, the EPA has concluded
there is a ``likely to be causal relationship'' between lead exposure
and both cognitive function decrements and psychopathological effects
in adults. The 2013 Lead ISA also concludes that there is a ``causal
relationship'' between lead exposure and decreased red blood cell
survival and function, altered heme synthesis, and male reproductive
function. The EPA has also concluded there is a ``likely to be causal
relationship'' between lead exposure and decreased host resistance,
resulting in increased susceptibility to bacterial infection and
suppressed delayed type hypersensitivity, and cancer.\294\
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    \294\ EPA (2013) ISA for Lead. Table ES-1. ``Summary of causal
determinations for the relationship between exposure to Pb and
health effects.'' pp. lxxxiv-lxxxvii. EPA, Washington, DC, EPA/600/
R-10/075F, 2013.

---------------------------------------------------------------------------

    Additionally, in 2013 EPA concluded that the evidence is
``suggestive of a causal relationship'' between lead exposure and some
additional effects. These include auditory function decrements in
adults and subclinical atherosclerosis, reduced kidney function, birth
outcomes (e.g., low birth weight, spontaneous abortion), and female
reproductive function.\295\
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    \295\ EPA (2013) ISA for Lead. Table ES-1. ``Summary of causal
determinations for the relationship between exposure to Pb and
health effects.'' pp. lxxxiv-lxxxvi. EPA, Washington, DC, EPA/600/R-
10/075F, 2013.
---------------------------------------------------------------------------

    The EPA has identified factors that may increase the risk of health
effects of lead exposure due to susceptibility and/or vulnerability;
these are termed ``at-risk'' factors. The 2013 Lead ISA describes the
systematic approach the EPA uses to evaluate the coherence of evidence
to determine the biological plausibility of associations between at-
risk factors and increased vulnerability and/or susceptibility. An
overall weight of evidence is used to determine whether a specific
factor results in a population being at increased risk of lead-related
health effects.\296\ The 2013 Lead ISA concludes that ``there is
adequate evidence that several factors--childhood, race/ethnicity,
nutrition, residential factors, and proximity to [lead] sources--confer
increased risk of lead-related health effects.'' \297\
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    \296\ EPA (2013) ISA for Lead. Chapter 5. ``Approach to
Classifying Potential At-Risk Factors.'' p. 5-2. EPA, Washington,
DC, EPA/600/R-10/075F, 2013.
    \297\ EPA (2013) ISA for Lead. Section 5.4. ``Summary.'' p. 5-
44. EPA, Washington, DC, EPA/600/R-10/075F, 2013.
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3. Welfare Effects and Lead Air Pollution
    The 2013 Lead ISA characterizes the causal nature of relationships
between lead exposure and welfare effects using a five-level hierarchy
that classifies the overall weight of evidence.\298\ We summarize here
the welfare effects for which the EPA has concluded that the evidence
supports a determination of either a ``causal relationship,'' or a
``likely to be causal relationship,'' with exposure to lead, or that
the evidence is ``suggestive of a causal relationship'' with lead
exposure. The discussion that follows is organized to first provide a
summary of the effects of lead in the terrestrial environment, followed
by a summary of effects of lead in freshwater and saltwater ecosystems.
The 2013 Lead ISA further describes the scales or levels at which these
determinations between lead exposure and effects on plants,
invertebrates, and vertebrates were made (i.e., community-level,
ecosystem-level, population-level, organism-level or sub-organism
level).\299\
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    \298\ Causal determinations for ecological effects were based on
integration of information on biogeochemistry, bioavailability,
biological effects, and exposure-response relationships of lead in
terrestrial, freshwater, and saltwater environments. This framework
employs a five-level hierarchy that classifies the overall weight of
evidence with respect to the causal nature of relationships between
criteria pollutant exposures and health and welfare effects using
the categorizations described in the 2013 Lead NAAQS.
    \299\ EPA (2013) ISA for Lead. Table ES-2. ``Schematic
representation of the relationships between the various MOAs by
which Pb exerts its effects.'' p. lxxxii. EPA, Washington, DC, EPA/
600/R-10/075F, 2013.
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    In terrestrial environments, the EPA determined that ``causal
relationships'' exist between lead exposure and reproductive and
developmental effects in vertebrates and invertebrates, growth in
plants, survival for invertebrates, hematological effects in
vertebrates, and physiological stress in plants.\300\ The EPA also
determined that there were ``likely to be causal relationships''
between lead exposure and community and ecosystem effects, growth in
invertebrates, survival in vertebrates, neurobehavioral effects in
invertebrates and vertebrates, and physiological stress in
invertebrates and vertebrates.
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    \300\ EPA (2013) ISA for Lead. Table ES-2. ``Summary of causal
determinations for the relationship between Pb exposure and effects
on plants, invertebrates, and vertebrates.'' p. xc. EPA, Washington,
DC, EPA/600/R-10/075F, 2013.
---------------------------------------------------------------------------

    In freshwater environments, the EPA found that ``causal
relationships'' exist between lead exposure and reproductive and
developmental effects in vertebrates and invertebrates, growth in
invertebrates, survival for vertebrates and invertebrates, and
hematological effects in vertebrates. The EPA also determined that
there were ``likely to be causal relationships'' between lead exposure
and community and ecosystem effects, growth in plants, neurobehavioral
effects in invertebrates and vertebrates, hematological effects in
invertebrates, and physiological stress in plants, invertebrates, and
vertebrates.\301\
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    \301\ EPA (2013) ISA for Lead. Table ES-2. ``Summary of causal
determinations for the relationship between Pb exposure and effects
on plants, invertebrates, and vertebrates.'' p. xc. EPA, Washington,
DC, EPA/600/R-10/075F, 2013.
---------------------------------------------------------------------------

    The EPA also determined that the evidence for saltwater ecosystems
was ``suggestive of a causal relationship'' between lead exposure and
reproductive and developmental effects in invertebrates, hematological
effects in vertebrates, and physiological stress in invertebrates.\302\
---------------------------------------------------------------------------

    \302\ EPA (2013) ISA for Lead. Table ES-2. ``Summary of causal
determinations for the relationship between Pb exposure and effects
on plants, invertebrates, and vertebrates.'' p. xc. EPA, Washington,
DC, EPA/600/R-10/075F, 2013.
---------------------------------------------------------------------------

    The 2013 Lead ISA concludes, ``With regard to the ecological
effects of [lead], uptake of [lead] into fauna and subsequent effects
on reproduction, growth and survival are established and are further
supported by more recent evidence. These may lead to effects at the
population, community, and ecosystem level of biological organization.
In both terrestrial and aquatic organisms, gradients in response are
observed with increasing concentration of [lead] and some studies
report effects within the range of [lead] detected in environmental
media over the past several decades. Specifically, effects on
reproduction, growth, and survival in sensitive freshwater
invertebrates are well-characterized from controlled studies at
concentrations at or near [lead] concentrations occasionally
encountered in U.S. fresh surface waters. Hematological and stress
related responses in some terrestrial and aquatic species were also
associated with elevated [lead] levels in polluted areas. However, in
natural environments, modifying factors affect [lead] bioavailability
and toxicity and there are considerable uncertainties associated with
generalizing effects observed in controlled studies to effects at
higher levels of biological organization. Furthermore, available
studies on community and ecosystem-level effects are usually from
contaminated areas where [lead] concentrations are much higher than
typically encountered in the environment. The contribution of
atmospheric [lead] to specific sites is not clear and the connection
between air concentration of [lead] and ecosystem exposure continues to
be poorly characterized.'' \303\
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    \303\ EPA (2013) ISA for Lead. ``Summary.'' p. xcvi. EPA,
Washington, DC, EPA/600/R-10/075F, 2013.
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B. Final Endangerment Finding

    The Administrator finds, for purposes of CAA section 231(a)(2)(A),
that lead air pollution may reasonably be anticipated to endanger the
public health and welfare. This finding is based on consideration of
the extensive scientific evidence, described in this section, that has
been amassed over decades and rigorously peer reviewed by CASAC, as
well as consideration of public comments on the proposal.

V. The Final Cause or Contribute Finding Under CAA Section 231

    In this action, the Administrator finds that engine emissions of
lead from

certain aircraft cause or contribute to the lead air pollution that may
reasonably be anticipated to endanger public health and welfare under
section 231(a)(2)(A) of the Clean Air Act. This section describes the
definition of the air pollutant and the data and information supporting
the Administrator's final determination. Public comments on the cause
or contribute finding were largely supportive of the EPA's proposal,
though some commenters opposed finalizing the finding. After
consideration of the comments on this topic, the EPA concludes that the
scientific evidence supports finalizing the finding as proposed. The
Agency's responses to certain public comments on the cause or
contribute finding can be found in section V.C. of this document, and
responses to additional comments on the cause or contribute finding can
be found in the Response to Comments document for this action.

A. Definition of the Air Pollutant

    Under section 231, the Administrator is to determine whether
emissions of any air pollutant from any class or classes of aircraft
engines cause or contribute to air pollution which may reasonably be
anticipated to endanger public health or welfare. As in the 2016
Findings that the EPA made under section 231 for greenhouse gases, in
making this cause or contribute finding under section 231(a)(2), the
Administrator first defines the air pollutant being evaluated. The
Administrator has reasonably and logically considered the relationship
between the lead air pollution and the air pollutant when considering
emissions of lead from engines used in covered aircraft. The
Administrator defines the air pollutant to match the definition of the
air pollution, such that the air pollutant analyzed for contribution
mirrors the air pollution considered in the endangerment finding.
Accordingly, for purposes of this action, the Administrator defines the
``air pollutant'' referred to in section 231(a)(2)(A) as lead, which we
also refer to as the lead air pollutant in this document.\304\ As noted
in section II.A.2. of this document, lead emitted to the air from
covered aircraft engines is predominantly in particulate form as lead
dibromide; however, some chemical compounds of lead that are expected
in the exhaust from these engines, including alkyl lead compounds,
would occur in the air in gaseous form.
---------------------------------------------------------------------------

    \304\ The lead air pollutant can occur as elemental lead or in
lead-containing compounds, and this definition of the air pollutant
recognizes the range of chemical forms of lead emitted by engines in
covered aircraft.
---------------------------------------------------------------------------

B. The Data and Information Used To Evaluate the Final Cause or
Contribute Finding

    The Administrator's assessment of whether emissions from the
engines used in covered aircraft cause or contribute to lead air
pollution was informed by estimates of lead emissions from the covered
aircraft, lead concentrations in air at and near airports that are
attributable to lead emissions from piston engines used in covered
aircraft, and projected future conditions.
    As used in this final action, the term ``covered aircraft'' refers
to all aircraft and ultralight vehicles equipped with covered engines
which, in this context, means any aircraft engine that is capable of
using leaded avgas. Examples of covered aircraft would include smaller
piston-powered aircraft such as the Cessna 172 (single-engine aircraft)
and the Beechcraft Baron G58 (twin-engine aircraft), as well as the
largest piston-engine aircraft such as the Curtiss C-46 and the Douglas
DC-6. Other examples of covered aircraft would include rotorcraft, such
as the Robinson R44 helicopter, light-sport aircraft, and ultralight
vehicles equipped with piston engines. The vast majority of covered
aircraft are piston-engine powered.
    In recent years, covered aircraft are estimated to be the largest
single source of lead to air in the U.S. Since 2008, as described in
section II.A.2.b. of this document, lead emissions from covered
aircraft are estimated to have contributed over 50 percent of all lead
emitted to the air nationally. The EPA estimates 470 tons of lead were
emitted by covered aircraft in 2017, comprising 70 percent of lead
emitted to air nationally that year.\305\ \306\ In approximately 1,000
counties in the U.S., the EPA's emissions inventory identifies covered
aircraft as the sole source of lead emissions. Among the 1,872 counties
in the U.S. for which the inventory identifies multiple sources of lead
emissions, including engine emissions from covered aircraft, the
contribution of aircraft engine emissions ranges from 0.00005 to 4.3
tons per year, comprising 0.15 to 98 percent (respectively) of total
lead emissions to air in those counties.\307\
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    \305\ The lead inventories for 2008, 2011 and 2014 are provided
in the EPA (2018b) Report on the Environment Exhibit 2.
Anthropogenic lead emissions in the U.S. Available at https://cfpub.epa.gov/roe/indicator.cfm?i=13#2. The lead inventories for
2017 are available at https://www.epa.gov/air-emissions-inventories/2017-national-emissions-inventory-nei-data#dataq.
    \306\ As described in section II.A.2., the EPA estimates 427
tons of lead were emitted by aircraft engines operating on leaded
fuel in 2020. Due to the Covid-19 pandemic, a substantial decrease
in activity by aircraft occurred in 2020, impacting the total lead
emissions for this year. The 2020 NEI is available at: https://www.epa.gov/air-emissions-inventories/2020-national-emissions-inventory-nei-data.
    \307\ Airport lead annual emissions data used were reported in
the 2017 NEI. Available at https://www.epa.gov/air-emissions-inventories/2017-national-emissions-inventory-nei-data. In addition
to the triennial NEI, the EPA collects from state, local, and Tribal
air agencies point source data for larger sources every year (see
https://www.epa.gov/air-emissions-inventories/air-emissions-reporting-requirements-aerr for specific emissions thresholds).
While these data are not typically published as a new NEI, they are
available publicly upon request and are also included in https://www.epa.gov/air-emissions-modeling/emissions-modeling-platforms,
which are created for years other than the triennial NEI years.
County estimates of lead emissions from non-aircraft sources used in
this action are from the 2019 inventory. There are 3,012 counties
and statistical equivalent areas where EPA estimates engine
emissions of lead occur.
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    Covered aircraft activity, as measured by the number of hours flown
nationwide, increased nine percent in the period from 2012 through
2019.\308\ General aviation activity, largely conducted by covered
aircraft, increased up to 52 percent at airports that are among the
busiest in the U.S.\309\ In future years, while piston-engine aircraft
activity overall is projected to decrease slightly, this change in
activity is not projected to occur uniformly across airports in the
U.S.; some airports are forecast to have increased activity by general
aviation aircraft, the majority of which is conducted by piston-engine
aircraft.\310\ Although there is some uncertainty in these projections,
they indicate that lead emissions from covered aircraft may increase at
some airports in the future.\311\
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    \308\ FAA. General Aviation and Part 135 Activity Surveys--CY
2019. Chapter 3: Primary and Actual Use. Table 1.3--General Aviation
and Part 135 Total Hours Flown by Aircraft Type 2008-2019 (Hours in
Thousands). Retrieved on Dec., 27, 2021 at https://www.faa.gov/data_research/aviation_data_statistics/general_aviation/CY2019/.
    \309\ Geidosch. Memorandum to Docket EPA-HQ-OAR-2022-0389. Past
Trends and Future Projections in General Aviation Activity and
Emissions. June 1, 2022. Docket ID EPA-HQ-2022-0389.
    \310\ Geidosch. Memorandum to Docket EPA-HQ-OAR-2022-0389. Past
Trends and Future Projections in General Aviation Activity and
Emissions. June 1, 2022. Docket ID EPA-HQ-2022-0389.
    \311\ FAA TAF Fiscal Years 2020-2045 describes the forecast
method, data sources, and review process for the TAF estimates. The
documentation for the TAF is available at https://taf.faa.gov/Downloads/TAFSummaryFY2020-2045.pdf.
---------------------------------------------------------------------------

    Additionally, engine emissions of lead from covered aircraft may
deposit in the local environment and, due to the small size of the
lead-bearing particles emitted by engines in covered aircraft, these
particles may disperse widely in

the environment. Therefore, because lead is a persistent pollutant in
the environment, we anticipate current and future emissions of lead
from covered aircraft engines may contribute to exposures and uptake by
humans and biota into the future.
    In evaluating the contributions of engine emissions from covered
aircraft to the lead air pollution, as defined in section IV.A. of this
document, the EPA also considered three types of information about lead
concentrations in the ambient air: monitored concentrations, modeled
concentrations, and model-extrapolated estimates of lead
concentrations. Lead concentrations monitored in the ambient air
typically quantify lead compounds collected as suspended particulate
matter. The information gained from air monitoring and air quality
modeling provides insight into how lead emissions from piston engines
used in covered aircraft can affect lead concentrations in air.
    As described in section II.A.3. of this document, the EPA has
conducted air quality modeling at two airports and extrapolated modeled
estimates of lead concentrations to 13,000 airports with piston-engine
aircraft activity. These studies indicate that over a three-month
averaging time (the averaging time for the Lead NAAQS), the engine
emissions of lead from covered aircraft are estimated to contribute to
air lead concentrations to a distance of at least 500 meters downwind
from a runway.\312\ \313\ Additional studies have reported that lead
emissions from covered aircraft may have increased concentrations of
lead in air by one to two orders of magnitude at locations proximate to
aircraft emissions compared to nearby locations not impacted by a
source of lead air emissions.\314\ \315\ \316\
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    \312\ Carr et al., 2011. Development and evaluation of an air
quality modeling approach to assess near-field impacts of lead
emissions from piston-engine aircraft operating on leaded aviation
gasoline. Atmospheric Environment, 45 (32), 5795-5804. DOI: https://dx.doi.org/10.1016/j.atmosenv.2011.07.017.
    \313\ EPA (2020) Model-extrapolated Estimates of Airborne Lead
Concentrations at U.S. Airports. Table 6. EPA-420-R-20-003, 2020.
Available at https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100YG52.pdf.
    \314\ Carr et al., 2011. Development and evaluation of an air
quality modeling approach to assess near-field impacts of lead
emissions from piston-engine aircraft operating on leaded aviation
gasoline. Atmospheric Environment, 45 (32), 5795-5804. DOI: https://dx.doi.org/10.1016/j.atmosenv.2011.07.017.
    \315\ Heiken et al., 2014. Quantifying Aircraft Lead Emissions
at Airports. ACRP Report 133. Available at https://www.nap.edu/catalog/22142/quantifying-aircraft-lead-emissions-at-airports.
    \316\ Hudda et al., 2022. Substantial Near-Field Air Quality
Improvements at a General Aviation Airport Following a Runway
Shortening. Environmental Science & Technology. DOI: 10.1021/
acs.est.1c06765.
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    In 2008 and 2010, the EPA enhanced the lead monitoring network by
requiring monitors to be placed in areas with sources such as
industrial facilities and airports, as described further in section
II.A.3. of this document.\317\ \318\ As part of this 2010 requirement
to expand lead monitoring nationally, the EPA required a 1-year
monitoring study of 15 additional airports with estimated lead
emissions between 0.50 and 1.0 ton per year in an effort to better
understand how these emissions affect concentrations of lead in the air
at and near airports. Further, to help evaluate airport characteristics
that could lead to ambient lead concentrations that approach or exceed
the lead NAAQS, airports for this 1-year monitoring study were selected
based on factors such as the level of activity of covered aircraft and
the predominant use of one runway due to wind patterns. Monitored lead
concentrations in ambient air are highly sensitive to monitor location
relative to the location of the run-up areas for piston-engine aircraft
and other localized areas of elevated lead concentrations relative to
the air monitor locations.
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    \317\ 73 FR 66965 (Nov. 12, 2008).
    \318\ 75 FR 81126 (Dec. 27, 2010).
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    The lead monitoring study at airports began in 2011. In 2012, air
monitors were placed in close proximity to the run-up areas at the San
Carlos Airport (measurements started on March 10, 2012) and the
McClellan-Palomar Airport (measurements started on March 16, 2012). The
concentrations of lead measured at both of these airports in 2012 were
above the level of the lead NAAQS, with the highest measured levels of
lead in total suspended particles over a rolling three-month average of
0.33 micrograms per cubic meter of air at the San Carlos Airport and
0.17 micrograms per cubic meter of air at the McClellan-Palomar
Airport. These concentrations violate the primary and secondary lead
NAAQS, which are set at a level of 0.15 micrograms per cubic meter of
air measured in total suspended particles, as an average of three
consecutive monthly concentrations.
    In recognition of the potential for lead concentrations to exceed
the lead NAAQS in ambient air near the area of maximum concentration at
airports, the EPA further conducted an assessment of airports
nationwide, titled ``Model-extrapolated Estimates of Airborne Lead
Concentrations at U.S. Airports'' and described in section II.A.3. of
this document.\319\ The model-extrapolated lead concentrations
estimated in this study are attributable solely to emissions from
engines in covered aircraft operating at the airports evaluated and did
not include other sources of lead emissions to air. The EPA identified
four airports with the potential for lead concentrations above the lead
NAAQS due to lead emissions from engines used in covered aircraft.
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    \319\ EPA (2020) Model-extrapolated Estimates of Airborne Lead
Concentrations at U.S. Airports Table 6. EPA-420-R-20-003, 2020.
Available at https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100YG52.pdf.
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    Additional information regarding the contribution of engine
emissions of lead from covered aircraft to lead air pollution is
provided by the EPA's Air Toxics Screening Assessment. As described and
summarized in section II.A.3. of this document, the EPA's Air Toxics
Screening Assessment estimates that piston engines used in aircraft
contribute more than 50 percent of the estimated lead concentrations in
over half of the census tracts in the U.S.\320\
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    \320\ EPA's 2019 AirToxScreen is available at https://www.epa.gov/AirToxScreen/2019-airtoxscreen.
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    The EPA also notes that lead is emitted from engines in covered
aircraft in three of the ten areas in the U.S. currently designated as
nonattainment for the 2008 lead NAAQS. These areas are Arecibo, PR, and
Hayden, AZ, each of which include one airport servicing covered
aircraft, and the Los Angeles County-South Coast Air Basin, CA, which
contains at least 22 airports within its nonattainment area
boundary.\321\ \322\ Although the lead emissions from aircraft are not
the predominant source of airborne lead in these areas, the emissions
from covered aircraft may increase ambient air lead concentrations in
these areas.
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    \321\ South Coast Air Quality Management District (2012)
Adoption of 2012 Lead SIP Los Angeles County by South Coast
Governing Board, p.3-11, Table 3-3. Available at https://www.aqmd.gov/home/air-quality/clean-air-plans/lead-state-implementation-plan. The South Coast Air Quality Management District
identified 22 airports in the Los Angeles County-South Coast Air
Basin nonattainment area; the Whiteman Airport is among those in the
nonattainment area and the EPA estimated activity at this airport
may increase lead concentrations to levels above the lead NAAQS in
the report, Model-extrapolated Estimates of Airborne Lead
Concentrations at U.S. Airports. Table 7. EPA, Washington, DC, EPA-
420-R-20-003, 2020. Available at https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100YG52.pdf.
    \322\ EPA provides updated information regarding nonattainment
areas at this website: https://www.epa.gov/green-book/green-book-lead-2008-area-information.

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C. Response to Certain Comments on the Cause or Contribute Finding

    The EPA received comments related to the contribution of lead
emissions from engines in covered aircraft to lead air pollution.
Commenters provided both support for and opposition to the EPA's
proposed cause or contribute finding, with specific comments regarding
the amount of lead emitted by aircraft operating on leaded fuel and the
contribution of aircraft engine emissions to lead concentrations in the
air.
    Numerous commenters state their support for the proposed cause or
contribute finding, in some cases noting that ample evidence supports
this finding and highlighting the important role that lead emissions
from covered aircraft engines have in local environments in many areas
of the U.S. Additional commenters express concern regarding monitored
lead concentrations that exceed the NAAQS at some airports. The
comments expressing support for the proposed cause or contribute
finding and EPA's responses are described in greater detail in the
Response to Comment document for this action. We acknowledge these
comments and the support expressed for the EPA's cause or contribute
finding, and we agree with the commenters that lead emissions from
engines in covered aircraft contribute to lead air pollution.
    Commenters stating opposition to the cause or contribute finding
based on the amount of lead emitted by aircraft operating on leaded
fuel, assert that lead emissions today are 425 times less than lead
emissions of the 1970s or that the emissions of lead from aircraft are
less than one quarter of one percent of the emissions from cars in the
1970s. Some commenters also state that it only stands to reason that
covered aircraft engine emissions of lead represent a high percentage
of current lead emissions because lead is no longer being emitted by
motor vehicles. At least one additional commenter states that given the
number of hours flown by covered aircraft, they do not contribute
enough lead to affect air pollution.
    Commenters stating opposition to the cause or contribute finding
based on the concentrations of lead in air from engine emissions by
covered aircraft state that concentrations of lead exceeding the lead
NAAQS are rare, representing two of 17 airports studied. One commenter
also notes that Table 2 (in section II.A.3. of this document) does not
address the localized conditions of the airports studied and that the
airports where lead concentrations violated the lead NAAQS may have
unique conditions that resulted in the concentrations measured.
Additionally, some commenters state that there is no evidence that
engine emissions of lead are creating a hazard, and that the lead
emitted is not toxic in the small amount emitted by aircraft engines.
    In response to commenters comparing emissions of lead from covered
aircraft to lead emitted by motor vehicles in the 1970s, the EPA
acknowledges that more lead was emitted by motor vehicles in the 1970s
than is emitted by covered aircraft engines currently. This cause or
contribute finding is focused on emissions of lead from covered
aircraft engines, a different category of mobile sources from motor
vehicles, and the commenters do not explain why the fact that
historical emissions were higher from a different source category means
that current emissions from covered aircraft engines are not
contributing to the existing lead air pollution. Similarly, the
historical contributions of lead emitted by motor vehicles is not
germane to the present-day analysis of the contribution of lead
emissions from covered aircraft engines to the total lead released to
the air annually in the U.S. Indeed, nothing in CAA section 231(a)
precludes EPA from making a cause or contribute finding for emissions
from aircraft engines where such a finding is warranted, even if
emissions from other sources regulated elsewhere in the CAA or under
other Federal programs may also contribute to that air pollution or
have historically contributed to it. See Massachusetts v. E.P.A., 549
U.S. 497, 533 (2007) (the alleged efficacy of other ``Executive Branch
programs'' in addressing the air pollution problem is not a valid
reason for declining to make an endangerment finding). As noted
previously, in making a cause or contribute finding, CAA section 231
does not require the EPA to find that the contribution from the
relevant source category is ``significant,'' let alone the sole or
major cause of the endangering air pollution. As described in section
V.B., the lead emissions from engines used in covered aircraft clearly
contribute to the endangering lead air pollution, as these emissions
contributed over 50 percent of lead emissions to air starting in 2008,
when approximately 560 tons of lead were emitted by engines in covered
aircraft, and more recently, in 2017, when approximately 470 tons of
lead were emitted by engines in covered aircraft. In the EPA's view,
both the quantity and percentage of lead emitted by covered aircraft
engines amply demonstrate that this source contributes to lead air
pollution in the U.S.
    In response to commenters stating that the number of hours flown by
covered aircraft do not contribute enough lead to affect air pollution,
the EPA notes that the commenters made a conclusory allegation and did
not provide data or analysis supporting their claim. The EPA disagrees
with this comment, and we present data in section V.B. of this document
demonstrating that the activity by covered aircraft, which includes the
number of hours flown, contributes to lead air pollution as described
in the preceding paragraph.
    In response to commenters asserting that concentrations of lead
exceeding the lead NAAQS are rare, representing two of 17 airports
studied, as an initial matter, the EPA notes that nothing in section
231(a) of the CAA premises the cause or contribute finding on emissions
from the relevant classes of aircraft engines contributing to such
exceedances in a minimum number of air quality regions. More
importantly, the EPA notes that the purpose of this airport monitoring
study was not to determine the frequency with which potential
violations of the lead NAAQS occur at or near airports, but to
understand the potential range in lead concentrations at a small sample
of airports and the factors that influence those concentrations. As
described in section II.A.3. of this document, the concentrations of
lead monitored at and near highly active general aviation airports is
largely determined by the placement of the monitor relative to the run-
up area, and monitor placement relative to the run-up area was not
uniform across the airports studied. The EPA fully explains the basis
on which the Administrator finds that emissions of lead from covered
aircraft engine emissions cause or contribute to lead air pollution.
The data that support this finding are presented in section V.B. and,
as articulated in section V.D., where, among other data, the
Administrator takes into account the fact that in some situations lead
emissions from covered aircraft have contributed and may continue to
contribute to air concentrations that exceed the lead NAAQS. Given that
the lead NAAQS are established to provide requisite protection of
public health and welfare, the Administrator expresses particular
concern with contributions to concentrations that exceed the lead
NAAQS, and those contributions are part of the support for the
conclusion that lead emissions from engines in covered aircraft cause
or contribute to the endangering air pollution.
    In response to the comment regarding the assertion that the two
airports where lead concentrations violated the lead NAAQS may have
unique conditions

that resulted in the concentrations measured, the EPA notes that the
commenter did not specify or explain what localized conditions might
lead to this result; nor did they provide supporting evidence for
localized conditions occurring in these areas that could explain these
lead concentrations presented in Table 2 of this document. The EPA
describes in section II.A.3. of this document that at both of these
airports, monitors were located in close proximity to the area at the
end of the runway most frequently used for pre-flight safety checks
(i.e., run-up), and monitor placement relative to the run-up area is a
key factor in evaluating the maximum impact location attributable to
lead emissions from piston-engine aircraft. Additionally, as described
in section II.A.3. of this document, air lead concentrations at and
downwind from airports can be influenced by factors such as the use of
more than one run-up area, wind speed, and the number of operations
conducted by single- versus twin-engine aircraft.\323\ At the two
airports at which concentrations of lead violated the lead NAAQS, the
EPA observed a similar fleet composition of single- versus twin-engine
aircraft compared with other airports where on-site measurements were
taken; wind speeds, which are inversely proportional to lead
concentration, were not lower at the airports with lead concentrations
violating the lead NAAQS compared with other airports; and these
airports were not unique in that the activity by piston-engine aircraft
was in the range of activity by these aircraft at the majority of
airports where monitors were located.\324\ The EPA thus concludes that
these two airports do not have unique conditions responsible for the
concentrations of lead that violated the lead NAAQS.
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    \323\ The data in Table 2 represent concentrations measured at
one location at each airport and monitors were not consistently
placed in close proximity to the run-up areas. As described in
section II.A.3., monitored concentrations of lead in air near
airports are highly influenced by proximity of the monitor to the
run-up area. In addition to monitor placement, there are individual
airport factors that can influence lead concentrations (e.g., the
use of multiple run-up areas at an airport, fleet composition, and
wind speed). The monitoring data reported in Table 2 reflect a range
of lead concentrations indicative of the location at which
measurements were made and the specific operations at an airport.
    \324\ EPA (2020) Model-extrapolated Estimates of Airborne Lead
Concentrations at U.S. Airports Appendix B, Table B-2. EPA-420-R-20-
003, 2020. Available at https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100YG52.pdf.
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    In response to the comments that there is no evidence that engine
emissions of lead are creating a hazard,\325\ and that the lead emitted
is not toxic in the small amount emitted by aircraft engines, we note
that these comments conflate the endangerment and cause or contribute
steps of the analysis. The text in section 231(a)(2) provides for the
EPA to make a finding based on a determination that emissions of the
air pollutant from the covered aircraft engine ``causes, or contributes
to'' the air pollution. In making a cause or contribute finding, the
EPA need not additionally and separately make a determination as to
whether the emissions from covered aircraft engines alone cause
endangerment. In section IV. of this document, the EPA explained why
the Administrator is finding that the lead air pollution endangers
public health and welfare. The only remaining issue at the second step
of the analysis is whether emissions from the analyzed class or classes
of aircraft engines cause or contribute to the air pollution that may
reasonably be anticipated to endanger public health and welfare. For
the reasons described in section V. of this document, in the
Administrator's judgment, emissions of the lead air pollutant from
engines in the covered aircraft cause or contribute to the lead air
pollution.
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    \325\ While the comment does not clearly explain what it is
referring to with the phrase ``creating a hazard,'' we understand
that phrase to align with the ``cause'' portion of the cause or
contribute findings.
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    Additional comments were submitted to the EPA regarding the
emissions, deposition, transport, and fate of lead emitted by covered
aircraft engines. The EPA responds to these comments in the Response to
Comments Document for this action.

D. Final Cause or Contribute Finding for Lead

    Taking into consideration the data and information summarized in
section V. of this document, and the public comments received on the
proposed finding, the Administrator finds that engine emissions of the
lead air pollutant from covered aircraft cause or contribute to the
lead air pollution that may reasonably be anticipated to endanger
public health and welfare. In reaching this conclusion, the
Administrator noted that piston-engine aircraft operate on leaded
avgas. That operation emits lead-containing compounds into the air,