with compromised respiratory function (e.g., emphysema, asthma) are
expected to be at increased risk of developing adverse responses to
strong respiratory irritants such as acrolein. EPA does not currently
have an acute reference concentration for acrolein. The available
health effect reference values for acrolein have been summarized by EPA
and include an ATSDR MRL for acute exposure to acrolein of 7 [micro]g/
m\3\ for 1-14 days exposure; and Reference Exposure Level (REL) values
from the California Office of Environmental Health Hazard Assessment
(OEHHA) for one-hour and 8-hour exposures of 2.5 [micro]g/m\3\ and 0.7
[micro]g/m\3\, respectively.\212\
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    \209\ U.S. EPA. (2003). Toxicological review of acrolein in
support of summary information on Integrated Risk Information System
(IRIS) National Center for Environmental Assessment, Washington, DC.
EPA/635/R-03/003. p. 10. Available online at: http://www.epa.gov/ncea/iris/toxreviews/0364tr.pdf.
    \210\ U.S. EPA. (2003). Toxicological review of acrolein in
support of summary information on Integrated Risk Information System
(IRIS) National Center for Environmental Assessment, Washington, DC.
EPA/635/R-03/003. Available online at: http://www.epa.gov/ncea/iris/toxreviews/0364tr.pdf.
    \211\ Morris JB, Symanowicz PT, Olsen JE, et al. (2003).
Immediate sensory nerve-mediated respiratory responses to irritants
in healthy and allergic airway-diseased mice. J Appl Physiol
94(4):1563-1571.
    \212\ U.S. EPA. (2009). Graphical Arrays of Chemical-Specific
Health Effect Reference Values for Inhalation Exposures (Final
Report). U.S. Environmental Protection Agency, Washington, DC, EPA/
600/R-09/061, 2009. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=211003.
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iii. Benzene
    EPA's Integrated Risk Information System (IRIS) database lists
benzene as a known human carcinogen (causing leukemia) by all routes of
exposure, and concludes that exposure is associated with additional
health effects, including genetic changes in both humans and animals
and increased proliferation of bone marrow cells in
mice.213 214 215 EPA states in its IRIS database that data
indicate a causal relationship between benzene exposure and acute
lymphocytic leukemia and suggest a

relationship between benzene exposure and chronic non-lymphocytic
leukemia and chronic lymphocytic leukemia. EPA's IRIS documentation for
benzene also lists a range of 2.2 x 10-6 to 7.8 x 10-6 per [micro]g/
m\3\ as the unit risk estimate (URE) for benzene.216 217 The
International Agency for Research on Cancer (IARC) has determined that
benzene is a human carcinogen, and the U.S. Department of Health and
Human Services (DHHS) has characterized benzene as a known human
carcinogen.218 219
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    \213\ U.S. EPA. (2000). Integrated Risk Information System File
for Benzene. This material is available electronically at: https://cfpub.epa.gov/ncea/iris2/chemicalLanding.cfm?substance_nmbr=276.
    \214\ International Agency for Research on Cancer. (1982). IARC
monographs on the evaluation of carcinogenic risk of chemicals to
humans, Volume 29, Some industrial chemicals and dyestuffs,
International Agency for Research on Cancer, World Health
Organization, Lyon, France 1982.
    \215\ Irons, R.D.; Stillman, W.S.; Colagiovanni, D.B.; Henry,
V.A. (1992). Synergistic action of the benzene metabolite
hydroquinone on myelopoietic stimulating activity of granulocyte/
macrophage colony-stimulating factor in vitro, Proc. Natl. Acad.
Sci. 89:3691-3695.
    \216\ A unit risk estimate is defined as the increase in the
lifetime risk of cancer of an individual who is exposed for a
lifetime to 1 [micro]g/m\3\ benzene in air.
    \217\ U.S. EPA. (2000). Integrated Risk Information System File
for Benzene. This material is available electronically at: https://cfpub.epa.gov/ncea/iris2/chemicalLanding.cfm?substance_nmbr=276.
    \218\ International Agency for Research on Cancer (IARC, 2018.
Monographs on the evaluation of carcinogenic risks to humans, volume
120. World Health Organization--Lyon, France. http://publications.iarc.fr/Book-And-Report-Series/Iarc-Monographs-On-The-Identification-Of-Carcinogenic-Hazards-To-Humans/Benzene-2018.
    \219\ NTP (National Toxicology Program). 2016. Report on
Carcinogens, Fourteenth Edition.; Research Triangle Park, NC: U.S.
Department of Health and Human Services, Public Health Service.
https://ntp.niehs.nih.gov/go/roc14.
---------------------------------------------------------------------------

    A number of adverse noncancer health effects, including blood
disorders such as preleukemia and aplastic anemia, have also been
associated with long-term exposure to benzene.220 221 The
most sensitive noncancer effect observed in humans, based on current
data, is the depression of the absolute lymphocyte count in
blood.222 223 EPA's inhalation reference concentration (RfC)
for benzene is 30 [micro]g/m\3\. The RfC is based on suppressed
absolute lymphocyte counts seen in humans under occupational exposure
conditions. In addition, studies sponsored by the Health Effects
Institute (HEI) provide evidence that biochemical responses occur at
lower levels of benzene exposure than previously
known.224 225 226 227 EPA's IRIS program has not yet
evaluated these new data. EPA does not currently have an acute
reference concentration for benzene. The Agency for Toxic Substances
and Disease Registry (ATSDR) Minimal Risk Level (MRL) for acute
exposure to benzene is 29 [micro]g/m\3\ for 1-14 days
exposure.228 229
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    \220\ Aksoy, M. (1989). Hematotoxicity and carcinogenicity of
benzene. Environ. Health Perspect. 82: 193-197. EPA-HQ-OAR-2011-
0135.
    \221\ Goldstein, B.D. (1988). Benzene toxicity. Occupational
medicine. State of the Art Reviews. 3: 541-554.
    \222\ Rothman, N., G.L. Li, M. Dosemeci, W.E. Bechtold, G.E.
Marti, Y.Z. Wang, M. Linet, L.Q. Xi, W. Lu, M.T. Smith, N. Titenko-
Holland, L.P. Zhang, W. Blot, S.N. Yin, and R.B. Hayes. (1996).
Hematotoxicity among Chinese workers heavily exposed to benzene. Am.
J. Ind. Med. 29: 236-246.
    \223\ U.S. EPA (2002). Toxicological Review of Benzene
(Noncancer Effects). Environmental Protection Agency, Integrated
Risk Information System (IRIS), Research and Development, National
Center for Environmental Assessment, Washington DC. This material is
available electronically at https://cfpub.epa.gov/ncea/iris/iris_documents/documents/toxreviews/0276tr.pdf.
    \224\ Qu, O.; Shore, R.; Li, G.; Jin, X.; Chen, C.L.; Cohen, B.;
Melikian, A.; Eastmond, D.; Rappaport, S.; Li, H.; Rupa, D.;
Suramaya, R.; Songnian, W.; Huifant, Y.; Meng, M.; Winnik, M.; Kwok,
E.; Li, Y.; Mu, R.; Xu, B.; Zhang, X.; Li, K. (2003). HEI Report
115, Validation & Evaluation of Biomarkers in Workers Exposed to
Benzene in China.
    \225\ Qu, Q., R. Shore, G. Li, X. Jin, L.C. Chen, B. Cohen, et
al. (2002). Hematological changes among Chinese workers with a broad
range of benzene exposures. Am. J. Industr. Med. 42: 275-285.
    \226\ Lan, Qing, Zhang, L., Li, G., Vermeulen, R., et al.
(2004). Hematotoxically in Workers Exposed to Low Levels of Benzene.
Science 306: 1774-1776.
    \227\ Turtletaub, K.W. and Mani, C. (2003). Benzene metabolism
in rodents at doses relevant to human exposure from Urban Air.
Research Reports Health Effect Inst. Report No.113.
    \228\ U.S. Agency for Toxic Substances and Disease Registry
(ATSDR). (2007). Toxicological profile for benzene. Atlanta, GA:
U.S. Department of Health and Human Services, Public Health Service.
http://www.atsdr.cdc.gov/ToxProfiles/tp3.pdf.
    \229\ A minimal risk level (MRL) is defined as an estimate of
the daily human exposure to a hazardous substance that is likely to
be without appreciable risk of adverse noncancer health effects over
a specified duration of exposure.
---------------------------------------------------------------------------

    There is limited information from two studies regarding an
increased risk of adverse effects to children whose parents have been
occupationally exposed to benzene.230 231 Data from animal
studies have shown benzene exposures result in damage to the
hematopoietic (blood cell formation) system during
development.232 233 234 Also, key changes related to the
development of childhood leukemia occur in the developing fetus.\235\
Several studies have reported that genetic changes related to eventual
leukemia development occur before birth. For example, there is one
study of genetic changes in twins who developed T cell leukemia at nine
years of age.\236\
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    \230\ Corti, M; Snyder, CA. (1996) Influences of gender,
development, pregnancy and ethanol consumption on the hematotoxicity
of inhaled 10 ppm benzene. Arch Toxicol 70:209-217.
    \231\ McKinney P.A.; Alexander, F.E.; Cartwright, R.A.; et al.
(1991) Parental occupations of children with leukemia in west
Cumbria, north Humberside, and Gateshead, Br Med J 302:681-686.
    \232\ Keller, KA; Snyder, CA. (1986) Mice exposed in utero to
low concentrations of benzene exhibit enduring changes in their
colony forming hematopoietic cells. Toxicology 42:171-181.
    \233\ Keller, KA; Snyder, CA. (1988) Mice exposed in utero to 20
ppm benzene exhibit altered numbers of recognizable hematopoietic
cells up to seven weeks after exposure. Fundam Appl Toxicol 10:224-
232.
    \234\ Corti, M; Snyder, CA. (1996) Influences of gender,
development, pregnancy and ethanol consumption on the hematotoxicity
of inhaled 10 ppm benzene. Arch Toxicol 70:209-217.
    \235\ U.S. EPA. (2002). Toxicological Review of Benzene
(Noncancer Effects). National Center for Environmental Assessment,
Washington, DC. Report No. EPA/635/R-02/001F. https://cfpub.epa.gov/ncea/iris/iris_documents/documents/toxreviews/0276tr.pdf.
    \236\ Ford, AM; Pombo-de-Oliveira, MS; McCarthy, KP; MacLean,
JM; Carrico, KC; Vincent, RF; Greaves, M. (1997) Monoclonal origin
of concordant T-cell malignancy in identical twins. Blood 89:281-
285.
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iv. 1,3-Butadiene
    EPA has characterized 1,3-butadiene as carcinogenic to humans by
inhalation.237 238 The IARC has determined that 1,3-
butadiene is a human carcinogen and the U.S. DHHS has characterized
1,3-butadiene as a known human carcinogen.239 240 241 242
There are numerous studies consistently demonstrating that 1,3-
butadiene is metabolized into genotoxic metabolites by experimental
animals and humans. The specific mechanisms of 1,3-butadiene-induced
carcinogenesis are unknown; however, the scientific evidence strongly
suggests that the carcinogenic effects are mediated by genotoxic
metabolites. Animal data suggest that females may be more sensitive
than males for cancer effects associated with 1,3-butadiene exposure;
there are insufficient data in humans from which to draw conclusions
about sensitive subpopulations. The URE for 1,3-butadiene is 3 x 10-5
per [micro]g/m\3\.\243\

1,3-butadiene also causes a variety of reproductive and developmental
effects in mice; no human data on these effects are available. The most
sensitive effect was ovarian atrophy observed in a lifetime bioassay of
female mice.\244\ Based on this critical effect and the benchmark
concentration methodology, an RfC for chronic health effects was
calculated at 0.9 ppb (approximately 2 [micro]g/m\3\).
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    \237\ U.S. EPA. (2002). Health Assessment of 1,3-Butadiene.
Office of Research and Development, National Center for
Environmental Assessment, Washington Office, Washington, DC. Report
No. EPA600-P-98-001F. This document is available electronically at
https://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=54499.
    \238\ U.S. EPA. (2002) ``Full IRIS Summary for 1,3-butadiene
(CASRN 106-99-0)'' Environmental Protection Agency, Integrated Risk
Information System (IRIS), Research and Development, National Center
for Environmental Assessment, Washington, DC https://cfpub.epa.gov/ncea/iris2/chemicalLanding.cfm?substance_nmbr=139.
    \239\ International Agency for Research on Cancer (IARC).
(1999). Monographs on the evaluation of carcinogenic risk of
chemicals to humans, Volume 71, Re-evaluation of some organic
chemicals, hydrazine and hydrogen peroxide, World Health
Organization, Lyon, France.
    \240\ International Agency for Research on Cancer (IARC).
(2008). Monographs on the evaluation of carcinogenic risk of
chemicals to humans, 1,3-Butadiene, Ethylene Oxide and Vinyl Halides
(Vinyl Fluoride, Vinyl Chloride and Vinyl Bromide) Volume 97, World
Health Organization, Lyon, France.
    \241\ NTP (National Toxicology Program). 2016. Report on
Carcinogens, Fourteenth Edition.; Research Triangle Park, NC: U.S.
Department of Health and Human Services, Public Health Service.
https://ntp.niehs.nih.gov/go/roc14.
    \242\ International Agency for Research on Cancer (IARC).
(2012). Monographs on the evaluation of carcinogenic risk of
chemicals to humans, Volume 100F chemical agents and related
occupations, World Health Organization, Lyon, France.
    \243\ U.S. EPA. (2002). ``Full IRIS Summary for 1,3-butadiene
(CASRN 106-99-0)'' Environmental Protection Agency, Integrated Risk
Information System (IRIS), Research and Development, National Center
for Environmental Assessment, Washington, DC https://cfpub.epa.gov/ncea/iris2/chemicalLanding.cfm?substance_nmbr=139.
    \244\ Bevan, C.; Stadler, J.C.; Elliot, G.S.; et al. (1996).
Subchronic toxicity of 4-vinylcyclohexene in rats and mice by
inhalation. Fundam. Appl. Toxicol. 32:1-10.
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v. Ethylbenzene
    EPA's inhalation RfC for ethylbenzene is 1 mg/m\3\. This conclusion
on a weight of evidence determination and RfC are contained in the 1991
IRIS file for ethylbenzene.\245\ The RfC is based on developmental
effects. A study in rabbits found reductions in live rabbit kits per
litter at 1000 ppm. In addition, a study on rats found an increased
incidence of supernumerary and rudimentary ribs at 1000 ppm, and
elevated incidence of extra ribs at 100 ppm. In 1988, EPA concluded
that data were inadequate to give a weight of evidence characterization
for carcinogenic effects. EPA released an IRIS Assessment Plan for
Ethylbenzene in 2017 \246\ and EPA will be releasing the Systematic
Review Protocol for ethylbenzene in 2023.\247\
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    \245\ U.S. EPA. (1991). Integrated Risk Information System File
for Ethylbenzene. This material is available electronically at:
https://iris.epa.gov/ChemicalLanding/&substance_nmbr=51.
    \246\ U.S. EPA (2017). IRIS Assessment Plan for Ethylbenzene.
EPA/635/R-17/332. This document is available electronically at:
https://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=337468.
    \247\ U.S. EPA (2022). IRIS Program Outlook. June, 2022. This
material is available electronically at: https://www.epa.gov/system/files/documents/2022-06/IRIS%20Program%20Outlook_June22.pdf.
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    California EPA completed a cancer risk assessment for ethylbenzene
in 2007 and developed an inhalation unit risk estimate of 2.5 x 10-
6.\248\ This value was based on incidence of kidney cancer in male
rats. California EPA also developed a chronic inhalation noncancer
reference exposure level (REL) of 2000 [micro]g/m\3\, based on
nephrotoxicity and body weight reduction in rats, liver cellular
alterations, necrosis in mice, and hyperplasia of the pituitary gland
in mice.\249\
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    \248\ California OEHHA, 2007. Adoption of a Unit Risk Value for
Ethylbenzene. This material is available electronically at: https://oehha.ca.gov/air/report-hot-spots/adoption-unit-risk-value-ethylbenzene.
    \249\ California OEHHA, 2008. Technical Supporting Document for
Noncancer RELs, Appendix D3. This material is available
electronically at: https://oehha.ca.gov/media/downloads/crnr/appendixd3final.pdf.
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    ATSDR developed chronic Minimal Risk Levels (MRLs) for ethylbenzene
of 0.06 ppm based on renal effects, and an acute MRL of 5 ppm based on
auditory effects.
vi. Formaldehyde
    In 1991, EPA concluded that formaldehyde is a Class B1 probable
human carcinogen based on limited evidence in humans and sufficient
evidence in animals.\250\ An Inhalation URE for cancer and a Reference
Dose for oral noncancer effects were developed by EPA and posted on the
IRIS database. Since that time, the NTP and IARC have concluded that
formaldehyde is a known human carcinogen.251 252 253
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    \250\ EPA. Integrated Risk Information System. Formaldehyde
(CASRN 50-00-0) https://cfpub.epa.gov/ncea/iris2/chemicalLanding.cfm?substance_nmbr=419.
    \251\ NTP (National Toxicology Program). 2016. Report on
Carcinogens, Fourteenth Edition.; Research Triangle Park, NC: U.S.
Department of Health and Human Services, Public Health Service.
https://ntp.niehs.nih.gov/go/roc14.
    \252\ IARC Monographs on the Evaluation of Carcinogenic Risks to
Humans Volume 88 (2006): Formaldehyde, 2-Butoxyethanol and 1-tert-
Butoxypropan-2-ol.
    \253\ IARC Monographs on the Evaluation of Carcinogenic Risks to
Humans Volume 100F (2012): Formaldehyde.
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    The conclusions by IARC and NTP reflect the results of
epidemiologic research published since 1991 in combination with
previous animal, human, and mechanistic evidence. Research conducted by
the National Cancer Institute reported an increased risk of
nasopharyngeal cancer and specific lymphohematopoietic malignancies
among workers exposed to formaldehyde.254 255 256 A National
Institute of Occupational Safety and Health study of garment workers
also reported increased risk of death due to leukemia among workers
exposed to formaldehyde.\257\ Extended follow-up of a cohort of British
chemical workers did not report evidence of an increase in
nasopharyngeal or lymphohematopoietic cancers, but a continuing
statistically significant excess in lung cancers was reported.\258\
Finally, a study of embalmers reported formaldehyde exposures to be
associated with an increased risk of myeloid leukemia but not brain
cancer.\259\
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    \254\ Hauptmann, M.; Lubin, J.H.; Stewart, P.A.; Hayes, R.B.;
Blair, A. 2003. Mortality from lymphohematopoetic malignancies among
workers in formaldehyde industries. Journal of the National Cancer
Institute 95: 1615-1623.
    \255\ Hauptmann, M.; Lubin, J.H.; Stewart, P.A.; Hayes, R.B.;
Blair, A. 2004. Mortality from solid cancers among workers in
formaldehyde industries. American Journal of Epidemiology 159: 1117-
1130.
    \256\ Beane Freeman, L.E.; Blair, A.; Lubin, J.H.; Stewart,
P.A.; Hayes, R.B.; Hoover, R.N.; Hauptmann, M. 2009. Mortality from
lymphohematopoietic malignancies among workers in formaldehyde
industries: The National Cancer Institute cohort. J. National Cancer
Inst. 101: 751-761.
    \257\ Pinkerton, L.E. 2004. Mortality among a cohort of garment
workers exposed to formaldehyde: an update. Occup. Environ. Med. 61:
193-200.
    \258\ Coggon, D, EC Harris, J Poole, KT Palmer. 2003. Extended
follow-up of a cohort of British chemical workers exposed to
formaldehyde. J National Cancer Inst. 95:1608-1615.
    \259\ Hauptmann, M,; Stewart P.A.; Lubin J.H.; Beane Freeman,
L.E.; Hornung, R.W.; Herrick, R.F.; Hoover, R.N.; Fraumeni, J.F.;
Hayes, R.B. 2009. Mortality from lymphohematopoietic malignancies
and brain cancer among embalmers exposed to formaldehyde. Journal of
the National Cancer Institute 101:1696-1708.
---------------------------------------------------------------------------

    Health effects of formaldehyde in addition to cancer were reviewed
by the Agency for Toxics Substances and Disease Registry in 1999,
supplemented in 2010, and by the World Health
Organization.260 261 262 These organizations reviewed the
scientific literature concerning health effects linked to formaldehyde
exposure to evaluate hazards and dose response relationships and
defined exposure concentrations for minimal risk levels (MRLs). The
health endpoints reviewed included sensory irritation of eyes and
respiratory tract, reduced pulmonary function, nasal histopathology,
and immune system effects. In addition, research on reproductive and
developmental effects and neurological effects were discussed along
with several studies that suggest that formaldehyde may increase the
risk of asthma--particularly in the young.
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    \260\ ATSDR. 1999. Toxicological Profile for Formaldehyde, U.S.
Department of Health and Human Services (HHS), July 1999.
    \261\ ATSDR. 2010. Addendum to the Toxicological Profile for
Formaldehyde. U.S. Department of Health and Human Services (HHS),
October 2010.
    \262\ IPCS. 2002. Concise International Chemical Assessment
Document 40. Formaldehyde. World Health Organization.
---------------------------------------------------------------------------

    In June 2010, EPA released a draft Toxicological Review of
Formaldehyde--Inhalation Assessment through the IRIS program for peer
review by the National Research Council (NRC) and public comment.\263\
That draft assessment reviewed more recent research from animal and
human studies on cancer and other health effects. The NRC released
their review report in April 2011.\264\ EPA's draft

assessment, which addresses NRC recommendations, was suspended in
2018.\265\ The draft assessment was unsuspended in March 2021, and an
external review draft was released in April 2022.\266\ This draft
assessment is now undergoing review by the National Academy of
Sciences.\267\
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    \263\ EPA (U.S. Environmental Protection Agency). 2010.
Toxicological Review of Formaldehyde (CAS No. 50-00-0)--Inhalation
Assessment: In Support of Summary Information on the Integrated Risk
Information System (IRIS). External Review Draft. EPA/635/R-10/002A.
U.S. Environmental Protection Agency, Washington DC [online].
Available: http://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=223614.
    \264\ NRC (National Research Council). 2011. Review of the
Environmental Protection Agency's Draft IRIS Assessment of
Formaldehyde. Washington DC: National Academies Press. http://books.nap.edu/openbook.php?record_id=13142.
    \265\ U.S. EPA (2018). See https://cfpub.epa.gov/ncea/iris2/chemicalLanding.cfm?substance_nmbr=419.
    \266\ U.S. EPA. IRIS Toxicological Review of Formaldehyde-
Inhalation (Interagency Science Consultation Draft, 2021). U.S.
Environmental Protection Agency, Washington, DC, EPA/635/R-21/286,
2021.
    \267\ https://www.nationalacademies.org/our-work/review-of-epas-2021-draft-formaldehyde-assessment.
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vii. Naphthalene
    Naphthalene is found in small quantities in gasoline and diesel
fuels. Naphthalene emissions have been measured in larger quantities in
both gasoline and diesel exhaust compared with evaporative emissions
from mobile sources, indicating it is primarily a product of
combustion.
    Acute (short-term) exposure of humans to naphthalene by inhalation,
ingestion, or dermal contact is associated with hemolytic anemia and
damage to the liver and the nervous system.\268\ Chronic (long term)
exposure of workers and rodents to naphthalene has been reported to
cause cataracts and retinal damage.\269\ Children, especially neonates,
appear to be more susceptible to acute naphthalene poisoning based on
the number of reports of lethal cases in children and infants
(hypothesized to be due to immature naphthalene detoxification
pathways).\270\ EPA released an external review draft of a reassessment
of the inhalation carcinogenicity of naphthalene based on a number of
recent animal carcinogenicity studies.\271\ The draft reassessment
completed external peer review.\272\ Based on external peer review
comments received, EPA is developing a revised draft assessment that
considers inhalation and oral routes of exposure, as well as cancer and
noncancer effects.\273\ The external review draft does not represent
official agency opinion and was released solely for the purposes of
external peer review and public comment. The NTP listed naphthalene as
``reasonably anticipated to be a human carcinogen'' in 2004 on the
basis of bioassays reporting clear evidence of carcinogenicity in rats
and some evidence of carcinogenicity in mice.\274\ California EPA has
released a new risk assessment for naphthalene, and the IARC has
reevaluated naphthalene and re-classified it as Group 2B: possibly
carcinogenic to humans.\275\
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    \268\ U.S. EPA. 1998. Toxicological Review of Naphthalene
(Reassessment of the Inhalation Cancer Risk), Environmental
Protection Agency, Integrated Risk Information System, Research and
Development, National Center for Environmental Assessment,
Washington, DC. This material is available electronically at https://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=56434.
    \269\ U.S. EPA. 1998. Toxicological Review of Naphthalene
(Reassessment of the Inhalation Cancer Risk), Environmental
Protection Agency, Integrated Risk Information System, Research and
Development, National Center for Environmental Assessment,
Washington, DC. This material is available electronically at https://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=56434.
    \270\ U.S. EPA. (1998). Toxicological Review of Naphthalene
(Reassessment of the Inhalation Cancer Risk), Environmental
Protection Agency, Integrated Risk Information System, Research and
Development, National Center for Environmental Assessment,
Washington, DC. This material is available electronically at https://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=56434.
    \271\ U.S. EPA. (1998). Toxicological Review of Naphthalene
(Reassessment of the Inhalation Cancer Risk), Environmental
Protection Agency, Integrated Risk Information System, Research and
Development, National Center for Environmental Assessment,
Washington, DC. This material is available electronically at https://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=56434.
    \272\ Oak Ridge Institute for Science and Education. (2004).
External Peer Review for the IRIS Reassessment of the Inhalation
Carcinogenicity of Naphthalene. August 2004. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=84403.
    \273\ U.S. EPA. (2018) See: https://cfpub.epa.gov/ncea/iris2/chemicalLanding.cfm?substance_nmbr=436.
    \274\ NTP (National Toxicology Program). 2016. Report on
Carcinogens, Fourteenth Edition.; Research Triangle Park, NC: U.S.
Department of Health and Human Services, Public Health Service.
https://ntp.niehs.nih.gov/go/roc14.
    \275\ International Agency for Research on Cancer (IARC).
(2002). Monographs on the Evaluation of the Carcinogenic Risk of
Chemicals for Humans. Vol. 82. Lyon, France.
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    Naphthalene also causes a number of non-cancer effects in animals
following chronic and less-than-chronic exposure, including abnormal
cell changes and growth in respiratory and nasal tissues.\276\ The
current EPA IRIS assessment includes noncancer data on hyperplasia and
metaplasia in nasal tissue that form the basis of the inhalation RfC of
3 [micro]g/m\3\.\277\ The ATSDR MRL for acute and intermediate duration
oral exposure to naphthalene is 0.6 mg/kg/day based on maternal
toxicity in a developmental toxicology study in rats.\278\ ATSDR also
derived an ad hoc reference value of 6 x 10-2 mg/m\3\ for acute (<=24-
hour) inhalation exposure to naphthalene in a Letter Health
Consultation dated March 24, 2014 to address a potential exposure
concern in Illinois.\279\ The ATSDR acute inhalation reference value
was based on a qualitative identification of an exposure level
interpreted not to cause pulmonary lesions in mice. More recently, EPA
developed acute RfCs for 1-, 8-, and 24-hour exposure scenarios; the
<=24-hour reference value is 2 x 10-2 mg/m\3\.\280\ EPA's acute RfCs
are based on a systematic review of the literature, benchmark dose
modeling of naphthalene-induced nasal lesions in rats, and application
of a PBPK (physiologically based pharmacokinetic) model.
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    \276\ U.S. EPA. (1998). Toxicological Review of Naphthalene,
Environmental Protection Agency, Integrated Risk Information System,
Research and Development, National Center for Environmental
Assessment, Washington, DC. This material is available
electronically at https://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=56434.
    \277\ U.S. EPA. (1998). Toxicological Review of Naphthalene.
Environmental Protection Agency, Integrated Risk Information System
(IRIS), Research and Development, National Center for Environmental
Assessment, Washington, DC https://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=56434.
    \278\ ATSDR. Toxicological Profile for Naphthalene, 1-
Methylnaphthalene, and 2-Methylnaphthalene (2005). https://www.atsdr.cdc.gov/ToxProfiles/tp67-p.pdf.
    \279\ ATSDR. Letter Health Consultation, Radiac Abrasives, Inc.,
Chicago, Illinois (2014). https://www.atsdr.cdc.gov/HAC/pha/RadiacAbrasives/Radiac%20Abrasives,%20Inc.%20_%20LHC%20(Final)%20_%2003-24-
2014%20(2)_508.pdf.
    \280\ U. S. EPA. Derivation of an acute reference concentration
for inhalation exposure to naphthalene. Report No. EPA/600/R-21/292.
https://cfpub.epa.gov/ncea/risk/recordisplay.cfm?deid=355035.
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viii. POM/PAHs
    The term polycyclic organic matter (POM) defines a broad class of
compounds that includes the polycyclic aromatic hydrocarbon compounds
(PAHs). One of these compounds, naphthalene, is discussed separately in
Section II.C.7.vii. POM compounds are formed primarily from combustion
and are present in the atmosphere in gas and particulate form as well
as in some fried and grilled foods. Epidemiologic studies have reported
an increase in lung cancer in humans exposed to diesel exhaust, coke
oven emissions, roofing tar emissions, and cigarette smoke; all of
these mixtures contain POM compounds.281 282 In 1991 EPA
classified seven PAHs (benzo[a]pyrene, benz[a]anthracene, chrysene,
benzo[b]fluoranthene, benzo[k]fluoranthene, dibenz[a,h]anthracene, and

indeno[1,2,3-cd]pyrene) as Group B2, probable human carcinogens based
on the 1986 EPA Guidelines for Carcinogen Risk Assessment.\283\ Studies
in multiple animal species demonstrate that benzo[a]pyrene is
carcinogenic at multiple tumor sites (alimentary tract, liver, kidney,
respiratory tract, pharynx, and skin) by all routes of exposure. An
increasing number of occupational studies demonstrate a positive
exposure-response relationship with cumulative benzo[a]pyrene exposure
and lung cancer. The inhalation URE in IRIS for benzo[a]pyrene is 6 x
10-4 per [micro]g/m\3\ and the oral slope factor for cancer is 1 per
mg/kg-day.\284\
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    \281\ Agency for Toxic Substances and Disease Registry (ATSDR).
(1995). Toxicological profile for Polycyclic Aromatic Hydrocarbons
(PAHs). Atlanta, GA: U.S. Department of Health and Human Services,
Public Health Service. Available electronically at http://www.atsdr.cdc.gov/ToxProfiles/TP.asp?id=122&tid=25.
    \282\ U.S. EPA (2002). Health Assessment Document for Diesel
Engine Exhaust. EPA/600/8-90/057F Office of Research and
Development, Washington DC. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=29060.
    \283\ U.S. EPA (1991). Drinking Water Criteria Document for
Polycyclic Aromatic Hydrocarbons (PAHS). ECAO-CIN-0010. EPA Research
and Development.
    \284\ U.S. EPA (2017). Toxicological Review of Benzo[a]pyrene.
This material is available electronically at: https://cfpub.epa.gov/ncea/iris/iris_documents/documents/toxreviews/0136tr.pdf.
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    Animal studies demonstrate that exposure to benzo[a]pyrene is also
associated with developmental (including developmental neurotoxicity),
reproductive, and immunological effects. In addition, epidemiology
studies involving exposure to PAH mixtures have reported associations
between internal biomarkers of exposure to benzo[a]pyrene
(benzo[a]pyrene diol epoxide-DNA adducts) and adverse birth outcomes
(including reduced birth weight, postnatal body weight, and head
circumference), neurobehavioral effects, and decreased fertility. The
inhalation RfC for benzo[a]pyrene is 2 x 10-6 mg/m\3\ and
the RfD for oral exposure is 3 x 10-4 mg/kg-day.\285\
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    \285\ U.S. EPA (2017). Toxicological Review of Benzo[a]pyrene.
This material is available electronically at: https://cfpub.epa.gov/ncea/iris/iris_documents/documents/toxreviews/0136tr.pdf.
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8. Exposure and Health Effects Associated With Traffic
    Locations in close proximity to major roadways generally have
elevated concentrations of many air pollutants emitted from motor
vehicles. Hundreds of studies have been published in peer-reviewed
journals, concluding that concentrations of CO, CO2, NO,
NO2, benzene, aldehydes, particulate matter, black carbon,
and many other compounds are elevated in ambient air within
approximately 300-600 meters (about 1,000-2,000 feet) of major
roadways. The highest concentrations of most pollutants emitted
directly by motor vehicles are found at locations within 50 meters
(about 165 feet) of the edge of a roadway's traffic lanes.
    A large-scale review of air quality measurements in the vicinity of
major roadways between 1978 and 2008 concluded that the pollutants with
the steepest concentration gradients in vicinities of roadways were CO,
ultrafine particles, metals, elemental carbon (EC), NO, NOX,
and several VOCs.\286\ These pollutants showed a large reduction in
concentrations within 100 meters downwind of the roadway. Pollutants
that showed more gradual reductions with distance from roadways
included benzene, NO2, PM2.5, and
PM10. In reviewing the literature, Karner et al., (2010)
reported that results varied based on the method of statistical
analysis used to determine the gradient in pollutant concentration.
More recent studies continue to show significant concentration
gradients of traffic-related air pollution around major
roads.287 288 289 290 291 292 293 294 295 296 There is
evidence that EPA's regulations for vehicles have lowered the near-road
concentrations and gradients.\297\ Starting in 2010, EPA required
through the NAAQS process that air quality monitors be placed near
high-traffic roadways for determining concentrations of CO,
NO2, and PM2.5 (in addition to those existing
monitors located in neighborhoods and other locations farther away from
pollution sources). The monitoring data for NO2 indicate
that in urban areas, monitors near roadways often report the highest
concentrations of NO2.\298\ More recent studies of traffic-
related air pollutants continue to report sharp gradients around
roadways, particularly within several hundred meters.299 300
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    \286\ Karner, A.A.; Eisinger, D.S.; Niemeier, D.A. (2010). Near-
roadway air quality: synthesizing the findings from real-world data.
Environ Sci Technol 44: 5334-5344.
    \287\ McDonald, B.C.; McBride, Z.C.; Martin, E.W.; Harley, R.A.
(2014) High-resolution mapping of motor vehicle carbon dioxide
emissions. J. Geophys. Res. Atmos.,119, 5283-5298, doi:10.1002/
2013JD021219.
    \288\ Kimbrough, S.; Baldauf, R.W.; Hagler, G.S.W.; Shores,
R.C.; Mitchell, W.; Whitaker, D.A.; Croghan, C.W.; Vallero, D.A.
(2013) Long-term continuous measurement of near-road air pollution
in Las Vegas: seasonal variability in traffic emissions impact on
air quality. Air Qual Atmos Health 6: 295-305. DOI 10.1007/s11869-
012-0171-x.
    \289\ Kimbrough, S.; Palma, T.; Baldauf, R.W. (2014) Analysis of
mobile source air toxics (MSATs)--Near-road VOC and carbonyl
concentrations. Journal of the Air & Waste Management Association,
64:3, 349-359, DOI: 10.1080/10962247.2013.863814.
    \290\ Kimbrough, S.; Owen, R.C.; Snyder, M.; Richmond-Bryant, J.
(2017) NO to NO2 Conversion Rate Analysis and
Implications for Dispersion Model Chemistry Methods using Las Vegas,
Nevada Near-Road Field Measurements. Atmos Environ 165: 23-24.
    \291\ Hilker, N.; Wang, J.W.; Jong, C-H.; Healy, R.M.; Sofowote,
U.; Debosz, J.; Su, Y.; Noble, M.; Munoz, A.; Doerkson, G.; White,
L.; Audette, C.; Herod, D.; Brook, J.R.; Evans, G.J. (2019) Traffic-
related air pollution near roadways: discerning local impacts from
background. Atmos. Meas. Tech., 12, 5247-5261. https://doi.org/10.5194/amt-12-5247-2019.
    \292\ Grivas, G.; Stavroulas, I.; Liakakou, E.; Kaskaoutis,
D.G.; Bougiatioti, A.; Paraskevopoulou, D.; Gerasopoulos, E.;
Mihalopoulos, N. (2019) Measuring the spatial variability of black
carbon in Athens during wintertime. Air Quality, Atmosphere & Health
(2019) 12:1405-1417. https://doi.org/10.1007/s11869-019-00756-y.
    \293\ Apte, J.S.; Messier, K.P.; Gani, S.; Brauer, M.;
Kirchstetter, T.W.; Lunden, M.M.; Marshall, J.D.; Portier, C.J.;
Vermeulen, R.C.H.; Hamburg, S.P. (2017) High-Resolution Air
Pollution Mapping with Google Street View Cars: Exploiting Big Data.
Environ Sci Technol 51: 6999-7008. https://doi.org/10.1021/acs.est.7b00891.
    \294\ Dabek-Zlotorzynska, E.; Celo, V.; Ding, L.; Herod, D.;
Jeong, C-H.; Evans, G.; Hilker, N. (2019) Characteristics and
sources of PM2.5 and reactive gases near roadways in two
metropolitan areas in Canada. Atmos Environ 218: 116980. https://doi.org/10.1016/j.atmosenv.2019.116980.
    \295\ Apte, J.S.; Messier, K.R.; Gani, S.; et al. (2017) High-
resolution air pollution mapping with Google Street View cars:
exploiting big data. Environ Sci Technol 51: 6999-7018, [Online at
https://doi.org/10.1021/acs.est.7b00891.]
    \296\ Gu, P.; Li, H.Z.; Ye, Q.; et al. (2018) Intercity
variability of particulate matter is driven by carbonaceous sources
and correlated with land-use variables. Environ Sci Technol 52: 52:
11545-11554. [Online at http://dx.doi.org/10.1021/acs.est.8b03833.]
    \297\ Sarnat, J.A.; Russell, A.; Liang, D.; Moutinho, J.L;
Golan, R.; Weber, R.; Gao, D.; Sarnat, S.; Chang, H.H.; Greenwald,
R.; Yu, T. (2018) Developing Multipollutant Exposure Indicators of
Traffic Pollution: The Dorm Room Inhalation to Vehicle Emissions
(DRIVE) Study. Health Effects Institute Research Report Number 196.
[Online at: https://www.healtheffects.org/publication/developing-multipollutant-exposure-indicators-traffic-pollution-dorm-room-inhalation.]
    \298\ Gantt, B; Owen, R.C.; Watkins, N. (2021) Characterizing
nitrogen oxides and fine particulate matter near major highways in
the United States using the National Near-road Monitoring Network.
Environ Sci Technol 55: 2831-2838. [Online at https://doi.org/10.1021/acs.est.0c05851.]
    \299\ Apte, J.S.; Messier, K.R.; Gani, S.; et al. (2017) High-
resolution air pollution mapping with Google Street View cars:
exploiting big data. Environ Sci Technol 51: 6999-7018, [Online at
https://doi.org/10.1021/acs.est.7b00891.]
    \300\ Gu, P.; Li, H.Z.; Ye, Q.; et al. (2018) Intercity
variability of particulate matter is driven by carbonaceous sources
and correlated with land-use variables. Environ Sci Technol 52: 52:
11545-11554. [Online at http://dx.doi.org/10.1021/acs.est.8b03833.]
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    For pollutants with relatively high background concentrations
relative to near-road concentrations, detecting concentration gradients
can be difficult. For example, many carbonyls have high background
concentrations as a result of photochemical breakdown of precursors
from many different organic compounds. However, several studies have
measured carbonyls in multiple weather conditions and found higher
concentrations of many carbonyls downwind of
roadways.301 302 These

findings suggest a substantial roadway source of these carbonyls.
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    \301\ Liu, W.; Zhang, J.; Kwon, J.l; et l. (2006).
Concentrations and source characteristics of airborne carbonyl
compounds measured outside urban residences. J Air Waste Manage
Assoc 56: 1196-1204.
    \302\ Cahill, T.M.; Charles, M.J.; Seaman, V.Y. (2010).
Development and application of a sensitive method to determine
concentrations of acrolein and other carbonyls in ambient air.
Health Effects Institute Research Report 149. Available at https://www.healtheffects.org/system/files/Cahill149.pdf.
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    In the past 30 years, many studies have been published with results
reporting that populations who live, work, or go to school near high-
traffic roadways experience higher rates of numerous adverse health
effects, compared to populations far away from major roads.\303\ In
addition, numerous studies have found adverse health effects associated
with spending time in traffic, such as commuting or walking along high-
traffic roadways, including studies among
children.304 305 306 307 The health outcomes with the
strongest evidence linking them with traffic-associated air pollutants
are respiratory effects, particularly in asthmatic children, and
cardiovascular effects.
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    \303\ In the widely-used PubMed database of health publications,
between January 1, 1990 and December 31, 2021, 1,979 publications
contained the keywords ``traffic, pollution, epidemiology,'' with
approximately half the studies published after 2015.
    \304\ Laden, F.; Hart, J.E.; Smith, T.J.; Davis, M.E.; Garshick,
E. (2007) Cause-specific mortality in the unionized U.S. trucking
industry. Environmental Health Perspect 115:1192-1196.
    \305\ Peters, A.; von Klot, S.; Heier, M.; Trentinaglia, I.;
H[ouml]rmann, A.; Wichmann, H.E.; L[ouml]wel, H. (2004) Exposure to
traffic and the onset of myocardial infarction. New England J Med
351: 1721-1730.
    \306\ Zanobetti, A.; Stone, P.H.; Spelzer, F.E.; Schwartz, J.D.;
Coull, B.A.; Suh, H.H.; Nearling, B.D.; Mittleman, M.A.; Verrier,
R.L.; Gold, D.R. (2009) T-wave alternans, air pollution and traffic
in high-risk subjects. Am J Cardiol 104: 665-670.
    \307\ Adar, S.; Adamkiewicz, G.; Gold, D.R.; Schwartz, J.;
Coull, B.A.; Suh, H. (2007) Ambient and microenvironmental particles
and exhaled nitric oxide before and after a group bus trip. Environ
Health Perspect 115: 507-512.
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    Numerous reviews of this body of health literature have been
published. In a 2022 final report, an expert panel of the Health
Effects Institute (HEI) employed a systematic review focusing on
selected health endpoints related to exposure to traffic-related air
pollution.\308\ The HEI panel concluded that there was a high level of
confidence in evidence between long-term exposure to traffic-related
air pollution and health effects in adults, including all-cause,
circulatory, and ischemic heart disease mortality.\309\ The panel also
found that there is a moderate-to-high level of confidence in evidence
of associations with asthma onset and acute respiratory infections in
children and lung cancer and asthma onset in adults. This report
follows on an earlier expert review published by HEI in 2010, where it
found strongest evidence for asthma-related traffic impacts. Other
literature reviews have been published with conclusions generally
similar to the HEI panels'.310 311 312 313 Additionally, in
2014, researchers from the U.S. Centers for Disease Control and
Prevention (CDC) published a systematic review and meta-analysis of
studies evaluating the risk of childhood leukemia associated with
traffic exposure and reported positive associations between
``postnatal'' proximity to traffic and leukemia risks, but no such
association for ``prenatal'' exposures.\314\ The U.S. Department of
Health and Human Services' National Toxicology Program (NTP) published
a monograph including a systematic review of traffic-related air
pollution and its impacts on hypertensive disorders of pregnancy. The
NTP concluded that exposure to traffic-related air pollution is
``presumed to be a hazard to pregnant women'' for developing
hypertensive disorders of pregnancy.\315\
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    \308\ HEI Panel on the Health Effects of Long-Term Exposure to
Traffic-Related Air Pollution (2022) Systematic review and meta-
analysis of selected health effects of long-term exposure to
traffic-related air pollution. Health Effects Institute Special
Report 23. [Online at https://www.healtheffects.org/publication/systematic-review-and-meta-analysis-selected-health-effects-long-term-exposure-traffic.] This more recent review focused on health
outcomes related to birth effects, respiratory effects,
cardiometabolic effects, and mortality.
    \309\ Boogaard, H.; Patton. A.P.; Atkinson, R.W.; Brook, J.R.;
Chang, H.H.; Crouse, D.L.; Fussell, J.C.; Hoek, G.; Hoffman, B.;
Kappeler, R.; Kutlar Joss, M.; Ondras, M.; Sagiv, S.K.; Somoli, E.;
Shaikh, R.; Szpiro, A.A.; Van Vliet E.D.S.; Vinneau, D.; Weuve, J.;
Lurmann, F.W.; Forastiere, F. (2022) Long-term exposure to traffic-
related air pollution and selected health outcomes: a systematic
review and meta-analysis. Environ Intl 164: 107262. [Online at
https://doi.org/10.1016/j.envint.2022.107262.]
    \310\ Boothe, V.L.; Shendell, D.G. (2008). Potential health
effects associated with residential proximity to freeways and
primary roads: review of scientific literature, 1999-2006. J Environ
Health 70: 33-41.
    \311\ Salam, M.T.; Islam, T.; Gilliland, F.D. (2008). Recent
evidence for adverse effects of residential proximity to traffic
sources on asthma. Curr Opin Pulm Med 14: 3-8.
    \312\ Sun, X.; Zhang, S.; Ma, X. (2014) No association between
traffic density and risk of childhood leukemia: a meta-analysis.
Asia Pac J Cancer Prev 15: 5229-5232.
    \313\ Raaschou-Nielsen, O.; Reynolds, P. (2006). Air pollution
and childhood cancer: a review of the epidemiological literature.
Int J Cancer 118: 2920-9.
    \314\ Boothe, VL.; Boehmer, T.K.; Wendel, A.M.; Yip, F.Y. (2014)
Residential traffic exposure and childhood leukemia: a systematic
review and meta-analysis. Am J Prev Med 46: 413-422.
    \315\ National Toxicology Program (2019) NTP Monograph n the
Systematic Review of Traffic-related Air Pollution and Hypertensive
Disorders of Pregnancy. NTP Monograph 7. https://ntp.niehs.nih.gov/ntp/ohat/trap/mgraph/trap_final_508.pdf.
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    Health outcomes with few publications suggest the possibility of
other effects still lacking sufficient evidence to draw definitive
conclusions. Among these outcomes with a small number of positive
studies are neurological impacts (e.g., autism and reduced cognitive
function) and reproductive outcomes (e.g., preterm birth, low birth
weight).316 317 318 319 320
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    \316\ Volk, H.E.; Hertz-Picciotto, I.; Delwiche, L.; et al.
(2011). Residential proximity to freeways and autism in the CHARGE
study. Environ Health Perspect 119: 873-877.
    \317\ Franco-Suglia, S.; Gryparis, A.; Wright, R.O.; et al.
(2007). Association of black carbon with cognition among children in
a prospective birth cohort study. Am J Epidemiol. doi: 10.1093/aje/
kwm308. [Online at http://dx.doi.org].
    \318\ Power, M.C.; Weisskopf, M.G.; Alexeef, SE; et al. (2011).
Traffic-related air pollution and cognitive function in a cohort of
older men. Environ Health Perspect 2011: 682-687.
    \319\ Wu, J.; Wilhelm, M.; Chung, J.; et al. (2011). Comparing
exposure assessment methods for traffic-related air pollution in and
adverse pregnancy outcome study. Environ Res 111: 685-6692.
    \320\ Stenson, C.; Wheeler, A.J.; Carver, A.; et al. (2021) The
impact of traffic-related air pollution on child and adolescent
academic performance: a systematic review. Environ Intl 155: 106696
[Online at https://doi.org/10.1016/j.envint.2021.106696.]
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    In addition to health outcomes, particularly cardiopulmonary
effects, conclusions of numerous studies suggest mechanisms by which
traffic-related air pollution affects health. For example, numerous
studies indicate that near-roadway exposures may increase systemic
inflammation, affecting organ systems, including blood vessels and
lungs.321 322 323 324 Additionally, long-term exposures in
near-road environments have been associated with inflammation-
associated conditions, such as atherosclerosis and
asthma.325 326 327
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    \321\ Riediker, M. (2007). Cardiovascular effects of fine
particulate matter components in highway patrol officers. Inhal
Toxicol 19: 99-105. doi: 10.1080/08958370701495238.
    \322\ Alexeef, SE; Coull, B.A.; Gryparis, A.; et al. (2011).
Medium-term exposure to traffic-related air pollution and markers of
inflammation and endothelial function. Environ Health Perspect 119:
481-486. Doi:10.1289/ehp.1002560.
    \323\ Eckel. S.P.; Berhane, K.; Salam, M.T.; et al. (2011).
Residential Traffic-related pollution exposure and exhaled nitric
oxide in the Children's Health Study. Environ Health Perspect.
doi:10.1289/ehp.1103516.
    \324\ Zhang, J.; McCreanor, J.E.; Cullinan, P.; et al. (2009).
Health effects of real-world exposure diesel exhaust in persons with
asthma. Res Rep Health Effects Inst 138. [Online at http://www.healtheffects.org].
    \325\ Adar, S.D.; Klein, R.; Klein, E.K.; et al. (2010). Air
pollution and the microvasculature: a cross-sectional assessment of
in vivo retinal images in the population-based Multi-Ethnic Study of
Atherosclerosis. PLoS Med 7(11): E1000372. doi:10.1371/
journal.pmed.1000372. Available at http://dx.doi.org.
    \326\ Kan, H.; Heiss, G.; Rose, K.M.; et al. (2008). Prospective
analysis of traffic exposure as a risk factor for incident coronary
heart disease: The Atherosclerosis Risk in Communities (ARIC) study.
Environ Health Perspect 116: 1463-1468. doi:10.1289/ehp.11290.
Available at http://dx.doi.org.
    \327\ McConnell, R.; Islam, T.; Shankardass, K.; et al. (2010).
Childhood incident asthma and traffic-related air pollution at home
and school. Environ Health Perspect 1021-1026.
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    Several studies suggest that some factors may increase
susceptibility to

the effects of traffic-associated air pollution. Several studies have
found stronger respiratory associations in children experiencing
chronic social stress, such as in violent neighborhoods or in homes
with high family stress.328 329 330
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    \328\ Islam, T.; Urban, R.; Gauderman, W.J.; et al. (2011).
Parental stress increases the detrimental effect of traffic exposure
on children's lung function. Am J Respir Crit Care Med.
    \329\ Clougherty, J.E.; Levy, J.I.; Kubzansky, L.D.; et al.
(2007). Synergistic effects of traffic-related air pollution and
exposure to violence on urban asthma etiology. Environ Health
Perspect 115: 1140-1146.
    \330\ Chen, E.; Schrier, H.M.; Strunk, R.C.; et al. (2008).
Chronic traffic-related air pollution and stress interact to predict
biologic and clinical outcomes in asthma. Environ Health Perspect
116: 970-5.
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    The risks associated with residence, workplace, or schools near
major roads are of potentially high public health significance due to
the large population in such locations. The 2013 U.S. Census Bureau's
American Housing Survey (AHS) was the last AHS that included whether
housing units were within 300 feet of an ``airport, railroad, or
highway with four or more lanes.'' \331\ The 2013 survey reports that
17.3 million housing units, or 13 percent of all housing units in the
U.S., were in such areas. Assuming that populations and housing units
are in the same locations, this corresponds to a population of more
than 41 million U.S. residents within 300 feet (approximately 90
meters) of high-traffic roadways or other transportation sources.
According to the Central Intelligence Agency's World Factbook, based on
data collected between 2012-2014, the United States had 6,586,610 km of
roadways, 293,564 km of railways, and 13,513 airports. As such,
highways represent the overwhelming majority of transportation
facilities described by this factor in the AHS.
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    \331\ The variable was known as ``ETRANS'' in the questions
about the neighborhood.
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    We analyzed national databases that allowed us to evaluate whether
homes and schools were located near a major road and whether
disparities in exposure may be occurring in these environments. Until
2009, the AHS included descriptive statistics of over 70,000 housing
units across the nation and asked about transportation infrastructure
near respondents' homes every two years.332 333 We also
analyzed the U.S. Department of Education's Common Core of Data, which
includes enrollment and location information for schools across the
U.S.\334\
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    \332\ U.S. Department of Housing and Urban Development, & U.S.
Census Bureau. (n.d.). Age of other residential buildings within 300
feet. In American Housing Survey for the United States: 2009 (pp. A-
1). Retrieved from https://www.census.gov/programs-surveys/ahs/data/2009/ahs-2009-summary-tables0/h150-09.html.
    \333\ The 2013 AHS again included the ``etrans'' question about
highways, airports, and railroads within half a block of the housing
unit but has not maintained the question since then.
    \334\ http://nces.ed.gov/ccd/.
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    In analyzing the 2009 AHS, we focused on whether a housing unit was
located within 300 feet of a ``4-or-more lane highway, railroad, or
airport'' (this distance was used in the AHS analysis).\335\ We
analyzed whether there were differences between households in such
locations compared with those in locations farther from these
transportation facilities.\336\ We included other variables, such as
land use category, region of country, and housing type. We found that
homes with a non-White householder were 22-34 percent more likely to be
located within 300 feet of these large transportation facilities than
homes with White householders. Homes with a Hispanic householder were
17-33 percent more likely to be located within 300 feet of these large
transportation facilities than homes with non-Hispanic householders.
Households near large transportation facilities were, on average, lower
in income and educational attainment and more likely to be a rental
property and located in an urban area compared with households more
distant from transportation facilities.
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    \335\ This variable primarily represents roadway proximity.
According to the Central Intelligence Agency's World Factbook, in
2010, the United States had 6,506,204 km of roadways, 224,792 km of
railways, and 15,079 airports. Highways thus represent the
overwhelming majority of transportation facilities described by this
factor in the AHS.
    \336\ Bailey, C. (2011) Demographic and Social Patterns in
Housing Units Near Large Highways and other Transportation Sources.
Memorandum to docket.
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    In examining schools near major roadways, we used the Common Core
of Data from the U.S. Department of Education, which includes
information on all public elementary and secondary schools and school
districts nationwide.\337\ To determine school proximities to major
roadways, we used a geographic information system (GIS) to map each
school and roadways based on the U.S. Census's TIGER roadway file.\338\
We estimated that about 10 million students attend public schools
within 200 meters of major roads, about 20 percent of the total number
of public school students in the U.S.\339\ About 800,000 students
attend public schools within 200 meters of primary roads, or about 2
percent of the total. We found that students of color were
overrepresented at schools within 200 meters of primary roadways, and
schools within 200 meters of primary roadways had a disproportionate
population of students eligible for free or reduced-price lunches.\340\
Black students represent 22 percent of students at schools located
within 200 meters of a primary road, compared to 17 percent of students
in all U.S. schools. Hispanic students represent 30 percent of students
at schools located within 200 meters of a primary road, compared to 22
percent of students in all U.S. schools.
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    \337\ http://nces.ed.gov/ccd/.
    \338\ Pedde, M.; Bailey, C. (2011) Identification of Schools
within 200 Meters of U.S. Primary and Secondary Roads. Memorandum to
the docket.
    \339\ Here, ``major roads'' refer to those TIGER classifies as
either ``Primary'' or ``Secondary.'' The Census Bureau describes
primary roads as ``generally divided limited-access highways within
the Federal interstate system or under state management.'' Secondary
roads are ``main arteries, usually in the U.S. highway, state
highway, or county highway system.''
    \340\ For this analysis we analyzed a 200-meter distance based
on the understanding that roadways generally influence air quality
within a few hundred meters from the vicinity of heavily traveled
roadways or along corridors with significant trucking traffic. See
U.S. EPA, 2014. Near Roadway Air Pollution and Health: Frequently
Asked Questions. EPA-420-F-14-044. For a surrogate of lower
socioeconomic status (SES), we used student eligibility for the U.S.
Department of Agriculture's (USDA) National School Lunch Program.
---------------------------------------------------------------------------

    Research into the impact of traffic-related air pollution on school
performance is tentative. Two reviews of this literature found some
evidence that children exposed to higher levels of traffic-related air
pollution show poorer academic performance than those exposed to lower
levels of traffic-related air pollution.341 342 However,
this evidence was judged to be weak due to limitations in the
assessment methods.
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    \341\ Stenson, C.; Wheeler, A.J.; Carver, A.; et al. (2021) The
impact of traffic-related air pollution on child and adolescent
academic performance: a systematic review. Environ Intl 155: 106696.
[Online at https://doi.org/10.1016/j.envint.2021.106696.]
    \342\ Gartland, N; Aljofi, H.E.; Dienes, K.; Munford, L.A.;
Theakston, A.L.; van Tongeren, M. (2022) The effects of traffic air
pollution in and around schools on executive function and academic
performance in children: a rapid review. Int J Environ Res Public
Health 10: 749. [Online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8776123/.]
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    EPA also conducted a study to estimate the number of people living
near truck freight routes in the United States, which includes many
large highways and other routes where light- and medium-duty vehicles
operate.343 344 Based on a population

analysis using the U.S. Department of Transportation's (USDOT) Freight
Analysis Framework 4 (FAF4) and population data from the 2010 decennial
census, an estimated 72 million people live within 200 meters of these
FAF4 roads, which are used by all types of vehicles.345 346
This analysis includes the population living within twice the distance
of major roads compared with the analysis of housing units near major
roads described earlier in this section. The larger distance and other
methodological differences explain the difference in the two estimates
for populations living near major roads. Relative to the rest of the
population, people of color and those with lower incomes are more
likely to live near FAF4 roads.
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    \343\ U.S. EPA (2021). Estimation of Population Size and
Demographic Characteristics among People Living Near Truck Routes in
the Conterminous United States. Memorandum to the Docket.
    \344\ FAF4 includes the following roadway types: interstate
highways, other FHWA-designated routes in the National Highway
System (NHS), National Network (NN) routes not part of the NHS,
other rural and urban principal arterials, intermodal connectors,
rural minor arterials for those counties not served by either NHS or
NN routes, and urban bypass and streets as appropriate for network
connectivity. Full documentation of the FAF4 road network is found
at https://fafdev.ornl.gov/fafweb/data/Final%20Report_FAF4_August_2016_BP.pdf.
    \345\ FAF4 is a model from the USDOT's Bureau of Transportation
Statistics (BTS) and Federal Highway Administration (FHWA), which
provides data associated with freight movement in the U.S. It
includes data from the 2012 Commodity Flow Survey (CFS), the Census
Bureau on international trade, as well as data associated with
construction, agriculture, utilities, warehouses, and other
industries. FAF4 estimates the modal choices for moving goods by
trucks, trains, boats, and other types of freight modes. It includes
traffic assignments, including truck flows on a network of truck
routes. https://ops.fhwa.dot.gov/freight/freight_analysis/faf/.
    \346\ The same analysis estimated the population living within
100 meters of a FAF4 truck route is 41 million.
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    EPA's Exposure Factor Handbook also indicates that, on average,
Americans spend more than an hour traveling each day, bringing nearly
all residents into a high-exposure microenvironment for part of the
day.\347\ The duration of commuting results in another important
contributor to overall exposure to traffic-related air pollution.
Studies of health that address time spent in transit have found
evidence of elevated risk of cardiac impacts.348 349 350
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    \347\ EPA. (2011) Exposure Factors Handbook: 2011 Edition.
Chapter 16. Online at https://www.epa.gov/expobox/about-exposure-factors-handbook.
    \348\ Riediker, M.; Cascio, W.E.; Griggs, T.R.; et al. (2004)
Particulate matter exposure in cars is associated with
cardiovascular effects in healthy young men. Am J Respir Crit Care
Med 169. [Online at https://doi.org/10.1164/rccm.200310-1463OC.]
    \349\ Peters, A.; von Klot, S.; Heier, M.; et al. (2004)
Exposure to traffic and the onset of myocardial infarction. New Engl
J Med 1721-1730. [Online at https://doi.org/10.1056/NEJMoa040203.]
    \350\ Adar, S.D.; Gold, D.R.; Coull, B.A.; (2007) Focused
exposure to airborne traffic particles and heart rate variability in
the elderly. Epidemiology 18: 95-103 [Online at 351: https://doi.org/10.1097/01.ede.0000249409.81050.46.]
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D. Welfare Effects Associated With Exposure to Criteria and Air Toxics
Pollutants Impacted by the Proposed Standards

    This section discusses the welfare effects associated with
pollutants affected by this rule, specifically particulate matter,
ozone, NOX, SOX, and air toxics.
1. Visibility
    Visibility can be defined as the degree to which the atmosphere is
transparent to visible light.\351\ Visibility impairment is caused by
light scattering and absorption by suspended particles and gases. It is
dominated by contributions from suspended particles except under
pristine conditions. Visibility is important because it has direct
significance to people's enjoyment of daily activities in all parts of
the country. Individuals value good visibility for the well-being it
provides them directly, where they live and work, and in places where
they enjoy recreational opportunities. Visibility is also highly valued
in significant natural areas, such as national parks and wilderness
areas, and special emphasis is given to protecting visibility in these
areas. For more information on visibility see the final 2019
PMISA.\352\
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    \351\ National Research Council, (1993). Protecting Visibility
in National Parks and Wilderness Areas. National Academy of Sciences
Committee on Haze in National Parks and Wilderness Areas. National
Academy Press, Washington, DC. This book can be viewed on the
National Academy Press website at https://www.nap.edu/catalog/2097/protecting-visibility-in-national-parks-and-wilderness-areas.
    \352\ U.S. EPA. Integrated Science Assessment (ISA) for
Particulate Matter (Final Report, 2019). U.S. Environmental
Protection Agency, Washington, DC, EPA/600/R-19/188, 2019.
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    EPA is working to address visibility impairment. Reductions in air
pollution from implementation of various programs associated with the
Clean Air Act Amendments of 1990 provisions have resulted in
substantial improvements in visibility and will continue to do so in
the future. Nationally, because trends in haze are closely associated
with trends in particulate sulfate and nitrate due to the relationship
between their concentration and light extinction, visibility trends
have improved as emissions of SO2 and NOX have
decreased over time due to air pollution regulations such as the Acid
Rain Program.\353\ However, in the western part of the country, changes
in total light extinction were smaller, and the contribution of
particulate organic matter to atmospheric light extinction was
increasing due to increasing wildfire emissions.\354\
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    \353\ U.S. EPA. Integrated Science Assessment (ISA) for
Particulate Matter (Final Report, 2019). U.S. Environmental
Protection Agency, Washington, DC, EPA/600/R-19/188, 2019.
    \354\ Hand, JL; Prenni, AJ; Copeland, S; Schichtel, BA; Malm,
WC. (2020). Thirty years of the Clean Air Act Amendments: Impacts on
haze in remote regions of the United States (1990-2018). Atmos
Environ 243: 117865.
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    In the Clean Air Act Amendments of 1977, Congress recognized
visibility's value to society by establishing a national goal to
protect national parks and wilderness areas from visibility impairment
caused by manmade pollution.\355\ In 1999, EPA finalized the regional
haze program to protect the visibility in Mandatory Class I Federal
areas.\356\ There are 156 national parks, forests and wilderness areas
categorized as Mandatory Class I Federal areas.\357\ These areas are
defined in CAA section 162 as those national parks exceeding 6,000
acres, wilderness areas and memorial parks exceeding 5,000 acres, and
all international parks which were in existence on August 7, 1977.
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    \355\ See Section 169(a) of the Clean Air Act.
    \356\ 64 FR 35714, July 1, 1999.
    \357\ 62 FR 38680-38681, July 18, 1997.
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    EPA has also concluded that PM2.5 causes adverse effects
on visibility in other areas that are not targeted by the Regional Haze
Rule, such as urban areas, depending on PM2.5 concentrations
and other factors such as dry chemical composition and relative
humidity (i.e., an indicator of the water composition of the
particles). The secondary (welfare-based) PM NAAQS provide protection
against visibility effects. In recent PM NAAQS reviews, EPA evaluated a
target level of protection for visibility impairment that is expected
to be met through attainment of the existing secondary PM
standards.\358\
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    \358\ On June 10, 2021, EPA announced that it will reconsider
the decision to retain the PM NAAQS. https://www.epa.gov/pm-pollution/national-ambient-air-quality-standards-naaqs-pm.
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2. Ozone Effects on Ecosystems
    The welfare effects of ozone include effects on ecosystems, which
can be observed across a variety of scales, i.e., subcellular,
cellular, leaf, whole plant, population, and ecosystem. Ozone effects
that begin at small spatial scales, such as the leaf of an individual
plant, when they occur at sufficient magnitudes (or to a sufficient
degree) can result in effects being propagated along a continuum to
higher and higher levels of biological organization. For example,
effects at the individual plant level, such as altered rates of leaf
gas exchange, growth, and reproduction, can, when widespread, result in
broad changes in ecosystems, such as productivity, carbon storage,
water

cycling, nutrient cycling, and community composition.
    Ozone can produce both acute and chronic injury in sensitive plant
species depending on the concentration level and the duration of the
exposure.\359\ In those sensitive species,\360\ effects from repeated
exposure to ozone throughout the growing season of the plant can tend
to accumulate, so even relatively low concentrations experienced for a
longer duration have the potential to create chronic stress on
vegetation.361 362 Ozone damage to sensitive plant species
includes impaired photosynthesis and visible injury to leaves. The
impairment of photosynthesis, the process by which the plant makes
carbohydrates (its source of energy and food), can lead to reduced crop
yields, timber production, and plant productivity and growth. Impaired
photosynthesis can also lead to a reduction in root growth and
carbohydrate storage below ground, resulting in other, more subtle
plant and ecosystems impacts.\363\ These latter impacts include
increased susceptibility of plants to insect attack, disease, harsh
weather, interspecies competition and overall decreased plant vigor.
The adverse effects of ozone on areas with sensitive species could
potentially lead to species shifts and loss from the affected
ecosystems,\364\ resulting in a loss or reduction in associated
ecosystem goods and services. Additionally, visible ozone injury to
leaves can result in a loss of aesthetic value in areas of special
scenic significance like national parks and wilderness areas and
reduced use of sensitive ornamentals in landscaping.\365\ In addition
to ozone effects on vegetation, newer evidence suggests that ozone
affects interactions between plants and insects by altering chemical
signals (e.g., floral scents) that plants use to communicate to other
community members, such as attraction of pollinators.
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    \359\ 73 FR 16486, March 27, 2008.
    \360\ 73 FR 16491, March 27, 2008. Only a small percentage of
all the plant species growing within the U.S. (over 43,000 species
have been catalogued in the USDA PLANTS database) have been studied
with respect to ozone sensitivity.
    \361\ U.S. EPA. Integrated Science Assessment (ISA) for Ozone
and Related Photochemical Oxidants (Final Report). U.S.
Environmental Protection Agency, Washington, DC, EPA/600/R-20/012,
2020.
    \362\ The concentration at which ozone levels overwhelm a
plant's ability to detoxify or compensate for oxidant exposure
varies. Thus, whether a plant is classified as sensitive or tolerant
depends in part on the exposure levels being considered.
    \363\ 73 FR 16492, March 27, 2008.
    \364\ 73 FR 16493-16494, March 27, 2008. Ozone impacts could be
occurring in areas where plant species sensitive to ozone have not
yet been studied or identified.
    \365\ 73 FR 16490-16497, March 27, 2008.
---------------------------------------------------------------------------

    The Ozone ISA presents more detailed information on how ozone
affects vegetation and ecosystems.366 367 The Ozone ISA
reports causal and likely causal relationships between ozone exposure
and a number of welfare effects and characterizes the weight of
evidence for different effects associated with ozone.\368\ The ISA
concludes that visible foliar injury effects on vegetation, reduced
vegetation growth, reduced plant reproduction, reduced productivity in
terrestrial ecosystems, reduced yield and quality of agricultural
crops, alteration of below-ground biogeochemical cycles, and altered
terrestrial community composition are causally associated with exposure
to ozone. It also concludes that increased tree mortality, altered
herbivore growth and reproduction, altered plant-insect signaling,
reduced carbon sequestration in terrestrial ecosystems, and alteration
of terrestrial ecosystem water cycling are likely to be causally
associated with exposure to ozone.
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    \366\ U.S. EPA. Integrated Science Assessment (ISA) for Ozone
and Related Photochemical Oxidants (Final Report). U.S.
Environmental Protection Agency, Washington, DC, EPA/600/R-20/012,
2020.
    \367\ U.S. EPA. Integrated Science Assessment (ISA) for Ozone
and Related Photochemical Oxidants (Final Report). U.S.
Environmental Protection Agency, Washington, DC, EPA/600/R-20/012,
2020.
    \368\ The Ozone ISA evaluates the evidence associated with
different ozone related health and welfare effects, assigning one of
five ``weight of evidence'' determinations: causal relationship,
likely to be a causal relationship, suggestive of a causal
relationship, inadequate to infer a causal relationship, and not
likely to be a causal relationship. For more information on these
levels of evidence, please refer to Table II of the ISA.
---------------------------------------------------------------------------

3. Deposition
    The Integrated Science Assessment for Oxides of Nitrogen, Oxides of
Sulfur, and Particulate Matter--Ecological Criteria documents the
ecological effects of the deposition of these criteria air
pollutants.\369\ It is clear from the body of evidence that oxides of
nitrogen, oxides of sulfur, and particulate matter contribute to total
nitrogen (N) and sulfur (S) deposition. In turn, N and S deposition
cause either nutrient enrichment or acidification depending on the
sensitivity of the landscape or the species in question. Both
enrichment and acidification are characterized by an alteration of the
biogeochemistry and the physiology of organisms, resulting in harmful
declines in biodiversity in terrestrial, freshwater, wetland, and
estuarine ecosystems in the U.S. Decreases in biodiversity mean that
some species become relatively less abundant and may be locally
extirpated. In addition to the loss of unique living species, the
decline in total biodiversity can be harmful because biodiversity is an
important determinant of the stability of ecosystems and their ability
to provide socially valuable ecosystem services.
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    \369\ U.S. EPA. Integrated Science Assessment (ISA) for Oxides
of Nitrogen, Oxides of Sulfur and Particulate Matter Ecological
Criteria (Final Report). U.S. Environmental Protection Agency,
Washington, DC, EPA/600/R-20/278, 2020.
---------------------------------------------------------------------------

    Terrestrial, wetland, freshwater, and estuarine ecosystems in the
U.S. are affected by N enrichment/eutrophication caused by N
deposition. These effects have been consistently documented across the
U.S. for hundreds of species. In aquatic systems increased nitrogen can
alter species assemblages and cause eutrophication. In terrestrial
systems nitrogen loading can lead to loss of nitrogen-sensitive lichen
species, decreased biodiversity of grasslands, meadows and other
sensitive habitats, and increased potential for invasive species. For a
broader explanation of the topics treated here, refer to the
description in Chapter 9 of the DRIA.
    The sensitivity of terrestrial and aquatic ecosystems to
acidification from nitrogen and sulfur deposition is predominantly
governed by geology. Prolonged exposure to excess nitrogen and sulfur
deposition in sensitive areas acidifies lakes, rivers, and soils.
Increased acidity in surface waters creates inhospitable conditions for
biota and affects the abundance and biodiversity of fishes, zooplankton
and macroinvertebrates and ecosystem function. Over time, acidifying
deposition also removes essential nutrients from forest soils,
depleting the capacity of soils to neutralize future acid loadings and
negatively affecting forest sustainability. Major effects in forests
include a decline in sensitive tree species, such as red spruce (Picea
rubens) and sugar maple (Acer saccharum).
    Building materials including metals, stones, cements, and paints
undergo natural weathering processes from exposure to environmental
elements (e.g., wind, moisture, temperature fluctuations, sunlight,
etc.). Pollution can worsen and accelerate these effects. Deposition of
PM is associated with both physical damage (materials damage effects)
and impaired aesthetic qualities (soiling effects). Wet and dry
deposition of PM can physically affect materials, adding to the effects
of natural weathering processes, by potentially promoting or
accelerating the corrosion of metals, by degrading paints and by
deteriorating building materials such as

stone, concrete, and marble.\370\ The effects of PM are exacerbated by
the presence of acidic gases and can be additive or synergistic due to
the complex mixture of pollutants in the air and surface
characteristics of the material. Acidic deposition has been shown to
have an effect on materials including zinc/galvanized steel and other
metal, carbonate stone (as monuments and building facings), and surface
coatings (paints).\371\ The effects on historic buildings and outdoor
works of art are of particular concern because of the uniqueness and
irreplaceability of many of these objects. In addition to aesthetic and
functional effects on metals, stone, and glass, altered energy
efficiency of photovoltaic panels by PM deposition is also becoming an
important consideration for impacts of air pollutants on materials.
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    \370\ U.S. EPA. Integrated Science Assessment (ISA) for
Particulate Matter (Final Report, 2019). U.S. Environmental
Protection Agency, Washington, DC, EPA/600/R-19/188, 2019.
    \371\ Irving, P.M., e.d. 1991. Acid Deposition: State of Science
and Technology, Volume III, Terrestrial, Materials, Health, and
Visibility Effects, The U.S. National Acid Precipitation Assessment
Program, Chapter 24, page 24-76.
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4. Welfare Effects Associated With Air Toxics
    Emissions from producing, transporting, and combusting fuel
contribute to ambient levels of pollutants that contribute to adverse
effects on vegetation. VOCs, some of which are considered air toxics,
have long been suspected to play a role in vegetation damage.\372\ In
laboratory experiments, a wide range of tolerance to VOCs has been
observed.\373\ Decreases in harvested seed pod weight have been
reported for the more sensitive plants, and some studies have reported
effects on seed germination, flowering, and fruit ripening. Effects of
individual VOCs or their role in conjunction with other stressors
(e.g., acidification, drought, temperature extremes) have not been well
studied. In a recent study of a mixture of VOCs including ethanol and
toluene on herbaceous plants, significant effects on seed production,
leaf water content and photosynthetic efficiency were reported for some
plant species.\374\
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    \372\ U.S. EPA. (1991). Effects of organic chemicals in the
atmosphere on terrestrial plants. EPA/600/3-91/001.
    \373\ Cape JN, ID Leith, J Binnie, J Content, M Donkin, M
Skewes, DN Price AR Brown, AD Sharpe. (2003). Effects of VOCs on
herbaceous plants in an open-top chamber experiment. Environ.
Pollut. 124:341-343.
    \374\ Cape JN, ID Leith, J Binnie, J Content, M Donkin, M
Skewes, DN Price AR Brown, AD Sharpe. (2003). Effects of VOCs on
herbaceous plants in an open-top chamber experiment. Environ.
Pollut. 124:341-343.
---------------------------------------------------------------------------

    Research suggests an adverse impact of vehicle exhaust on plants,
which has in some cases been attributed to aromatic compounds and in
other cases to NOX.375 376 377 The impacts of
VOCs on plant reproduction may have long-term implications for
biodiversity and survival of native species near major roadways. Most
of the studies of the impacts of VOCs on vegetation have focused on
short-term exposure and few studies have focused on long-term effects
of VOCs on vegetation and the potential for metabolites of these
compounds to affect herbivores or insects.
---------------------------------------------------------------------------

    \375\ Viskari E-L. (2000). Epicuticular wax of Norway spruce
needles as indicator of traffic pollutant deposition. Water, Air,
and Soil Pollut. 121:327-337.
    \376\ Ugrekhelidze D, F Korte, G Kvesitadze. (1997). Uptake and
transformation of benzene and toluene by plant leaves. Ecotox.
Environ. Safety 37:24-29.
    \377\ Kammerbauer H, H Selinger, R Rommelt, A Ziegler-Jons, D
Knoppik, B Hock. (1987). Toxic components of motor vehicle emissions
for the spruce Picea abies. Environ. Pollut. 48:235-243.
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III. EPA Proposal for Light- and Medium-Duty Vehicle Standards for
Model Years 2027 and Later

A. Introduction and Background

    This Preamble Section III outlines the proposed GHG and criteria
pollutant standards and related provisions that are included in the
proposal.
    Throughout this section and elsewhere in this NPRM, EPA uses the
following conventions to identify specific vehicle technology types.
More information about these vehicle technologies may be found in the
2016 EPA Draft Technical Assessment Report.\378\
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    \378\ Draft Technical Assessment Report, EPA-420-D-16-900, July
2016.

 ICE vehicle: an internal combustion engine (ICE) vehicle with
no powertrain electrification
 BEV: Battery Electric Vehicle
 PHEV: Plug-in Hybrid Electric Vehicle
 PEV: Plug-in Electric Vehicle (refers collectively to BEVs and
PHEVs)
 HEV: Hybrid Electric Vehicle (or strong hybrid) \379\
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    \379\ Strong hybrids typically operate at high voltage (greater
than 60 volts and most often up to several hundred volts) to provide
significant engine assist and regenerative braking, and most
commonly occur in what are known as P2 and power-split or other
parallel/series drive configurations. See also Draft Technical
Assessment Report, EPA-420-D-16-900, July 2016, pp. 5-11 and 5-12.
---------------------------------------------------------------------------

 MHEV: Mild Hybrid Electric Vehicle \380\
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    \380\ Mild hybrids most commonly operate at or about 48 volts
and provide idle-stop capability and launch assistance. See also
Draft Technical Assessment Report, EPA-420-D-16-900, July 2016, p.
5-11.
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 Hybrid: refers collectively to HEVs (or strong hybrid) and
MHEVs
 FCEV: Fuel Cell Electric Vehicle
 Electrified: any of the preceding vehicle types with an
electric drive, including FCEV
 ZEV: Zero-Emission Vehicle (used primarily in reference to the
California ZEV program)

    Because ZEV has a specific meaning under the California program,
EPA in this proposal is generally refraining from using the term except
in reference to the California program. Executive Order (E.O.) 14037
also uses the term ``zero-emission vehicle'' to refer generally to
BEVs, FCEVs, and PHEVs, so EPA may also use ``ZEV'' when referencing
the E.O.
    Additionally, in the context of the criteria pollutant program, the
abbreviation LDV refers to light-duty vehicles that are not otherwise
designated as a light-duty truck (LDT) or medium-duty passenger vehicle
(MDPV).\381\ In this proposal, the new nomenclature ``medium-duty
vehicle'' (MDV) refers to Class 2b and 3 vehicles, as described in the
following section.
---------------------------------------------------------------------------

    \381\ Title 40 CFR 86.1803.
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1. What vehicle categories and pollutants are covered by the proposal?
    EPA is proposing emissions standards for both light-duty vehicles
and medium-duty (Class 2b and 3) vehicles. The light-duty vehicle
category includes passenger cars, light trucks, and medium-duty
passenger vehicles (MDPVs), consistent with previous EPA GHG and
criteria pollutant rules.\382\ In this proposed rule, Class 2b and 3
vehicles are referred to as ``medium-duty vehicles'' (MDVs) to
distinguish them from Class 4 and higher vehicles that remain under the
heavy-duty program in 40 CFR parts 1036 and 1037. EPA has not
previously used the MDV nomenclature, referring to these larger
vehicles in prior rules as either heavy-duty Class 2b and 3 vehicles or
heavy-duty pickups and vans.\383\ The MDV category includes large
pickups, vans, and incomplete vehicles, but excludes MDPVs. Examples of
vehicles in this

category include GM or Stellantis 2500 and 3500 series, and Ford 250
and 350 series, pickups and vans. EPA notes that it is proposing that
certain Class 2b and 3 vehicles would be subject to engine-based
criteria pollutant emissions standards under EPA's heavy-duty engine
standards rather than being included in the MDV category, as discussed
in Section III.C.
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    \382\ Light-duty trucks (LDTs) that have gross vehicle weight
ratings above 6,000 pounds and all MDVs are considered ``heavy-duty
vehicles'' under the CAA. See section 202(b)(3)(C). For regulatory
purposes, we generally refer to those LDTs which are above 6,000
pounds GVWR and at or below 8,500 pounds GVWR as ``heavy light-duty
trucks'' made up of LDT3s and LDT4s, and we have defined MDPVs
primarily as vehicles between 8,501 and 10,000 pounds GVWR designed
primarily for the transportation of persons. See 40 CFR 86.1803-01.
    \383\ See 76 FR 57106 and 79 FR 23414. Heavy-duty vehicles
subject to standards under 40 CFR part 86, subpart S, are defined at
40 CFR 86.1803-01 to include all vehicles above 8,500 pounds GVWR,
and also incomplete vehicles with lower GVWR if they have curb
weight above 6,000 pounds or basic vehicle frontal area greater than
45 square feet.
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    EPA is proposing new standards for emissions of GHGs and
hydrocarbons, oxides of nitrogen (NOX), and particulate
matter (PM). EPA's proposed standards are based on an assessment of all
available and potential vehicle emissions control technologies,
including advancements in gasoline vehicle technologies, strong
hybridization, and zero-emission technologies over the model years
affected by the proposal.
2. Light-Duty and Medium-Duty Vehicle Standards: Background and History
    Previously, EPA has addressed medium-duty vehicle emissions as part
of regulatory programs for GHG emissions along with the heavy-duty
sector, and for criteria pollutant emissions along with the light-duty
sector. As a result, the program structure for medium-duty vehicles is
similar to that of the light-duty program for criteria pollutants but
differs from that of light-duty program for GHG emissions. This section
provides a brief overview of the rules and the standards structures for
EPA's light-duty GHG emissions standards, MDV GHG emissions standards,
and criteria pollutant emissions standards. While the current proposal
is addressing both light- and medium-duty vehicles under a single
umbrella rulemaking, EPA is proposing standards for each class and for
each pollutant pursuant to the relevant statutory provisions for each
class and pollutant based on its assessment of the feasibility of more
stringent standards for each class and pollutant, and the programs
would continue to follow the basic structures EPA has previously
adopted.
i. GHG Standards
    EPA has issued four rules establishing light-duty vehicle GHG
standards, which EPA refers to in this proposal based on the year in
which the previous final rule was issued, as shown in Table 20.\384\
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    \384\ The first three rules were issued jointly with NHTSA,
while EPA issued the 2021 Rule in coordination with NHTSA but not as
a joint rulemaking.

                           Table 20--Previous GHG Light-Duty Vehicles Standards Rules
----------------------------------------------------------------------------------------------------------------
           Rule                    MYs covered                 Title               Federal Register  citation
----------------------------------------------------------------------------------------------------------------
2010 Rule.................  Initial 2010 rule         Light-Duty Vehicle       75 FR 25324, May 7, 2010.
                             established standards     Greenhouse Gas
                             for MYs 2012-2016 and     Emission Standards and
                             later.                    Corporate Average Fuel
                                                       Economy Standards.
2012 Rule.................  Set more stringent        2017 and Later Model     77 FR 62624, October 15, 2012.
                             standards for MYs 2017-   Year Light-Duty
                             2025 and later.           Vehicle Greenhouse Gas
                                                       Emissions and
                                                       Corporate Average Fuel
                                                       Economy Standards.
2020 Rule.................  Revised the standards     The Safer Affordable     85 FR 24174, April 30, 2020.
                             for MYs 2022-2025 to      Fuel-Efficient (SAFE)
                             make them less            Vehicles Rule for
                             stringent and             Model Years 2021-2026
                             established a new         Passenger Cars and
                             standard for MYs 2026     Light Trucks.
                             and later.
2021 Rule.................  Revised the standards     Revised 2023 and Later   86 FR 74434, December 30, 2021.
                             for MYs 2023-2026 to      Model Year Light-Duty
                             make them more            Vehicle Greenhouse Gas
                             stringent, with the MY    Emissions Standards.
                             2026 standards being
                             the most stringent GHG
                             standards established
                             by EPA to date.
----------------------------------------------------------------------------------------------------------------

    The GHG standards have all been based on fleet average
CO2 emissions. Each vehicle model is assigned a
CO2 target based on the vehicle's ``footprint'' in square
feet (ft\2\), generally consisting of the area of the rectangle formed
by the four points at which the tires rest on the ground. Generally,
vehicles with larger footprints have higher assigned CO2
emissions targets. The most recent set of footprint curves established
by the 2021 rule for model years 2023-2026 are shown in Figure 4 and
Figure 5, along with the curves for MYs 2021-2022, included for
comparison. As shown, passenger cars and light trucks have separate
footprint standards curves, which result in separate fleet average
standards for the two sets of vehicles. The fleet-average standards are
the production-weighted fleet average of the footprint targets for all
the vehicles in a manufacturer's fleet for a given model year. As a
result, the footprint-based fleet average standards, which
manufacturers are required to meet on an annual basis, will vary for
each manufacturer based on its actual production of vehicles in a given
model year. Individual vehicles are not required to meet their
footprint-based CO2 targets, although they are required to
demonstrate compliance with applicable in-use standards.
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BILLING CODE 6560-50-C
    For medium-duty vehicles,\385\ EPA has established GHG standards
previously as part of our heavy-duty vehicle GHG Phase 1 and 2 rules,
shown in Table 21.
---------------------------------------------------------------------------

    \385\ Note, the HD GHG rules referred to MDVs as HD pickups and
vans.

                               Table 21--Prior Heavy-Duty GHG Rules Covering MDVs
----------------------------------------------------------------------------------------------------------------
            Rule                  MYs covered                Title                Federal Register  citation
----------------------------------------------------------------------------------------------------------------
HD Phase 1.................  Initial MDV           Greenhouse Gas Emissions   76 FR 57106, September 15, 2011.
                              standards phased in   Standards and Fuel
                              over MYs 2014-2018.   Efficiency Standards for
                                                    Medium- and Heavy-Duty
                                                    Engines and Vehicles.
HD Phase 2.................  More stringent MDV    Greenhouse Gas Emissions   81 FR 73478, October 25, 2016.
                              standards phased in   and Fuel Efficiency
                              over MYs 2021-2027.   Standards for Medium-
                                                    and Heavy-Duty Engines
                                                    and Vehicles--Phase 2.
----------------------------------------------------------------------------------------------------------------

    The MDV standards are also attribute-based. However, they are based
on a ``work factor'' attribute rather than the footprint attribute used
in the light-duty vehicle program. Work-based measures such as payload
and towing capability are two key factors that characterize differences
in the design of vehicles, as well as differences in how the vehicles
are expected to be regularly used. The work factor attribute combines
vehicle payload capacity and vehicle towing capacity, in pounds (lb),
with an additional fixed adjustment for four-wheel drive vehicles. This
adjustment accounts for the fact that four-wheel drive, critical to
enabling heavy-duty work (payload or trailer towing) in certain road
conditions, adds roughly 500 pounds to the vehicle weight. The work
factor is calculated as follows:

75 percent maximum payload + 25 percent of maximum towing + 375 lb if
four-wheel drive.

--Maximum payload is calculated as GVWR minus curb weight
--Maximum towing is calculated as Gross Combined Weight Rating (GCWR)
minus GVWR

    Under this approach, GHG targets are determined for each vehicle
with a unique work factor (analogous to a target for each discrete
vehicle footprint in the light-duty vehicle rules). These targets are
then production weighted and summed to derive a manufacturer's annual
fleet average standard for its MDVs. The current program includes
separate standards for gasoline and diesel-fueled vehicles.\386\ The
Phase 2 work factors are shown in Figure 6 and Figure 7.
---------------------------------------------------------------------------

    \386\ See 81 FR 73736-73739.
---------------------------------------------------------------------------

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BILLING CODE 6560-50-C
ii. Criteria and Toxic Pollutant Emissions Standards
    Over the last several decades, EPA has set progressively more
stringent vehicle emissions standards for criteria pollutants. Most
recently, in 2014, EPA adopted Tier 3 emissions standards. Unlike GHG
standards, criteria pollutant standards are not attribute-based. The
Tier 3 rule included standards for both light-duty and medium-duty
vehicles. Similar to the prior Tier 2 standards, Tier 3 established
``bins'' of Federal Test Procedure (FTP) standards, shown in Table 22.
Each bin contains a milligrams per mile (mg/mile) standard for non-
methane organic gases (NMOG) plus oxides of nitrogen (NOX)
or NMOG+NOX, particulate matter (PM), carbon monoxide (CO),
and formaldehyde (HCHO).

                                Table 22--Tier 3 FTP Standards for LDVs and MDPVs
                                                    [mg/mile]
----------------------------------------------------------------------------------------------------------------
                                                     NMOG+NOX           PM              CO             HCHO
----------------------------------------------------------------------------------------------------------------
Bin 160.........................................             160               3             4.2               4
Bin 125.........................................             125               3             2.1               4
Bin 70..........................................              70               3             1.7               4
Bin 50..........................................              50               3             1.7               4
Bin 30..........................................              30               3             1.0               4
Bin 20..........................................              20               3             1.0               4
Bin 0...........................................               0               0               0               0
----------------------------------------------------------------------------------------------------------------

    Manufacturers select, or assign, a standards bin to each vehicle
model and vehicles must meet all of the standards in that bin over the
vehicle's full useful life. Each manufacturer must also meet a fleet
average NMOG + NOX standard each model year, which declines
over a phase-in period for the Tier 3 final standards. The declining
NMOG+NOX standards are shown in Table 23. As shown, the
fleet is split between two categories: (1) Passenger cars and small
light trucks and (2) larger light trucks and MDPVs, with final
NMOG+NOX fleet average standards of 30 mg/mile for both
vehicle categories.\387\
---------------------------------------------------------------------------

    \387\ Small light trucks are those vehicles in the LDT1 class,
while larger light trucks are those in the LDT2-4 classes.

             Table 23--Tier 3 NMOG+NOX Fleet Average FTP Standards for Light-Duty Vehicles and MDPVs
                                                    [mg/mile]
----------------------------------------------------------------------------------------------------------------
                                                                   Model year
                              ----------------------------------------------------------------------------------
                                                                                2025 and
                                 2017     2018     2019     2020     2021     2022     2023     2024     later
----------------------------------------------------------------------------------------------------------------
Passenger cars and small            86       79       72       65       58       51       44       37         30
 trucks......................
Large light trucks and MDPVs.      101       93       83       74       65       56       47       38         30
----------------------------------------------------------------------------------------------------------------

    The Tier 3 rule also established more stringent criteria pollutant
emissions standards for MDVs. The Tier 3 MDV standards are also based
on a bin structure, but with generally less stringent bin standards and
with less stringent NMOG+NOX fleet average standards. As
discussed in Section III.A.1, the MDV category consists of vehicles
with gross vehicle weight ratings (GVWR) between 8,501-14,000 pounds.
For Tier 3, EPA set separate standards for two sub-categories of
vehicles, Class 2b (8,501-10,000 pounds GVWR) and Class 3 (10,001-
14,000 pounds GVWR) vehicles. Table 24 provides the final Tier 3 FTP
standards bins for MDVs and Table 25 provides the NMOG+NOX
fleet average standards that apply to these vehicles in MYs 2018 and
later. It is important to note that MDVs are tested at a higher test
weight than light-duty vehicles, as discussed in Section III.B.3, and
as such the numeric standards are not directly comparable across the
light-duty and MDV categories.

                                  Table 24--MDV Tier 3 FTP Final Standards Bins
----------------------------------------------------------------------------------------------------------------
                                                     NMOG+NOX           PM              CO             HCHO
----------------------------------------------------------------------------------------------------------------
                                        Class 2b (10,001-14,000 lb GVWR)
----------------------------------------------------------------------------------------------------------------
Bin 250.........................................             250               8             6.4               6
Bin 200.........................................             200               8             4.2               6
Bin 170.........................................             170               8             4.2               6
Bin 150.........................................             150               8             3.2               6
Bin 0...........................................               0               0               0               0
----------------------------------------------------------------------------------------------------------------
                                         Class 3 (8.501-10,000 lb GVWR)
----------------------------------------------------------------------------------------------------------------
Bin 400.........................................             400              10             7.3               6
Bin 270.........................................             270              10             4.2               6
Bin 230.........................................             230              10             4.2               6
Bin 200.........................................             200              10             3.7               6
Bin 0...........................................               0               0               0               0
----------------------------------------------------------------------------------------------------------------

                              Table 25--MDV Final Fleet Average NMOG+NOX Standards
                                                    [mg/mile]
----------------------------------------------------------------------------------------------------------------
                                      2018            2019            2020            2021        2022 and later
----------------------------------------------------------------------------------------------------------------
Class 2b.......................             278             253             228             203              178
Class 3........................             451             400             349             298              247
----------------------------------------------------------------------------------------------------------------

    EPA has also established supplemental Federal test procedure (SFTP)
standards for light and medium-duty vehicles, as well as cold
temperature standards for CO and HC. These standards address emissions
outside of the FTP test conditions such as at high vehicle speeds and
differing ambient temperatures. EPA is not reopening the current SFTP
standards in this rulemaking.
3. EPA's Statutory Authority Under the Clean Air Act (CAA)
    Title II of the Clean Air Act provides for comprehensive regulation
of mobile sources, authorizing EPA to regulate emissions of air
pollutants from all mobile source categories, including motor vehicles
under CAA section 202(a). EPA is setting standards under multiple
provisions of CAA section 202(a). GHG standards for all motor vehicles
and light duty criteria pollutant standards are set under section
202(a)(1)-(2). Criteria pollutant standards for larger light-duty
trucks and MDVs, which are considered ``heavy-duty vehicles'' under the
CAA by virtue of having GVWR above 6,000 pounds, are being set pursuant
to section 202(a)(3), which requires that standards applicable to
emissions of hydrocarbons, NOX, CO, and PM from heavy-duty
vehicles (which includes MDVs) reflect the greatest degree of emission
reduction available for the model year to which such standards apply,
giving appropriate consideration to cost, energy, and safety. In turn,
CAA section 216(2) defines ``motor vehicle'' as ``any self-propelled
vehicle designed for transporting persons or property on a street or
highway.'' Congress has intentionally and consistently used the broad
term ``any self-propelled vehicle'' since the Motor Vehicle Control Act
of 1965 so as not to limit standards adopted under CAA section 202 to
vehicles running on a particular fuel, power source, or system of
propulsion. Congress's focus was on emissions from classes of motor
vehicles and the ``requisite technologies'' that could feasibly reduce
those emissions giving appropriate consideration to cost of compliance
and lead time, as opposed

to being limited to any particular type of vehicle.
    Section 202(a)(1) of the CAA states that ``the Administrator shall
by regulation prescribe (and from time to time revise) . . . standards
applicable to the emission of any air pollutant from any class or
classes of new motor vehicles . . . which in his judgment cause, or
contribute to, air pollution which may reasonably be anticipated to
endanger public health or welfare.'' CAA section 202(a)(1) also
requires that any standards promulgated thereunder ``shall be
applicable to such vehicles and engines for their useful life (as
determined under [CAA section 202(d)], relating to useful life of
vehicles for purposes of certification), whether such vehicle and
engines are designed as complete systems or incorporate devices to
prevent or control such pollution.''
    While emission standards set by the EPA under CAA section 202(a)(1)
generally do not mandate use of particular technologies, they are
technology-based, as the levels chosen must be premised on a finding of
technological feasibility. Thus, standards promulgated under CAA
section 202(a) are to take effect only ``after such period as the
Administrator finds necessary to permit the development and application
of the requisite technology, giving appropriate consideration to the
cost of compliance within such period.'' CAA section 202(a)(2); see
also NRDC v. EPA, 655 F. 2d 318, 322 (D.C. Cir. 1981). EPA must
consider costs to those entities which are directly subject to the
standards. Motor & Equipment Mfrs. Ass'n Inc. v. EPA, 627 F. 2d 1095,
1118 (D.C. Cir. 1979). Thus, ``the [s]ection 202(a)(2) reference to
compliance costs encompasses only the cost to the motor-vehicle
industry to come into compliance with the new emission standards, and
does not mandate consideration of costs to other entities not directly
subject to the proposed standards.'' Coalition for Responsible
Regulation, 684 F.3d at 128. EPA is afforded considerable discretion
under section 202(a) when assessing issues of technical feasibility and
availability of lead time to implement new technology. Such
determinations are ``subject to the restraints of reasonableness,''
which ``does not open the door to `crystal ball' inquiry.'' NRDC, 655
F. 2d at 328, quoting International Harvester Co. v. Ruckelshaus, 478
F. 2d 615, 629 (D.C. Cir. 1973). However, ``EPA is not obliged to
provide detailed solutions to every engineering problem posed in the
perfection of [a particular device]. In the absence of theoretical
objections to the technology, the agency need only identify the major
steps necessary for development of the device and give plausible
reasons for its belief that the industry will be able to solve those
problems in the time remaining. EPA is not required to rebut all
speculation that unspecified factors may hinder `real world' emission
control.'' NRDC, 655 F. 2d at 333-34. In developing such technology-
based standards, EPA has the discretion to consider different standards
for appropriate groupings of vehicles (``class or classes of new motor
vehicles''), or a single standard for a larger grouping of motor
vehicles. NRDC, 655 F.2d at 338.\388\
---------------------------------------------------------------------------

    \388\ Additionally, with respect to regulation of vehicular
greenhouse gas emissions, EPA is not ``required to treat NHTSA's . .
. regulations as establishing the baseline for the [section 202(a)
standards].'' Coalition for Responsible Regulation, 684 F.3d at 127
(noting that the section 202(a) standards provide ``benefits above
and beyond those resulting from NHTSA's fuel-economy standards'').
---------------------------------------------------------------------------

    Although standards under CAA section 202(a)(1) are technology-
based, they are not based exclusively on technological capability.
Pursuant to the broad grant of authority in section 202, when setting
emission standards for light duty vehicles EPA may also consider other
factors and has done so previously when setting such standards. For
instance, in recent light duty greenhouse gas rules, EPA has also
considered such issues as: Technology effectiveness; its cost (per
vehicle, per manufacturer, and per consumer); the feasibility and
practicability of potential standards in light of the lead time
available to implement the technology; the impacts of potential
standards on emissions reductions of both GHGs and criteria pollutants;
the impacts of standards on oil conservation and energy security; the
impacts of standards on fuel savings by consumers; as well as other
relevant factors such as safety.
    In addition, EPA has clear authority to set standards under CAA
section 202(a)(1)-(2) that are technology-forcing when EPA considers
that to be appropriate but is not required to do so (as compared to
standards under section 202(a)(3), which require the greatest degree of
emissions reduction achievable, giving appropriate consideration to
cost, energy and safety factors). CAA section 202(a) does not specify
the degree of weight to apply to each factor, and EPA accordingly has
discretion in choosing an appropriate balance among factors. See Sierra
Club v. EPA, 325 F.3d 374, 378 (D.C. Cir. 2003) (even where a provision
is technology-forcing, the provision ``does not resolve how the
Administrator should weigh all [the statutory] factors in the process
of finding the `greatest emission reduction achievable' ''); National
Petrochemical and Refiners Ass'n v. EPA, 287 F.3d 1130, 1135 (D.C. Cir.
2002) (EPA decisions, under CAA provision authorizing technology-
forcing standards, based on complex scientific or technical analysis
are accorded particularly great deference); see also Husqvarna AB v.
EPA, 254 F. 3d 195, 200 (D.C. Cir. 2001) (great discretion to balance
statutory factors in considering level of technology-based standard,
and statutory requirement ``to [give appropriate] consideration to the
cost of applying . . . technology'' does not mandate a specific method
of cost analysis); Hercules Inc. v. EPA, 598 F. 2d 91, 106 (D.C. Cir.
1978) (``In reviewing a numerical standard we must ask whether the
agency's numbers are within a zone of reasonableness, not whether its
numbers are precisely right.'').\389\
---------------------------------------------------------------------------

    \389\ See also; Permian Basin Area Rate Cases, 390 U.S. 747, 797
(1968) (same); Federal Power Commission v. Conway Corp., 426 U.S.
271, 278 (1976) (same); Exxon Mobil Gas Marketing Co. v. Federal
Energy Regulatory Comm'n, 297 F. 3d 1071, 1084 (D.C. Cir. 2002)
(same).
---------------------------------------------------------------------------

    With regard to the specific technologies that could be used to meet
the emission standards promulgated under the relevant statutory
authorities, EPA's rules have historically not required the use of any
particular technology, but rather have allowed manufacturers to use any
technology that demonstrates the engines or vehicles meet the standards
over the applicable test procedures. Similarly, in determining the
standards, EPA appropriately considers updated data and analysis on
pollution control technologies, without a priori limiting its
consideration to a particular set of technologies. Given the continuous
development of pollution control technologies since the early days of
the CAA, this approach means that EPA routinely considers novel and
projected technologies developed or refined since the time of the CAA's
enactment, including, for instance, electric vehicle technologies. This
forward-looking regulatory approach keeps pace with real-world
technological developments and comports with Congressional intent.
    Section 202 does not specify or expect any particular type of motor
vehicle propulsion system to remain prevalent, and it was clear as
early as the 1960s that ICE vehicles might be inadequate to achieve the
country's air quality goals. In 1967, the Senate Committees on Commerce
and Public Works held five days of hearings on ``electric vehicles and
other alternatives to the internal

combustion engine,'' which Chairman Magnuson opened by saying ``The
electric will help alleviate air pollution. . . . The electric car does
not mean a new way of life, but rather it is a new technology to help
solve the new problems of our age.'' \390\ In a 1970 message to
Congress seeking a stronger CAA, President Nixon stated he was
initiating a program to develop ``an unconventionally powered,
virtually pollution free automobile'' because of the possibility that
``the sheer number of cars in densely populated areas will begin
outrunning the technological limits of our capacity to reduce pollution
from the internal combustion engine.'' \391\
---------------------------------------------------------------------------

    \390\ Electric Vehicles and Other Alternatives to the Internal
Combustion Engine: Joint Hearings before the Comm. on Commerce and
the Subcomm. on Air and Water Pollution of the Comm. on Pub. Works,
90th Cong. (1967).
    \391\ Richard Nixon, Special Message to the Congress on
Environmental Quality (Feb. 10, 1970), https://www.presidency.ucsb.edu/documents/special-message-the-congress-environmental-quality.
---------------------------------------------------------------------------

    Since the earliest days of the CAA, Congress has emphasized that
the goal of section 202 is to address air quality hazards from motor
vehicles, not to simply reduce emissions from internal combustion
engines to the extent feasible. In the Senate Report accompanying the
1970 CAA Amendments, Congress made clear the EPA ``is expected to press
for the development and application of improved technology rather than
be limited by that which exists'' and identified several unconventional
technologies that could successfully meet air quality-based emissions
targets for motor vehicles.\392\ In the 1970 amendments Congress
further demonstrated its recognition that developing new technology to
ensure that pollution control keeps pace with economic development is
not merely a matter of refining the ICE, but requires considering new
types of motor vehicle propulsion. Congress provided EPA with authority
to fund the development of ``low emission alternatives to the present
internal combustion engine'' as well as a program to encourage Federal
purchases of ``low-emission vehicles.'' See CAA section 104(a)(2)
(previously codified as CAA section 212). Congress also adopted section
202(e) expressly to grant the Administrator discretion regarding the
certification of vehicles and engines based on ``new power source[s] or
propulsion system[s],'' that is to say, power sources and propulsion
systems beyond the existing internal combustion engine and fuels
available at the time of the statute's enactment, if those vehicles
emit pollutants which the Administrator judges contribute to dangerous
air pollution but has not yet established standards for under section
202(a). As the D.C. Circuit held in 1973, ``We may also note that it is
the belief of many experts-both in and out of the automobile industry-
that air pollution cannot be effectively checked until the industry
finds a substitute for the conventional automotive power plant-the
reciprocating internal combustion (i.e., ``piston'') engine. . . . It
is clear from the legislative history that Congress expected the Clean
Air Amendments to force the industry to broaden the scope of its
research-to study new types of engines and new control systems.''
International Harvester Co. v. Ruckelshaus, 478 F.2d 615, 634-35 (D.C.
Cir. 1973).
---------------------------------------------------------------------------

    \392\ S. Rep. No. 91-1196, at 24-27 (1970).
---------------------------------------------------------------------------

    Since that time, Congress has continued to emphasize the importance
of technology development to achieving the goals of the CAA. In the
1990 amendments, Congress instituted a clean fuel vehicles program to
promote further progress in emissions reductions and the adoption of
new technologies and alternative fuels, which also applied to motor
vehicles as defined under section 216, see CAA section 241(1), and
explicitly defined motor vehicles qualifying under the program as
including vehicles running on an alternative fuel or ``power source
(including electricity),'' CAA section 241(2). Congress also directed
EPA to phase-in certain section 202(a) standards, see CAA section
202(g), which confirms EPA's authority to promulgate standards, such as
fleet averages, phase-ins, and averaging, banking, and trading
programs, that are fulfilled through compliance over an entire fleet,
or a portion thereof, rather than through compliance by individual
vehicles.\393\
---------------------------------------------------------------------------

    \393\ EPA has a long history of exercising its authority to
include compliance flexibilities in standards. As early as 1983,
manufacturers could comply with criteria-pollutant standards using
averaging. EPA introduced banking and trading in 1990. Fleet average
standards were adopted for light duty vehicles in 2000. All of these
flexibilities have likewise been part of EPA's GHG standards program
since the program's inception in 2010, and consistently since then.
Averaging, banking, and trading is discussed further in Section
III.B.4 and additional history is discussed in EPA's Answering Brief
in Texas v. EPA (D.C. Cir., 22-1031).
---------------------------------------------------------------------------

    The recently enacted Inflation Reduction Act \394\ ``reinforces the
longstanding authority and responsibility of [EPA] to regulate GHGs as
air pollutants under the Clean Air Act,'' \395\ and ``the IRA clearly
and deliberately instructs EPA to use'' this authority by ``combin[ing]
economic incentives to reduce climate pollution with regulatory drivers
to spur greater reductions under EPA's CAA authorities.'' \396\ The IRA
specifically affirms Congress's previously articulated statements that
non-ICE technologies will be a key component of achieving emissions
reductions from the mobile source sector, and Congress provided a
number of significant financial incentives for PEVs and the
infrastructure necessary to support them.\397\ The Congressional Record
reflects that ``Congress recognizes EPA's longstanding authority under
CAA section 202 to adopt standards that rely on zero emission
technologies, and Congress expects that future EPA regulations will
increasingly rely on and incentivize zero-emission vehicles as
appropriate.'' \398\
---------------------------------------------------------------------------

    \394\ See Inflation Reduction Act, Public Law 117-169, at
Sec. Sec.  13403, 13404, 13501, 13502, 60101, 136 Stat. 1818,
(2022), available at https://www.congress.gov/117/bills/hr5376/BILLS-117hr5376enr.pdf.
    \395\ 168 Cong. Rec. E868-02 (daily ed. Aug. 12, 2022)
(statement of Rep. Pallone).
    \396\ 168 Cong. Rec. E879-02, at 880 (daily ed. Aug. 26, 2022)
(statement of Rep. Pallone).
    \397\ See Inflation Reduction Act, Public Law 117-169, at
Sec. Sec.  13403, 13404, 13501, 13502, 60101, 136 Stat. 1818,
(2022), available at https://www.congress.gov/117/bills/hr5376/BILLS-117hr5376enr.pdf.
    \398\ 168 Cong. Rec. E879-02, at 880 (daily ed. Aug. 26, 2022)
(statement of Rep. Pallone).
---------------------------------------------------------------------------

    Consistent with Congress's intent, EPA's CAA Title II emission
standards have been based on and stimulated the development of a broad
set of advanced automotive technologies, such as on-board computers and
fuel injection systems, which have been the building blocks of
automotive designs and have yielded not only lower pollutant emissions,
but improved vehicle performance, reliability, and durability.
Beginning in 2010, EPA has set standards under section 202 for GHGs and
manufacturers have responded by continuing to develop and deploy a wide
range of technologies, including more fuel-efficient engine designs,
transmissions, aerodynamics, tires, materials improvements for mass
reduction, as well as various levels of electrified vehicle
technologies including mild hybrids, strong and plug-in hybrids,
battery electric vehicles, and fuel cell electric vehicles. In
addition, the continued application of performance-based standards with
fleet-wide averaging provides an opportunity for all technology
improvements and innovation to be reflected in a vehicle manufacturer's
compliance results.
i. Testing Authority
    Under section 203 of the CAA, sales of vehicles are prohibited
unless the

vehicle is covered by a certificate of conformity. EPA issues
certificates of conformity pursuant to section 206 of the CAA, based on
(necessarily) pre-sale testing conducted either by EPA or by the
manufacturer. The Federal Test Procedure (FTP or ``city'' test) and the
Highway Fuel Economy Test (HFET or ``highway'' test) are used for this
purpose. Compliance with standards is required not only at
certification but throughout a vehicle's useful life, so that testing
requirements may continue post-certification. To assure each vehicle
complies during its useful life, EPA may apply an adjustment factor to
account for vehicle emission control deterioration or variability in
use (section 206(a)).
    EPA establishes the test procedures under which compliance with the
CAA emissions standards is measured. EPA's testing authority under the
CAA is broad and flexible. EPA has also developed tests with additional
cycles (the so-called 5-cycle tests) which are used for purposes of
fuel economy labeling, SFTP standards, and extending off-cycle credits
under the light-duty vehicle GHG program.
ii. Compliance and Enforcement Authority
    EPA oversees testing, collects and processes test data, and
performs calculations to determine compliance with CAA standards. CAA