Document ID: ./input/supremecourt_opinions/opinions/21pdf/20-1530_n758.pdf
Page Number: 9

Cite as:  597 U. S. ____ (2022) 

3 

Opinion of the Court 

reasonably be anticipated to endanger public health or wel-
fare,” and “the presence of which in the ambient air results
from  numerous  or  diverse  mobile  or  stationary  sources.”
§7408(a)(1).  After identifying such pollutants, EPA estab-
lishes a NAAQS for each.  The NAAQS represents “the max-
imum  airborne  concentration  of  [the]  pollutant  that  the
public health can tolerate.”  Whitman v. American Trucking 
Assns., Inc., 531 U. S. 457, 465 (2001); see §7409(b).  EPA, 
though,  does  not  choose  which  sources  must  reduce  their
pollution and by how much to meet the ambient pollution 
target.  Instead, Section 110 of the Act leaves that task in 
the first instance to the States, requiring each “to submit to
[EPA]  a  plan  designed  to  implement  and  maintain  such 
standards  within  its  boundaries.”  Train  v.  Natural  Re-
sources Defense Council, Inc., 421 U. S. 60, 65 (1975); §7410.
The second major program governing stationary sources
is the HAP program.  The HAP program primarily targets
pollutants, other than those already covered by a NAAQS,
that present “a threat of adverse human health effects,” in-
cluding  substances  known  or  anticipated  to  be  “carcino-
genic,  mutagenic,  teratogenic,  neurotoxic,”  or  otherwise 
“acutely or chronically toxic.”  §7412(b)(2).

EPA’s  regulatory  role  with  respect  to  these  toxic  pollu-
tants is different in kind from its role in administering the 
NAAQS program.  There, EPA is generally limited to deter-
mining the maximum safe amount of covered pollutants in
the  air.  As  to  each  hazardous  pollutant,  by  contrast,  the 
Agency must promulgate emissions standards for both new 
and existing major sources.  §7412(d)(1).  Those standards 
must  “require  the  maximum  degree  of  reduction  in  emis-
sions . . . that the [EPA] Administrator, taking into consid-
eration the cost of achieving such emission reduction, and 
any non-air quality health and environmental impacts and 
energy requirements, determines is achievable . . . through
application  of  measures,  processes,  methods,  systems  or 
techniques”  of  emission  reduction.    §7412(d)(2).  In  other