Document ID: ./input/supremecourt_opinions/opinions/21pdf/20-1530_n758.pdf
Page Number: 11

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

5 

Opinion of the Court 

environmental  impact  and  energy  requirements)  the 
[EPA] Administrator determines has been adequately 
demonstrated.”  §7411(a)(1). 

Thus, the statute directs EPA to (1) “determine[ ],” taking
into  account  various  factors,  the  “best  system  of  emission
reduction  which  . . .  has  been  adequately  demonstrated,” 
(2) ascertain the “degree of emission limitation achievable
through the application” of that system, and (3) impose an
emissions  limit  on  new  stationary  sources  that  “reflects” 
that  amount.  Ibid.;  see  also  80  Fed.  Reg.  64538  (2015). 
Generally  speaking,  a  source  may  achieve  that  emissions 
cap any way it chooses; the key is that its pollution be no
more than the amount “achievable through the application
of  the  best  system  of  emission  reduction  . . .  adequately
demonstrated,” or the BSER.  §7411(a)(1); see §7411(b)(5).
EPA  undertakes  this  analysis  on  a  pollutant-by-pollutant 
basis, establishing different standards of performance with
respect to different pollutants emitted from the same source 
category.  See, e.g., 73 Fed. Reg. 35838 (2008); 42 Fed. Reg. 
22510 (1977).

Although the thrust of Section 111 focuses on emissions
limits for new and modified sources—as its title indicates— 
the statute also authorizes regulation of certain pollutants
from existing sources.  Under Section 111(d), once EPA “has 
set new source standards addressing emissions of a partic-
ular pollutant under . . . section 111(b),” 80 Fed. Reg. 64711, 
it  must  then  address  emissions of  that  same  pollutant  by 
existing sources—but only if they are not already regulated
under the NAAQS or HAP programs.  §7411(d)(1).  Existing
power plants, for example, emit many pollutants covered by 
a NAAQS or HAP standard.  Section 111(d) thus “operates 
as a gap-filler,” empowering EPA to regulate harmful emis-
sions  not  already  controlled  under  the  Agency’s  other  au-
thorities.  American Lung Assn. v. EPA, 985 F. 3d 914, 932 
(CADC 2021).