Document ID: ./input/supremecourt_opinions/opinions/21pdf/20-1530_n758.pdf
Page Number: 27.0

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

21 

Opinion of the Court 

operate more cleanly.  See, e.g., 41 Fed. Reg. 48706 (requir-
ing “degree of control achievable through the application of
fiber mist eliminators”); see also supra, at 6.  It had never 
devised  a  cap  by  looking  to  a  “system”  that  would  reduce
pollution simply by “shifting” polluting activity “from dirt-
ier to cleaner sources.”  80 Fed. Reg. 64726; see id., at 64738 
(“[O]ur traditional interpretation . . . has allowed regulated 
entities to produce as much of a particular good as they de-
sire provided that they do so through an appropriately clean 
(or  low-emitting)  process.”).    And  as  Justice  Frankfurter 
has noted, “just as established practice may shed light on 
the  extent  of  power  conveyed  by  general  statutory  lan-
guage, so the want of assertion of power by those who pre-
sumably would be alert to exercise it, is equally significant 
in  determining  whether  such  power  was  actually  con-
ferred.”  FTC  v.  Bunte  Brothers,  Inc.,  312  U. S.  349,  352 
(1941).

The Government quibbles with this description of the his-
tory of Section 111(d), pointing to one rule that it says relied 
upon a cap-and-trade mechanism to reduce emissions.  See 
70 Fed. Reg. 28616 (2005) (Mercury Rule).  The legality of 
that choice was controversial at the time and was never ad-
dressed by a court.  See New Jersey v. EPA, 517 F. 3d 574 
(CADC 2008) (vacating on other grounds).  Even assuming
the Rule was valid, though, it still does not help the Gov-
ernment.  In that regulation, EPA set the actual “emission 
cap”—i.e., the limit on emissions that sources would be re-
quired to meet—“based on the level of [mercury] emissions 
reductions that w[ould] be achievable by” the use of “tech-
nologies [that could be] installed and operational on a na-
tionwide  basis”  in  the  relevant  timeframe—namely,  wet
scrubbers.  70 Fed. Reg. 28620–28621.  In other words, EPA 
set the cap based on the application of particular controls, 
and  regulated  sources  could  have  complied  by  installing 
them.  By contrast, and by design, there is no control a coal