Document ID: ./input/supremecourt_opinions/opinions/23pdf/23a349_0813.pdf
Page Number: 15.0

12 

OHIO v. EPA 

Opinion of the Court 

facts found and the choice made.”  Motor Vehicle Mfrs. Assn. 
of  United  States,  Inc.  v.  State  Farm  Mut.  Automobile  Ins. 
Co., 463 U. S. 29, 43 (1983) (internal quotation marks omit-
ted).  Accordingly, an agency cannot simply ignore “an im-
portant aspect of the problem.”  Ibid. 
  We agree with the applicants that EPA’s final FIP likely 
runs  afoul  of  these  long-settled  standards.    The  problem 
stems  from  the  way  EPA  chose  to  determine  which  emis-
sions “contribute[d] significantly” to downwind States’ dif-
ficulty  meeting  national  ozone  standards.    42  U. S. C. 
§7410(a)(2)(D)(i)(I).  Recall that EPA’s plan rested on an as-
sumption that all 23 upwind States would adopt emissions-
reduction tools up to a “uniform” level of “costs” to the point 
of diminishing returns.  87 Fed. Reg. 20076, 20095; 88 Fed. 
Reg. 36661, 36683–36684, 36719; see Part I–C, supra.  But 
as the applicants ask:  What happens—as in fact did hap-
pen—when  many  of  the  upwind  States  fall  out  of  the 
planned  FIP  and  it  may  now  cover  only  a  fraction  of  the 
States  and  emissions  EPA  anticipated?    See,  e.g.,  States’ 
Application 16–21; Application for American Forest & Pa-
per  Association  et al.  14–15,  19–20.    Does  that  affect  the 
“knee  in  the  curve,”  or  the  point  at  which  the  remaining 
States  might  still  “maximiz[e]  cost-effectiv[e]”  downwind 
ozone-level  improvements?    87  Fed.  Reg.  20055.    As  “the 
mix of states changes, . . . and their particular technologies 
and  industries  drop  out  with  them,”  might  the  point  at 
which  emissions-control  measures  maximize  cost-effective 
downwind air-quality improvements also shift?  Tr. of Oral 
Arg. 6. 
  Although commenters posed this concern to EPA during 
the notice and comment period, see Part I–C, supra, EPA 
offered no reasoned response.  Indeed, at argument the gov-
ernment acknowledged that it could not represent with cer-
tainty whether the cost-effectiveness analysis it performed 
collectively for 23 States would yield the same results and