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Page Number: 5

2 

OHIO v. EPA 

Opinion of the Court 

Here, the federal government announced its intention to re-
ject over 20 States’ plans for controlling ozone pollution.  In 
their place, the government sought to impose a single, uni-
form  federal  plan.    This  litigation    concerns  whether,  in 
adopting that plan, the federal government complied with 
the terms of the Act. 

I 
A 
  “The  Clean  Air  Act  regulates  air  quality  through  a  
federal-state collaboration.”  EME Homer City Generation, 
L.P. v. EPA, 795 F. 3d 118, 124 (CADC 2015).  Periodically, 
the Environmental Protection Agency (EPA) sets standards 
for common air pollutants, as necessary to “protect the pub-
lic  health.”    §§7409(a)(1),  (b)(1).    Once  EPA  sets  a  new 
standard, the clock starts ticking:  States have three years 
to  design  and  submit  a  plan—called  a  State  Implementa-
tion  Plan,  or  SIP—providing  for  the  “implementation, 
maintenance, and enforcement” of that standard in their ju-
risdictions.  §7410(a)(1); see EPA v. EME Homer City Gen-
eration,  L.  P.,  572  U. S.  489,  498  (2014).    Under  the  Act, 
States  decide  how  to  measure  ambient  air  quality.  
§7410(a)(2)(B).  States pick “emission limitations and other 
control measures.”  §7410(a)(2)(A).  And States provide for 
prescribed  measures.  
their 
the 
§7410(a)(2)(C). 
  At  the  same  time,  States  must  design  these  plans  with 
their  neighbors  in  mind.    Because  air  currents  can  carry 
pollution across state borders, emissions in upwind States 
sometimes affect air quality in downwind States.  See EME 
Homer, 572 U. S., at 496.  To address that externality prob-
lem, under the Act’s “Good Neighbor Provision,” state plans 
must prohibit emissions “in amounts which will . . . contrib-
ute  significantly  to  nonattainment  in,  or  interfere  with 
maintenance by, any other State” of the relevant air-quality 
standard.  §7410(a)(2)(D)(i)(I). 

enforcement 

of