6 3 the ▁United ▁States ' s ▁compreh ensive ▁law ▁reg ulating ▁air ▁poll ution . ▁The ▁bill ▁changed ▁the ▁law ▁so ▁that ▁ ▁all ▁companies ▁in ▁the ▁United ▁States ▁that ▁planned ▁to ▁build ▁or ▁install ▁any ▁major ▁source ▁of ▁air ▁poll ut ants ▁were ▁required ▁to ▁go ▁through ▁an ▁elaborate ▁" new - source ▁review " ▁process ▁before ▁they ▁could ▁proceed . ▁The ▁bill ▁did ▁not ▁precisely ▁define ▁what ▁constit uted ▁a ▁" source " ▁of ▁air ▁poll ut ants , ▁and ▁so ▁the ▁Environment al ▁Prote ction ▁Agency ▁( E PA ) ▁form ulated ▁a ▁definition ▁as ▁part ▁of ▁implementing ▁the ▁changes ▁to ▁the ▁law . ▁The ▁E PA ' s ▁initial ▁definition ▁of ▁a ▁" source " ▁of ▁air ▁poll ut ants ▁covered ▁essentially ▁any ▁significant ▁change ▁or ▁addition ▁to ▁a ▁plant ▁or ▁factory , ▁but ▁in ▁ 1 9 8 1 ▁it ▁changed ▁its ▁definition ▁to ▁be ▁simply ▁a ▁plant ▁or ▁factory ▁in ▁its ▁entire ty . ▁This ▁allowed ▁companies ▁to ▁avoid ▁the ▁" new - source ▁review " ▁process ▁entirely ▁if , ▁when ▁increasing ▁their ▁plant ' s ▁em issions ▁through ▁building ▁or ▁modifying , ▁they ▁simultaneously ▁modified ▁other ▁parts ▁of ▁their ▁plant ▁to ▁reduce ▁em issions ▁so ▁that ▁the ▁overall ▁change ▁in ▁the ▁plant ' s ▁em issions ▁was ▁zero . ▁The ▁Natural ▁Resources ▁Def ense ▁Council , ▁an ▁American ▁non - pro fit ▁environmental ▁advoc acy ▁organization , ▁then ▁file d ▁a ▁law suit ▁challeng ing ▁the ▁leg ality ▁of ▁the ▁E PA ' s ▁new