Sec. 161. In accordance  with the policy of section  101(b)(1),
each applicable implementation plan shall contain emission
limitations  and  such other  measures  as may  be  necessary, as
determined  under regulations  promulgated  under this  part,  to
prevent significant  deterioration of air quality  in each region
(or  portion  thereof)  designated  pursuant to  section  107  as
attainment or unclassifiable.
[42 U.S.C. 7471]