The United States Supreme Court has held that individual variances from a state's implementation plan which do not violate national ambient air standards are permissible as "revisions," rather than "postponements," under the Clean Air Act Amendments of 1970.
Train v. Natural Resources Defense Council, Inc., 421 U.S. 60 (1975).
John A. Fries,
Environmental Law - Clean Air Act Amendments - State Implementation Plans - Postponements - Revisions,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol14/iss1/15