History of PM Standards

1971- EPA issued the first National Ambient Air Quality Standards for Total Suspended Particles (TSP).
1987- EPA revised the standards and replaced TSP with PM₁₀ (targeting particles smaller than 10 micrometers).
1997- EPA revised the standards and included PM_2.5 (targeting particles smaller than 2.5 micrometers).
1997- Several industry and other organizations, some state governments challenged EPA’s standards in the U.S. Court of Appeals for the D.C. Circuit.
2001- The U.S. Supreme Court upheld EPA’s authority under the Clean Air Act to set standards, clarified that EPA cannot consider cost in setting standards. Remanded several issues to the appellate court.
2002- The DC Circuit Court rejected all remaining legal challenges to EPA’s 1997 standards for PM_2.5.
2004- EPA designated 39 areas as not meeting the standards for PM_2.5.