We examine whether counties designated as out of attainment with the NAAQS under the 1970 CAA experienced larger reductions in TSP during the 1970s than attainment counties. We answer this question using the official designation of nonattainment status which, between 1972 and 1978, was by Air Quality Control Region (AQCR). Data from balanced panels of TSP monitors in operation from 1969-78 and in operation from 1971-78 are used to examine the impact of nonattainment status on TSP. We also examine the impact of nonattainment on TSP using the definition in the literature, which designates a county as out of attainment if any of its monitors violated the NAAQS. Using the official designation of nonattainment, TSP, on average, fell by over 9 μg/m3 more in non-attainment than in attainment counties, controlling for county and year fixed effects, county population, employment, and per capita income. The average treatment effect is 10.2 μg/m3 using the 1969 panel and 9.1 μg/m3 using the 1971 panel. Using the definition of nonattainment in the literature yields similar, albeit smaller, average treatment effects 6.0 μg/m3 using the 1969-78 panel and 7.7 μg/m3 using the 1971-78 panel.