We examine the effects of the 1990 Clean Air Act Amendments (CAAAs) on ambient concentrations of PM10 in the United States between 1990 and 2005. We find that non-attainment designation has no effect on the ‘average monitor’ in non-attainment counties, after controlling for weather and socioeconomic characteristics at the county level. In sharp contrast, if we allow for heterogeneous treatment by type of monitor and county, we do find that the 1990 CAAAs produced substantial effects. Our best estimate suggests that PM10 concentrations at monitors with concentrations above the national annual standard dropped by between 7?g/m3 and 9?g/m3, which is roughly equivalent to a 11-14% drop. We also show that monitors which were in violation of the daily standard experience two fewer days in violation of the daily standard the following year. Empirical results suggest that this treatment effect is independent of whether the EPA has finalized the non-attainment designation.