We assess the US Clean Air Act standards for fine particulate matter (PM 2.5 ). Using high-resolution data, we find that the 2005 regulation reduced PM 2.5 levels by 0.4 μg/m 3 over five years, with larger effects in more polluted areas. Standard difference-in-differences overstates these effects by a factor of three because time trends differ by baseline pollution, a bias we overcome with three alternative approaches. We show that the regulation contributed to narrowing Urban-Rural and Black-White PM 2.5 exposure disparities, but less than difference-in-differences suggest. Pollution damages capitalized into house prices, however, appear larger than previously thought when leveraging regulatory variation. (JEL D63, K32, Q52, Q53, Q58, R31)