Clean Identification? The Effects of the Clean Air Act on Air Pollution, Exposure Disparities, and House Prices†

We assess the US Clean Air Act standards for fine particulate matter (PM2.5). Using high-resolution data, we find that the 2005 regulation reduced PM2.5 levels by 0.4 mu g/m3 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 PM2.5 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)