Do Smog Checks Affect Smog? Emissions Inspections, Station Quality and Local Air Pollution

Abstract

Personal automobile emissions are a major source of urban air pollution. Many U.S. states control emissions through mandated vehicle inspections and repairs. But there is little empirical evidence directly linking mandated inspections, maintenance, and local air pollution levels. Using individual-level data from 1998-2012 from California's inspection program, we estimate the contemporaneous effect of inspections on local air quality by exploiting day-to-day, within-county variation in the number of vehicles nominally repaired and recertified after failing an initial inspection. Additional re-inspections of pre-1985 model year vehicles reduce local carbon monoxide, nitrogen oxide, and particulate matter levels, while re-inspections of newer vehicles with more modern engine technology have no economically significant effect on air pollution. This suggests emissions inspections become less effective at reducing local air pollution as more high-polluting vehicles from the 1970s and 1980s leave the road. We also estimate the importance of station quality, using a metric devised for California's new STAR certification program. We show re-inspections of older vehicles conducted by low quality inspection stations do not change air pollution, while inspections at high quality stations have a moderate effect on pollution concentrations. We find little effect on ambient ozone levels, regardless of station quality or vehicle age.