This paper identifies the achievement impact of installing air filters in classrooms for the first time. To do so, I leverage a unique setting arising from the largest gas leak in United States history, whereby the offending gas company installed air filters in every classroom, office and common area for all schools within five miles of the leak (but not beyond). This variation allows me to compare student achievement in schools receiving air filters relative to those that did not using a spatial regression discontinuity design. I find substantial improvements in student achievement: air filter exposure led to a 0.20 standard deviation increase in mathematics and English scores, with test score improvements persisting into the following year. Air testing conducted inside schools during the leak (but before air filters were installed) showed no presence of natural gas pollutants, implying that the effectiveness of air filters came from removing common air pollutants and so these results should extend to other settings. The results indicate that air filter installation is a highly cost-effective policy to raise student achievement and, given that underprivileged students attend schools in highly polluted areas, one that can reduce the pervasive test score gaps that plague public education.