We randomize the installation of air purifiers across primary school classrooms to reduce children’s exposure to air pollution. The intervention reduces indoor P M2.5 concentrations by 32% and decreases student absenteeism by 12.5%. Effects are larger among students with higher pre-treatment absenteeism. The impact is greater when outdoor air pollution is relatively low and diminishes as outdoor pollution intensifies, consistent with non-linear marginal effects of air quality on health. The treatment students report fewer respiratory symptoms and exhibit greater awareness of air quality. The cost per absence day avoided is approximately AC11, resulting in a conservative cost-benefit ratio of one-to-nine.