Nearly the entire world's population breathes air exceeding WHO pollution guidelines, but the extent to which that exposure impairs the accumulation of human capital is not well understood. We study this using longitudinal PSAT data on nearly 10 million U.S. high school students, comparing the same student's scores across attempts preceded by differing air quality and instrumenting for local PM2.5 with smoke from distant wildfires. A year of observed pollution exposure reduces learning by 0.04-0.06 standard deviations, or 14-19% of typical annual score growth. The damage comes almost entirely from moderate pollution days (8–12 μg/m3), below the EPA's historical standard, and from exposure during the school year rather than summer, pointing to instructional disruption as a mechanism. Effects are three times larger in disadvantaged schools and among Black and Hispanic students, who are harmed more by the same exposure. Exposure to air pollution widens achievement gaps.