2019 No https://doi.org/10.1016/j.envres.2018.12.012 Majd, Ehsan North America Observational Exposure Study Students Elementary (K-5) Middle/Jr High (6-8) Carbon Monoxide (CO) Particulate Matter (PM) Nitrogen Dioxide (NO2) Other Indoor air quality Exposure Building characteristics Environmental factors PM2.5, NO2, and CO were measured in 16 urban U.S. schools across seasons. Indoor concentrations often exceeded guidelines. Building defects and nearby pollution sources were linked to poor IAQ. Results support school facility improvements.