2016 No https://doi.org/10.1007/s11356-016-6555-y Rufo, João Cavaleiro Europe Observational Exposure Study Students Elementary (K-5) Particulate Matter (PM) Other Ultrafine particles Primary schools Indoor air Children exposure Traffic-related UFP Rural environment Ultrafine particles (UFPs) were measured in 8 Portuguese primary schools. Urban classrooms had nearly double the UFP counts of rural ones. Source factors included traffic proximity and materials. Results support building design to reduce UFP exposure in children.