2020 Yes https://doi.org/10.1016/j.envres.2020.110291 Tong, Zheming North America Exposure Study Students Elementary (K-5) Carbon Dioxide (CO2) Particulate Matter (PM) PM2.5 and CO₂ were monitored in two NYC schools to assess filtration impact. CO₂ increased with reduced outdoor air; PM2.5 levels dropped significantly when filters were used. Findings support improved HVAC filtration to reduce indoor pollutant exposure.