2018 No https://doi.org/10.1016/j.buildenv.2018.03.040 Johnson, David L. North America Observational Exposure Study Modeling Students Elementary (K-5) Temperature Humidity Carbon Dioxide (CO2) Carbon Monoxide (CO) Particulate Matter (PM) Volatile Organic Compounds (VOCs) Nitrogen Dioxide (NO2) Other Ventilation Indoor air quality Transient mass balance IAQ in 12 Oklahoma elementary schools was assessed using seasonal 24-h sampling of PM, CO₂, CO, NO₂, VOCs, formaldehyde, temp, and humidity. A transient mass balance model estimated effective fresh air ventilation. Many rooms lacked sufficient ventilation, suggesting seasonal HVAC variation needs policy attention.