Translating CO ₂ decay into ventilation and occupancy guidance using a low-cost IAQ monitor: Particulate matter and air quality in a university weight room

Indoor Air Quality (IAQ) in gyms is a concern for both athletic performance and for reducing airborne transmission of viruses, particularly during peak usage hours. To reduce the build-up of breath exhalate and other bodily emissions, facilities typically force outside fresh air through the indoor space, but building operators often limit this air flow to minimize air conditioning costs. Small enterprises and non-profits often lack affordable ways to test fresh air ventilation. Grounded in first principles, this study presents a low-cost monitoring approach for evaluating fresh air adequacy by using the carbon dioxide (CO<sub>2</sub>) tracer gas decay method to estimate Air Change Rates (ACR), translate them into occupancy thresholds, and assess Particulate Matter (PM). Monitoring was performed in the American University of Beirut weight room over two weeks in July–August 2025. A low-cost sensor system was designed and built to measure CO<sub>2</sub> and particulate matter (PM₁.₀, PM₂.₅, PM₁₀). ACR was estimated by fitting nighttime CO<sub>2</sub> decay after gym closure to a mass-balance model. Compliance with the standards of the American Society for Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) was assessed, and CO<sub>2</sub>–PM associations were tested with Spearman’s correlation. Across 10 valid nights, ACR ranged from 1.00–1.94 h⁻¹ (mean 1.51 ± 0.37 h⁻¹), a value that would accommodate ~17 occupants according to ASHRAE 62.1. The weight room frequently hosts more than double that number during the evening hours of the Fall and Spring semesters, indicating insufficient fresh air flow for the number of users. The mean indoor CO<sub>2</sub> during open hours was ~792 ppm and tracked gym occupancy patterns. PM concentrations did not correlate with occupancy, and peaked during the hours the gym was closed, indicating a different mechanism for PM build-up. This work demonstrates that public facilities can assess ventilation with inexpensive monitoring, offering a practical alternative to costly HVAC engineering services. The approach enables resource-constrained operators to identify ventilation gaps and pursue evidence-based improvements to IAQ without disrupting normal operations.