<p>Indoor air quality (IAQ) is a critical public health concern, especially in sports facilities where high occupancy and physical activity increase exposure risks. This study assessed IAQ in a Chicago sports center using low-cost Air Quality Egg sensors, deployed in the basketball courts, rock-climbing area, and weight room from 14 March to 2 April 2024, followed by a colocation test to ensure sensor consistency. Results showed PM_2.5 concentrations across all locations remained below the World Health Organization’s 24-h guideline of 15&#xa0;µg&#xa0;m⁻³ but demonstrated distinct diurnal patterns associated with occupancy levels. Indoor PM_2.5 was positively associated with CO₂ and relative humidity (RH), while temperature effects varied by location. Multiple regression models using CO₂, RH, and temperature explained 34% of PM_2.5 variability, with CO₂ being the strongest predictor. Infiltration analysis showed weak correlations (<i>R</i>² = 0.04) between indoor and outdoor PM_2.5, indicating that indoor sources were dominant and the building was relatively well sealed. The estimated infiltration coefficients ranged from 0.07 to 0.12, and indoor-generated PM_2.5 levels (1.11–1.66&#xa0;µg&#xa0;m⁻³) were relatively lower than those reported in similar buildings. Among the three locations, the weight room recorded the highest levels of PM_2.5, CO₂, and RH during both peak and evening hours, despite having the highest air exchange rate based on CO₂ decay analysis. The findings suggest that enhanced ventilation may still be needed to improve IAQ in the weight room. This study demonstrates the utility of low-cost sensors in capturing spatial and temporal IAQ variability. The results emphasize the importance of aligning ventilation strategies with space usage in recreational environments.</p> Graphical Abstract <p></p>