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NCl3 is formed as a disinfection byproduct in chlorinated swimming pools and can partition between the liquid and gas phases. Exposure to gas-phase NCl3 has been linked to asthma and can irritate the eyes and respiratory airways, thereby affecting the health and athletic performance of swimmers. This study involved an investigation of the spatiotemporal dynamics of gas-phase NCl3 in an aquatic center during a collegiate swim meet. Real-time (up to 1 Hz) measurements of gas-phase NCl3 were made via a novel on-line derivatization cavity ring-down spectrometer and a proton transfer reaction time-of-flight mass spectrometer. Significant temporal variations in gas-phase NCl3 and CO2 concentrations were observed across varying time scales, from seconds to hours. Gas-phase NCl3 concentrations increased with the number of active swimmers due to swimming-enhanced liquid-to-gas transfer of NCl3, with peak concentrations between 116 and 226 ppb. Strong correlations between concentrations of gas-phase NCl3 with concentrations of CO2 and water (relative humidity) were found and attributed to similar features in their physical transport processes in pool air. A vertical gradient in gas-phase NCl3 concentrations was periodically observed above the water surface, demonstrating that swimmers can be exposed to elevated levels of NCl3 beyond those measured in the bulk air.