Determinants of chlorination by-products in indoor swimming pools

Water and air quality of fifteen swimming pools using chlorine disinfection was measured during eight sampling campaigns in Rennes, France. Concentrations of trihalomethanes (THMs) and trichloramine (NCl3) in air as well as concentrations of parameters, including total organic carbon (TOC), chlorine (free, combined and total), pH, Kjeldhal nitrogen (KN), chloride ions and THMs in water and air temperature, were measured. Water and air samples were collected during 3 h in the morning and afternoon, during high and low attendance days, in summer and winter. Data analysis was perfomed by multivariate ordinary least square (OLS) and quantile (QR) regressions, from both data quality and other parameters such as the ratio between the number of visitors and the volume of water available V/W, the proportion of air re-circulated and the number of swimmers during sampling. The geometric means (GM ± 95% CIs) of the sum of THMs (TTHM) and NCl3 in air were 74.9 ± 6.7 μg/m3 and 190 ± 10 μg/m3, respectively. As expected, concentrations of TTHM in air were correlated to concentrations of TTHM in water. Concentrations of TTHM and NCl3 in air measured in winter were significantly higher than in summer, depicting that low air exchange rate in winter lead to accumulation of pollutants. NCl3 (especially at higher quantiles) and TTHM concentrations in air were associated to the number of swimmers. NCl3 was also linked to air temperature and pH. Moreover TTHM and NCl3 in air seemed to be affected by additional determinants including the volume of hall and the ratio V/W. In a process of reducing exposure, the OLS regression provides useful information on key parameters influencing concentrations of THMs and NCl3 in indoor swimming pools. In addition, QR allows identifying factors associated to elevated levels of contamination.