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Characterization of water quality from indoor swimming pools, using chorine-based disinfection techniques, was performed during a 6-mo period to study the occurrence, distribution, and concentration factors of trihalomethanes (THM). Several parameters such as levels of water THM, water and air chloroform, water bromodichloromethane (BDCM), water dibromochloromethane (DBCM), water bromoform (BF), free residual chlorine (FrCl), pH, water and air temperature, and permanganate water oxidizability (PWO) were determined in each pool during that period. Chloroform (CFW) was the THM detected at higher concentrations in all pools, followed by BDCM, DBCM, and BF detected at 99, 34, and 6% of the samples, respectively. Water THM concentrations ranged from 10.1 to 155 μg/L, with 6.5% of the samples presenting values above 100 μg/L (parametric value established in Portuguese law DL 306/2007). In this study, air chloroform (CFAir) concentrations ranged from 45 to 373 μg/m3 with 24% of the samples presenting values above 136 μg/m3 (considered high exposure value). Several significant correlations were observed between total THM and other parameters, namely, CFW, CFAir, FrCl, water temperature (TW), and PWO. These correlations indicate that FrCl, TW and PWO are parameters that influence THM formation. The exposure criterion established for water THM enabled the inclusion of 67% of Lisbon pools in the high exposure group, which reinforces the need for an improvement in pool water quality. [ABSTRACT FROM PUBLISHER]