Objective To investigate the spatiotemporal distribution and influencing factors of trihalomethanes (THMs) in indoor swimming venues in Shenzhen, China.

Methods According to the requirements of the Examination Methods for Public Places—Part 6: Technical Specifications of Health Monitoring (GB/T 18204.6-2013), the swimming pool water and air of six representative indoor swimming venues in Shenzhen were monitored for four consecutive days for THMs (including trichloromethane TCM, bromodichloromethane DCBM, dibromochloromethane DBCM, and bromoform TBM) during the peak swimming period in summer (July to August 2023) and winter (January to February 2024). Samples were collected from the water in the deep water area and shallow water area, as well as air in the deep water area, shallow water area and rest area of the swimming pool once each day. The THMs concentrations in swimming pool water and air of indoor swimming venues in different seasons were compared using Mann-Whitney U test, and the correlation of THMs in swimming pool water with environmental factors (residual chlorine, temperature, and pH) and THMs in air were analyzed using the Spearman rank correlation method.

Results A total of 96 pool water samples (48 in winter and 48 in summer) and 142 air samples (71 in winter and 71 in summer) were collected from indoor swimming venues in Shenzhen. The detection rate of THMs was 100.00%, with a qualified rate of THMs in swimming pool water of 95.83%. Among the four THMs, the detection rate of TCM was the highest, with detection rates of TCM in pool water and air samples of 100.00% and 99.30%, respectively. Among the four THMs, TCM had the highest concentration, with median concentrations of 48.7 μg/L and 0.134 μg/L in pool water and air samples, respectively. The concentrations of THMs in both pool water and air samples were higher in winter than in summer (Z=-3.36 and -9.96, respectively, P < 0.05). The concentration of TBM in swimming pool water was positively correlated with free residual chlorine (r_s=0.250, P < 0.05), and the concentrations of DCBM and DBCM were positively correlated with the pH of swimming pool water (r_s=0.375 and 0.242, respectively, P < 0.05). The concentrations of TCM and DBCM in swimming pool water were positively correlated with the concentrations of TCM and DBCM in the air of indoor swimming venues (r_s=0.463 and 0.373, respectively, P < 0.05).

Conclusion TCM is the main by-product of chlorination disinfection in pool water and air samples in indoor swimming venues in Shenzhen. Health education and supervisory testing of swimming venues should be strengthened to reduce the production of chlorinated disinfection by-products such as THMs and protect the health of swimming-related populations.