Once again, the bathing season is almost upon us. To ensure that swimmers (and non‑swimmers) have a safe and enjoyable experience, public pools have to meet strict water-quality standards. As contaminants such as hair, skin flakes, sweat and urine – but also bacteria and viruses – are inevitably introduced by pool users, the water in public pools is continuously filtered and disinfected. The most commonly used disinfectant is chlorine, usually applied in the form of hypochlorous acid or calcium hypochlorite. But, as environmental chemist Fabian Soltermann of Eawag’s Water Resources and Drinking Water department points out, chlorination also has a downside: “Several hundred compounds are formed as by-products.” In many cases, the effects of these substances are unknown, but some can be harmful to health or even (at critical concentrations) carcinogenic.
Increased levels more likely in indoor pools
Among the disinfection by-products of concern is trichloramine, which can irritate the skin, eyes and respiratory tract and has been implicated in the development of asthma. Pool staff, swimming instructors and young children are believed to be particularly at risk. Trichloramine is formed when chlorine in pool water reacts with nitrogenous compounds such as urea. A certain amount is released as a gas, which can accumulate in enclosed spaces. Soltermann explains: “The trichloramine issue mainly arises at indoor pools. At outdoor pools, there’s plenty of fresh air, and the compound is also broken down by sunlight.” A legal limit has yet to be specified in Switzerland. In practice, however, a standard issued by the Swiss Society of Engineers and Architects – SIA 385/9 – is widely employed. The SIA recommends an upper limit of 0.2 milligrams of trichloramine per cubic metre of air. According to a study published in 2012, this level was exceeded at 5 of 30 indoor pools investigated in Switzerland.
If exposure to trichloramine is to be minimized, it is important to understand how the compound is formed in water. For his doctoral dissertation, Fabian Soltermann developed a low-cost, readily applicable method for measuring trichloramine in pool water. Using this method, he was able to identify factors that influence the production and degradation of the substance. His study involved a total of 30 different pools at a number of indoor swimming facilities in Switzerland. At one facility, long-term measurements were performed in various pools [Fig. 2].