Ever wonder why swim centers ask users to shower before entering the pool? The standard argument is that dirt, including flecks of dead skin, can react with the water’s chlorine and limit how much of the chemical is available to kill germs. There’s another good reason for the showers: Dirt-chlorine reactions create potentially toxic chemical by-products. A British study now shows that the concentrations of these so-called disinfection by-products in pools generally increases with the number of swimmers and the grunge that they bring into the water.
Mark J. Nieuwenhuijsen of Imperial College of Science, Technology, and Medicine and Hilary Chu of the Royal School of Mines, both in London, sampled water at least three times in as many weeks at each of eight local pools. They measured total organic compounds–the grunge–and trihalomethanes (THMs), a group of volatile, potentially carcinogenic disinfection by-products that includes chloroform.
Concern about the uptake of these compounds by people isn’t hypothetical. Previous work by other researchers has shown that an hour’s swim can boost blood concentrations of chloroform 10-fold or more (SN: 1/7/95, p. 5). Moreover, some studies have linked a woman’s ingestion of THMs in drinking water with elevated risk of birth defects and miscarriages.
In the April Occupational and Environmental Medicine, the London researchers report that chloroform was by far the predominant THM in the pools. The concentrations of chloroform reported in the study varied broadly day to day but clustered around 110 micrograms per liter. These THM concentrations are higher than in earlier studies, most of which were of single pools.
Because the London researchers visited many pools, each on multiple occasions, they claim they could tease out what factors most influenced THM production. Nieuwenhuijsen says he was surprised to find that these include elevated water temperature, large numbers of swimmers, and high concentrations of organic compounds. This suggests, he says, that swimming in cool water after a good shower deters generation of toxic pollutants.