How safe are swimming pools?

Breathing large doses of chlorine-based disinfectants is known to cause lung damage, but what about smaller levels, such as the 0.02-0.2 parts per million in the air surrounding indoor pools? According to a report in the June 2003 issue of Occupational and Environmental Medicine, cumulative exposure to such levels increases the prevalence of asthma, particularly for young children. The likely culprit is nitrogen trichloride (trichloramine), a highly reactive by-product created when chlorine reacts with organic matter such as urine and sweat.

In the three-part study, researchers led by Alfred Bernard, a professor of toxicology at the Catholic University of Louvain in Brussels, took blood samples from 235 Belgian children who attended schools requiring weekly or biweekly swimming. The researchers measured lung-specific proteins--including surfactant-associated protein A (SP-A) and surfactant-associated protein B (SP-B)--whose presence in the blood is associated with cellular damage.

Cumulative time spent at indoor pools was the independent variable most consistently associated with higher concentrations of SP-A and SP-B. These proteins are produced primarily deep in the lung epithelium, and their rising levels in the blood indicate that epithelial membranes have been damaged. Children with the highest pool attendance over time showed a 50% increase in serum SP-A and SP-B, increases similar to those found in regular smokers and reported by Bernard and colleagues in the November 2002 European Respiratory Journal.

In an epidemiological portion of the study, the researchers analyzed data on health status, lifestyle, and pool attendance gathered from 1,881 primary school children. Then they screened the children for asthma and found that total asthma prevalence was significantly correlated with cumulative pool attendance. The strongest association was seen in children in kindergarten and first grade, who showed a correlation of 64% even after adjusting for other asthma risk factors such as exposure to pets and environmental tobacco smoke.

Finally, the researchers measured changes in blood levels of SP-A and SP-B in 16 children and 13 adults after the subjects spent two hours in and around an indoor pool. Strikingly, after just one hour, the adults' levels rose significantly, and the children's rose significantly after two hours. Commenting on the paper, Kaye H. Kilburn, director of the Environmental Sciences Laboratory at the University of Southern California, says, "People who complain of problems after exposure to chlorine are not being overly sensitive. Damage is being done."

To prevent this damage, Bernard suggests several solutions, such as better ventilation for indoor pool facilities and requiring swimmers to shower before entering the pool, to reduce the amount of by-products created. Better methods for measuring chlorine and its by-products in the air are needed, he says. He adds that if you can detect a strong smell of chlorine in the air, the level is probably too high. He also suggests exploring nonchlorine-based disinfectants such as ozone or copper.