April 8, 2002 -- For decades, chlorine has been used to disinfect public drinking water and swimming pools. Now, British researchers say that a chemical by-product of chlorine -- found in very high levels in swimming pools -- may be hazardous for pregnant women and their unborn babies.
In the study, levels of this by-product -- called trihalomethanes (or THMs) -- were considerably higher in swimming pools than in tap water, writes study author Mark J. Nieuwenhuijsen, PhD, an environmental science researcher at Imperial College of Science, Technology, and Medicine in London.
"[Miscarriage], low birth weight, neural tube defects, urinary tract defects and others have been associated with exposure to THMs, but the evidence so far has been inconsistent and inconclusive," he writes.
His report appears in the current issue of Occupational and Environmental Medicine.
However, in U.S. pools, extra chlorine is added periodically during peak swim hours as a "shock treatment" to destroy microorganisms and decrease by-products such as THMs through a complex chemical reaction, says Tom Bennett, REHS, an environmental consultant with the Georgia Division of Public Health. He agreed to review the British study for WebMD.
"Don't draw any conclusions" from this study, says Bennett.
When chlorine is added to water, it reacts with organic matter in the water -- skin cells and body care products -- to form various by-products, including THMs. In recent years, there has been increasing concern that THMs may be linked with reproductive problems.
In his study, Nieuwenhuijsen tested the water in eight indoor swimming pools, taking at least one sample every week for three consecutive weeks. Also, he tested a few samples of tap water for comparison. He kept track of factors such as water temperature and number of people in the pool when the samples were collected.
He found relatively high concentrations of THMs in the pools -- especially on days when pools were crowded and there was much turbulence in the water. Also, as the pool's water and air temperature rose, so did the THMs level. "Therefore more [THMs] are likely to be formed in both water and air" on those days, he writes.
Swimmers are at great risk because they are exposed for long periods of time and because THMs can be absorbed through the skin and inhaled from the air around the pool, he says.
Studies have shown that a one-hour swim in a chlorinated pool could result in a THM dose 141 times higher than what you get taking a shower, or a dose 93 times higher than drinking a glass of tap water.
Further research is needed to understand the link between THMs and potential health risks, writes Nieuwenhuijsen. In the meantime, he calls for swimming pool owners to reduce THM levels "as far as possible ... while maintaining effective control against waterborne microbiological disease."
The other studies that Nieuwenhuijsen mentions have actually found lower levels of THMs than this study did, Bennett says.
Also, all studies of THMs have been done in Europe, he says. "We don't know whether they chlorinate the same as we do in U.S. pools, whether they maintain pools like we do."
"I see this as more of a water quality problem than a swimming pool problem," Bennett says. "If the chemical is that bad, we need to be looking at how it got into drinking water. I would be much more concerned about that."
Pregnant lifeguards at indoor swimming pools have been known to suffer from different illnesses -- like certain lung problems -- because of the continuous exposure to chlorinated water, he tells WebMD. Whether THMs are a risk for pregnant women who are not lifeguards, "I just don't know," he says. "You can't say from this study that it is."