DNA-damaging disinfection by-products found in pool water

Chemicals used to disinfect swimming pools generate other chemicals, some of which damage DNA in cells, according to a trio of new papers that quantified this effect not only in the lab but also in 49 swimmers.

Chlorine and other disinfecting agents are designed to kill germs. But through interactions with other water pollutants, disinfectants can produce mutagenic chemicals. Low concentrations of such DNA-damaging chemicals have been isolated from drinking water in the past, but only one 30-year-old paper hinted at their presence in pool water.

Rates of DNA mutations in swimmers are low and might eventually be fixed by natural repair mechanisms, says environmental epidemiologist Cristina Villanueva of the Municipal Institute of Medical Research in Barcelona, who is a coauthor of all three papers. “Our study was only evaluating short-term exposures,” she notes. The development of cancers, she emphasizes, would likely require extended, chronic exposures to such mutagens.

Swimmers underwent a battery of tests before and after 40 minutes of laps in an indoor Barcelona pool. Markers of DNA damage known as micronuclei increased in white blood cells following the swim, Villanueva’s group reports in one of three new papers published online September 12 in Environmental Health Perspectives.

When cells are stained in the lab to highlight genetic material, sometimes small satellite dots appear. These micronuclei are commonly found when a mutagen breaks a chromosome, explains coauthor David DeMarini of EPA’s National Health and Environmental Effects Research Laboratory in Research Triangle Park, N.C. And that’s the assumed explanation for micronuclei seen here, the genetic toxicologist says.

Elevated micronuclei were correlated with higher concentrations of certain disinfection by-products called trihalomethanes that most likely entered the body through breathing or skin penetration and were exhaled several minutes after leaving the pool.

“The genotoxicity that we found in the swimmers was associated with concentrations of brominated trihalomethanes” but not with chlorinated ones, DeMarini says. And even among the brominated by-products, two stood out: Exhaled bromoform and bromodichloromethane together accounted for roughly one-quarter of the postswim increase in micronuclei measured in blood.

Bacterial assays also quantified the direct mutagenicity of pool water. Initially, DeMarini says, it was difficult to get any data because almost all of the bacteria tested died. The pool water turned out to be too toxic, he explains — roughly twice as toxic as tap water, which he suspects is due to excess germ-killing free chlorine in the pool’s water. Once he diluted pool water to bring its toxicity to that of tap water, enough bacteria survived to establish for the first time that pool water is mutagenic.

Indeed, it proved almost exactly as big a threat to DNA as drinking water is. The surprise, DeMarini concedes, is that pool water “isn’t more mutagenic.”

For years, only five chlorinated by-products generated by the use of chlorine have been quantified in pool water. In their new research, Susan Richardson of EPA’s National Exposure Research Laboratory in Athens, Ga., and her colleagues expand the number of such disinfection by-products associated with pool water to roughly 100.

Several of the newfound chemicals in pool water contained nitrogen. Their suspected source: reactions of pool chemicals with the nitrogen in swimmers’ skin, sweat and urine.

Earlier studies of pool disinfection by-products focused primarily on chloroform, observes DeMarini. But because chloroform is not mutagenic, “We have shown it’s the most irrelevant,” he says. In the new study, bromoform — a bromine-based analog of chloroform — correlated most strongly with mutagenicity.

Over the years, Richardson has identified more than half of the roughly 600 known by-products that have been associated with drinking water disinfection. Her team’s new paper for the first time expands the survey for many of these to pool water. In addition to those identified by name and structure, analyses turned up even more that await identification, DeMarini says. Indeed, “we have no reason to doubt that there are hundreds,” he says, because swimming pool water “fundamentally isn’t much different than drinking water.”

Greatly expanding the known disinfection by-products associated with pool water is “interesting” but probably not all that surprising, notes Thomas Lachocki, head of the National Swimming Pool Foundation based in Colorado Springs, Colo. “Once you start putting people in the water along with suntan oils and air-blown debris you’re going to find disinfection by-products.”

That the resulting water “is no more mutagenic than drinking water — well, that’s fabulous,” he contends.

But the chemist expressed disappointment that the new studies reported so little information on how pool disinfection systems were controlled — where chlorine levels were maintained, for example, whether automatic controllers were used and what the rates of ventilation had been. Such unknowns, he worries, could make attempts to replicate this study difficult.

Although swimming is viewed as a healthful activity, “there are some negative attributes that are not very well recognized in the swimming community — or even in the scientific community,” observes Ernest Blatchley III of Purdue University in West Lafayette, Ind. He says the new papers not only draw attention to risks “but also contribute to the scientific understanding of those issues.”

One take-home message, he says, “is that swimmers need to be more aware of their personal hygiene.” Taking bathroom breaks, for instance, would “improve swimmer health tremendously,” he maintains. The reason: “The urea in urine reacts with chlorine to produce some of these disinfection by-products. In fact,” he notes, “it’s the constituents of human sweat and urine that are largely responsible for their formation.”