Open water, open mouths: scuba divers face infection risks.

Circling sharks and empty air tanks may haunt scuba divers' imaginations, but ordinary microbes are a far more probable hazard. A new study takes a stab at quantifying the risks that waterborne bacteria and viruses pose to divers.

While scientists regularly measure bacterial concentrations in waters used by beachgoers, they don't test all the sites visited by divers, surfers, and kayakers. What's more, researchers don't know how much of the water these people swallow, says microbiologist and mathematical modeler Jack Schijven of the National Institute of Public Health and the Environment in Bilthoven, the Netherlands.

To begin measuring the microbial risk to divers, Sehijven and his institute colleague Ana Maria de Roda Husman provided a questionnaire to 233 professional divers and posted a similar survey online for about 26,000 recreational divers in the Netherlands. Thirty-seven pros--who do underwater-construction or search-and-rescue work, for example--and 483 amateurs responded. They supplied data on illnesses they'd had in the past year, how many dives they'd made in various aquatic environments, and what volume of water they'd swallowed on a typical dive.

The researchers focused on skin, ear, eye, respiratory, or gastrointestinal symptoms, which might have been caused by infections acquired during dives. Most respondents said that they'd had at least one such illness. Diarrhea and ear problems topped the list.

"Only 20 percent of the divers stated that they did not have any complaint at all," Schijven says. "We were really astonished."

The study didn't include a comparison group of nondivers, so it's unclear what portion of the ailments resulted from diving, he cautions.

Other data from the questionnaires suggest that recreational divers face a gastrointestinal infection risk of up to 1.1 percent per marine dive and 1.5 percent per freshwater dive.

The recreational divers tended either to swallow no water or to swallow about the volume of a shot glass. Professional divers, who often wear full face masks, generally swallowed a few drops of water or less. From the survey information and data on pathogen abundance, the researchers estimate in the May Environmental Health Perspectiws that professionals face the highest risks.

The pros "have to dive in any kind of water, even wastewater," Schijven says.

Overall, divers reported more ear complaints during the summer months than the winter months. That's "a strong hint" that diving is to blame, Schijven says, because the bacterial suspects in such infections prefer warm water. By contrast, gastrointestinal problems, which are caused by pathogens that survive longer in cold water, are most frequent during winter.

The study takes a "great approach" to examining overlooked aspects of divers' health, says Richard E. Moon, a Duke University physician and senior medical consultant for the Durham, N.C.--based Divers Alert Network. "It should raise divers' level of awareness of this potential risk," he adds. However, he says, the results could be biased because people with health complaints may have responded more readily to the survey.