Murky waters: Freshwater swimming tied to higher risk of skin rashes and stomach trouble
Laine BergesonAugust 31, 2026
E coli
Public Health
Jumping in a cold lake on a hot day is one of summer’s many pleasures, but swimming in lakes and rivers may increase the risk of gastrointestinal (GI) illness and skin ailments. That’s according to a new study that compared the outcomes of those who swam in a lake or river for 10 minutes with those who stood on the shore.
The findings of the large randomized controlled trial, published yesterday in the International Journal of Hygiene and Environmental Health, showed that swimmers were more than twice as likely to suffer skin ailments, like rashes and itching, compared with those who stayed dry. The data also showed that as levels of Escherichia coli (E coli) and other viruses in the water rose, so did swimmers' risk of stomach troubles.
“This work provides some of the strongest evidence yet that contamination in freshwater recreational sites can translate into real-world illness among bathers,” Paul Hunter, MD, study author and professor in medicine at the University of East Anglia, in a news release.
Higher contamination levels tied to greater risk
For the study, a team led by researchers from University of East Anglia examined data from 2,368 participants, roughly half of whom were randomized to swim at one of four freshwater swimming areas in Hungary and the other half to stay on the shore and not make contact with the water. The demographic and behavioral characteristics of the participants were well-balanced between the groups and there were no statistically significant differences in prior illnesses.Water samples collected during the trial were tested for E coli, intestinal enterococci, and somatic coliphages—viruses that can indicate fecal contamination in the water.
After one week, GI illness was reported in 24 swimmers (2.12%) and 24 non-swimmers (1.94%), making swimming itself not significantly associated with GI issues. But among swimmers, the risk rose alongside concentrations of E coli and somatic coliphages in the water.
After adjustment for age, recent GI illness, and behavioral factors, each one-unit increase in E coli concentration was associated with a 77% increase in the risk of GI illness (relative risk [RR]: 1.77). An increase in somatic coliphage concentration was associated with a 48% increase in risk (RR: 1.48).
Swimmers were more than twice as likely as non-swimmers to report a skin ailment like rash or itching (RR: 2.17). None of the three microbial indicators, however, was significantly associated with skin symptoms. “This could be due to exposure to naturally occurring organisms not linked to sewage pollution,” Hunter says. “Agricultural or industrial contaminants may also play a role.”
Respiratory, ear, and eye infections were uncommon, with only 9, 13, and 13 cases, respectively.
Findings may extend beyond Hungary
While the current findings apply to freshwater sites in Hungary, they can be interpreted more broadly, according to Hunter.“Our results were consistent across the four Hungarian study sites, which differed in water quality and water types. Our results were also consistent with a previous study in Germany,” he says. “So overall, this suggests that the relationships that we have identified are generalisable and likely applicable in UK waters.” Open-air swimming has surged in popularity in the UK in recent years.
Overall, the findings support the use of E coli concentrations to measure the risk of GI illness in freshwater and suggest that somatic coliphages may be useful markers of water quality, indicating that monitoring of freshwater swimming areas is important for public health.