This summer, Olympic swimmers are poised to compete in the Seine. Swimming in the Parisian river has been banned for over a century due to boat traffic, pollution, and bacteria. Now, the French government has invested over a billion dollars to try to clean it up in time for the Olympic Games. But after all their efforts, the river is still not viable to swim in due to unsafe levels of Escherichia coli, aka E. coli.
The Olympics aren’t the only event hampered by the bacteria. On June 29, the first legal swim in D.C.’s Anacostia River was postponed because of “dangerous levels” of E. coli. And in the past year, beaches, lakes, and other waterways across the United States have been closed because of the bacteria. (This isn’t necessarily atypical—though contamination could be increasing with climate change.)
E. coli isn’t inherently bad; most forms of the bacteria are not harmful to humans. In our gut, it can help with digestion. But the problem is what high levels of E. coli can do—as well as what its presence can indicate. Found in the intestines of humans and most warm-blooded animals, the bacteria is used as a proxy for, well, poop. In addition to being plain gross, feces has all sorts of germs that can make people sick. The most common illness from swimming in polluted water is gastroenteritis. The water usually has to be swallowed or enter through the nose, eyes, ears, or an open wound to lead to illness. Because of this, the Environmental Protection Agency has recommendations for threshold levels of E. coli that are generally safe to swim in (as do agencies in other countries). These thresholds are usually calculated by looking at the relationship between E. coli concentrations and illness rates.
Just as mask mandates during the pandemic could differ from location to location, states and counties can use these thresholds to guide their decisionmaking, but can also go their own way. There’s no hard-and-fast line where water goes from safe to unsafe, says Daniel Nidzgorski, an ecologist with King County Natural Resources in Seattle. He also notes that hospitalizations from swimming in dirty water are pretty rare, and that sickness usually just manifests as a tummy ache that goes away after a few days. (The EPA also acknowledges that illness from swimming is usually not that severe.)
There’s some further uncertainty in the process that can make judgment calls difficult. For one, there is often a delay (of several hours to a day) between the time water samples were collected and when results are available. “In that time frame, the water quality can change either for the better or for the worse,” says Phanikumar Mantha, a professor of civil and environmental engineering at Michigan State University.
Lakes and rivers are not controlled environments. Weather can easily impact water quality and conditions. On a bright sunny day, the sun’s ultraviolet rays can inactivate bacteria in the water, making it less likely that swimmers will get sick. But on the other end of the spectrum, rainy weather can elevate bacterial concentrations. When it rains, poop buildup on the land can get washed into nearby bodies of water. There’s also potential for sewer overflow, as rain can overflow sewage systems and cause small releases of sewage and stormwater into rivers, lakes, or bays. And as the water level rises, contamination from the bottom of the water can bubble up to the surface. The Associated Press reports that rainfall in Paris “has exacerbated the issue” of water quality in the Seine, noting that officials are looking for “summer weather” to help clear up the condition of the water.
Another wrinkle: Even if E. coli levels are high, it doesn’t necessarily always mean the other viruses that contribute to illness risk are there. “That’s the difficult part of this equation, which is figuring out when elevated levels of E. coli really impact human health,” Mantha says.
Chances are high that if you’re going to go swimming in some natural water this summer, there will be E. coli. But there are ways to reduce your chances of getting sick. Nidzgorski advises looking for cues on land before taking a dip. If there’s goose or bird poop around a beach or on the banks of a river, it’s likely more abundant in the water, too. “Take a look and smell around where you’re going to swim,” he says.
Paying attention to the weather is also key, given how it can quickly impact conditions. “I would give your readers the same advice that I follow myself: I would avoid beaches, for example, here in the Great Lakes region, if it rains the previous day or in the last two days,” Mantha says.
And even if it’s a nice, clear day, there are simple steps that individuals can take to avoid contamination for future swimmers. “Don’t feed the geese and ducks, and don’t bring your dog to the swimming beach,” Nidzgorski says, noting that both can result in higher poop levels in the surrounding area. “Take a shower, rinse off before swimming, and babies, toddlers, and others should use a high-quality swim diaper. We all do our part to help keep poop out of the water like that.”