Americans may want to think twice before that next summer swim. Many of the country’s idyllic beaches conceal a filthy secret below the sand and waves: dangerous levels of bacteria that put thousands of bathers at risk every year.

In a recently published study conducted by the Environment America Research and Policy Center, researchers found that the water at beaches in 29 coastal and Great Lakes states contained concentrations of bacteria well in excess of Environmental Protection Agency (EPA) standards – including the harmful organisms that inhabit human feces – which they said could sicken millions of beachgoers annually.

“It’s hard to believe that 47 years after we passed the Clean Water Act that we are still concerned with poop in the water when people want to go swimming,” John Rumpler, clean water program director for the center, told USA Today.

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Bacteria levels can spike to dangerous levels on certain days, such as when rain washes waste from the streets into creeks or streams, which flow into larger bodies of water. Some areas fare much worse than others, however.

In Chicago, all of the 19 beaches sampled were considered unsafe for at least one day in 2018, while the city’s South Shore Beach registered contamination in 93 of the 98 samples collected. California’s Cabrillo Beach also came in high on the list, considered unsafe in 85 out of 175 samples in one section.

On the cleaner end of the spectrum, Georgia’s Jekyll Driftwood Beach provided only 2 contaminated samples out of 46, and Florida’s Bird Key Park resulted in 17 dirty samples out of 67, suggesting that even the tidier beaches can be risky on some days.

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The harmful microbes found in the water can cause a litany of unpleasant symptoms in their human hosts, ranging from gastrointestinal illness, respiratory disease, as well as ear, eye and skin infections. However, the most nightmarish surprise lurking below the depths, and one not covered by the center’s study, goes by the name of necrotizing fasciitis – or flesh-eating bacteria. While the horrifying condition is rare, some of the 30 or so organisms that cause the illness are native to America’s lakes and coasts. Two recent cases in Florida left one man dead, while another man nearly lost a leg.

Fortunately, things might not be all bad when it comes to America’s squalid beaches. Luke Ginger, water quality scientist for environmental organization Heal The Bay, told USA Today that the national trends were moving in the right direction, particularly through local government initiatives to treat stormwater runoff, among other anti-pollution measures.

“There are improvements that can be made but for the most part, there’s really good water quality,” Ginger said. “There are just a few beaches that have issues.”

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