A day at the beach is not always a day at the beach. According to a new report on water quality at 4523 beaches across the United States, more than half demonstrated potentially unsafe levels of bacteria for at least one day in 2018. The main culprit? Fecal contamination. If that makes you feel like packing up your beach umbrella and towels, there are some caveats to keep in mind.

The report [PDF] was prepared by Environment America Research and Policy Center, which examined the water quality of beaches in 29 states and Puerto Rico in 2018 using data collected by local, state, and federal agencies. Of the sites evaluated, 2620 were considered unsafe to swim in for at least one day. Six hundred and five sites were unsafe on 25 percent of the days that samples were collected. The determination was made by examining bacteria levels according to the Environmental Protection Agency’s threshold, or Beach Action Value (BAV). That guideline specifies contamination as being unsafe if levels could conceivably make 32 out of 1000 swimmers ill.

The triggering element? Poop. Specifically, bacteria from fecal matter that’s present in water as a result of storm runoff from urban areas. The rainwater can pick up bacteria from yards or streets and flow into water; sewage leaks can also be at fault. Germs like Escherichia coli or Enterococcus seep into the water and can then be introduced to swimmers' gastrointestinal systems when they swallow water, causing illness. The bacteria can also prompt skin infections or respiratory disease.

In Florida, 180 of 263 beach sites tested exceeded the BAV limits. In New York, 276 of 422 sites were deemed unsafe for at least one testing day.

Environment America recommends increased federal oversight that can provide same-day testing and post water quality warnings when appropriate, as well as bring older sewage systems up to date.

Naturally, not everyone agrees with the report’s conclusions. Officials in Maine, for example, which had water quality issues at roughly half of 85 sampled sites, responded to the findings by saying 93 percent of their samples collected in 2018 were below the approved threshold for safe recreation and that 97.2 percent of beach days were free from any contamination advisories or closures. Maine and other states often have a higher threshold for bacteria before waters are deemed unsafe. In Maine’s case, it’s 104 colony-forming units (CFUs) of enterococcus bacteria per 100 milliliters of water, higher than the blanket 60 CFUs of the Beach Action Value. There’s not necessarily a “wrong” number, as the EPA approves of both. Maine officials also argued that tested sites would have higher numbers shortly after heavy precipitation.

Anyone concerned about swimming in public beaches should be aware of recent rainfall that could lead to increased urban runoff, which could result in more bacteria in the water. Swimmers should also avoid swallowing water, and make a habit of showering after going to the beach.

[h/t Forbes]