Swimming in pools, lakes and the ocean is a great way to beat the summer heat − but not if the water is dirty and can make you sick.

Dirty pools commonly lead to urinary tract infections, ear infections and stomach bugs, and bacteria and germs in the water can cause your eyes to sting − ruining the fun of swimming.

Symptoms caused by polluted water look similar to food poisoning or a cold, said Mara Dias, senior water quality manager at the Surfrider Foundation, an environmental preservation group with more than 80 chapters nationwide. Beyond pool contamination you can see, it's easy for pool-goers to forget the water can be polluted, she said.

"It’s really hard to tease out symptoms from getting exposed to polluted recreational water," Dias told USA TODAY. "People aren’t always aware of it because it’s hard to pinpoint."

Other options for swimming, like lakes and the sea, come with different pollution risks, which would-be swimmers have very little control over, like animal waste and other harmful substances seeping into the water. It's usually much easier to get info about how clean a pool is, said Primrose Freestone, a clinical microbiologist at the University of Leicester in the United Kingdom who writes about recreational water safety.

"There's so many unknowns," Freestone said. "There’s a risk with swimming in all wild waters."

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Wherever you go swimming, research done ahead of time goes a long way to ensuring your safety. Dias recommends checking the water quality at your favorite swim spot the same way you'd check the weather forecast.

“When we go to the beach or spend a day at the pool, we look at whether it’s going to storm. Water quality is a condition you can be concerned about as well," Dias said.

Is my pool clean?

Pools are staying open longer as summers get hotter, and they're more crowded, said Dias, who is based in East Hampton, New York. When a greater number of people share a small volume of swimming water, the risk for catching something like a virus goes up, she said.

 "One way germs can get into the water is coming off of people, like if people are sick," Dias said. "Pools are tested probably every day, and chlorine will kill germs, but I wouldn’t say the exposure is zero."

If a pool's water is not crystal clear, there's a chance it's not clean enough to swim in, according to pool experts, who recommend doing a quick visual test before jumping in. Chlorine should lead pool water to look clear and blue, according to San Diego Pools, which specializes in making custom pools and spas in southern California.

If your pool's water is cloud or murky, or if there are leaves or debris on the surface, it may not be clean. Dull-looking water, or when you can't see all the way to the bottom are other indicators of uncleanliness, San Diego Pools says.

A pool's water should also feel very wet, and not slimy, according to the company.

"If you’re noticing a slimy feeling on your hand, it means the chlorine isn’t working," the pool design contractor's website says.

What's safer − pool, lake, river or ocean?

Any natural body of water will run the risk of being less safe than a pool, because sewage, pet waste and agricultural run-off can seep into lakes, rivers and oceans, Freestone said.

Natural bodies of water may also be less likely to have lifeguards on duty, another safety concern, she said. If you're worried about water temperature, pools will always be "warmer and more pleasant" than water found in nature, Freestone said.

In a river, any pollutant from upstream could be flowing directly your way, Freestone said, and if you can see pipes along the river's banks, get out immediately, she warned, because they may be carrying human sewage. Since there isn't a steady flow of water in and out of a lake, experts warn, any fecal matter, bacteria, algae or other pollutant could stick around for longer.

"Lakes would hold on to the pollution longer, whereas in an ocean, there would be more mixing with the larger volume of ocean water," Dias said.

But even in the ocean, if harmful substances are dumped into the water, they will flow "right back toward you" because of the tides, Freestone said, noting, "pollutants will travel."

"Because of the sheer volume of water in the oceans, it is probably overall safer to swim in the ocean," Freestone said. "But having said that, you can pick up some very nasty infections from sea water because even though it's very salty, it doesn't actually kill off pathogens, bacteria and viruses."

One of the worst cases of sewage contamination in the U.S. is in San Diego County, where sewage from Mexico has led to beach closures and states of emergency.

"Southernmost San Diego County has been plagued by sewage," said Chris Brewster, chair of the national certification committee at the United States Livesaving Association, a national lifeguard group.

How can I swim safely?

Ask lifeguards for the latest water quality test results before you get in the water, and remember that bodies of water can be the most polluted 24 to 48 hours after heavy rain, Dias said.

After swimming, make sure to rinse off with fresh water − no matter if you'd been in a pool, lake, river or the ocean, she said.

"That will help remove any potential pollution from your body," she said, adding that hands are the top area to wash thoroughly with soap and water after swimming, because we use them to eat and touch our face.

Everyone should be able to access the benefits of exercise, time outdoors and relief from the heat that comes with swimming, Dias said, and those positive health impacts are easily ruined by unknowingly swimming in polluted water.

"It has consequences, you want to enjoy a day at the beach, you don’t want to pay for it with feeling like you have food poisoning that night,” Dias said. “Swimming should give you a good night’s sleep."