Our local water park opened over Memorial Day weekend, and of course the kids begged us to take them. My husband and I easily relented — we enjoy zipping down waterslides on hot days, too — but it was as crowded as we had ever seen it. Eyeing the wave pool as it teemed with sweaty adults and snot-encrusted kids, I couldn’t help wondering what kinds of gross germs lurked in it.

And what about other crowded pools and lakes in the summertime — are they more like cesspools than swimming pools?

It is hard to get a clear answer: Nobody knows how many Americans get sick from germs they are exposed to while swimming. The Centers for Disease Control and Prevention reports between 20 and 60 infectious disease outbreaks related to swimming and recreational water use every year, which doesn’t sound like a lot, but those numbers represent “a fraction of what is actually happening out there,” said Michele Hlavsa, chief of the CDC’s Healthy Swimming Program.

“We miss a lot of outbreaks,” she said, because people who get sick after they swim often don’t go to the doctor, let alone report their illness to health authorities. And since people can travel long distances to visit pools, lakes, oceans and water parks, water-caused illnesses rarely get traced back to their source.

Still, most people don’t get sick after they swim — and none of us, thankfully, felt ill after our crowded Memorial Day adventure. But some watering holes, I learned, are riskier than others.

Persistent parasites

Among the known outbreaks caused by germs in U.S. pools, hot tubs and water parks between 2015 and 2019, one-third were traced to hotels and motels. The CDC isn’t sure why resort pools are so germ-laden, but one theory is that the person responsible for maintaining water quality at a hotel is also “responsible for 100 other things,” Hlavsa said.

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When researchers at the CDC conducted routine inspections of thousands of public U.S. swimming pools and hot tubs as part of a 2016 study, they found that 9.2% of pools, and 19.2% of hot tubs, violated disinfection requirements — for instance, by not having enough chlorine in them. Many of the pools’ pH levels didn’t meet recommended guidelines, which is worrying because pH affects how well chlorine disinfects. (The CDC recommends that pools have at least 1 part per million of chlorine in them and that hot tubs contain at least 3 parts per million. The pH of water should be between 7.2 and 7.8. You can buy chlorine and pH test kits or strips at hardware and pool supply stores.)

Even when pools do contain enough chlorine, the chemical doesn’t kill germs immediately, said Dr. Marirose Trimmier, a family medicine physician at the University of South Alabama College of Medicine, who recently co-wrote a research paper on swimming-related illnesses.

Some microbes can persist in chlorinated water for quite a while. The parasite Cryptosporidium, which was responsible for nearly half of all pool-related infectious disease outbreaks between 2015 and 2019, and causes diarrhea, fever and fatigue, can survive in chlorinated water for more than a week, Hlavsa said. Noroviruses, which are a common cause of stomach bugs, can also be chlorine-resistant, as can tiny diarrhea-causing parasites known as Giardia.

One thing that is clear from the data is that hot tubs contain the grossest water of all: A whopping 70% of recent outbreaks were traced to them, according to the CDC. Hot tubs can be germ-laden for several reasons, Hlavsa said. Chlorine breaks down especially quickly in hot water, and the aerosolization of water from tub jets renders the chlorine even less effective, she said. (Some hot tub operators now use bromine, rather than chlorine, to kill germs, as it is more heat-stable.) Also, certain bacteria grow quite well in hot water — like Legionella, the cause of pneumonialike Legionnaires’ disease, which recently killed two people in an outbreak in New York City.

Another germ that is often found in hot tubs is Pseudomonas, a bacterium that causes body rashes, said Dr. Allen Perkins, a family medicine doctor at the University of South Alabama. This rash, which appears as scattered red bumps, will typically appear 48 hours after a hot tub soak and will be worst on the areas of skin your swimsuit covered because the suit traps the bacteria close to your skin, he said. These bacteria have been shown to be increasingly resistant to chlorine, too.

Thankfully, most of these infections are treatable.

Assessing water parks

Circling back to my beloved local water park: Just how gross are these popular attractions? Hlavsa said she wouldn’t be too concerned.

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“Actually, I am planning on taking my kiddos to a water park this coming weekend,” she told me when we spoke.

Because water parks are operated by professionals, they are usually careful to follow disinfection guidelines, she said. I was also relieved to learn that in some states, including New York, the water used in splash pads, which spray water on users and are frequented by younger (i.e., germier) kids, are not only treated with chlorine but also with ultraviolet light, which kills chlorine-resistant microbes.) Hlavsa pointed out that concerned water-park-goers (or poolgoers) should also be able to access public health inspection data. Inspections may be done by the county, the city or another municipality. She suggested calling your local health department for more information.

Watch out for murky water

If you are planning to swim in a lake, river, stream or ocean, look for physical signs indicating that the water is either safe or unsafe for swimming. (Or check your state’s water quality conditions online.) This is especially important to do after heavy rain, because as it flows and drains, rain picks up everything it comes in contact with — including animal feces — and can deposit it into swimming areas, Perkins said. Avoid areas that have pipes draining into them, too.

If you have an open wound or scratch, it is best to avoid swimming or wading, especially in saltwater where rivers flow into the ocean, Trimmier said. At the very least, cover wounds with a waterproof bandage. The bacteria Vibrio can enter the body through minor wounds and cause serious, but rare, infections.

“The rule of thumb is, if the water looks murky, don’t swim in it,” Perkins said. Don’t swim in it if it smells bad; even if it smells fine, don’t drink it.