Swimming pools might help you survive the summer heatwave. But some pools make for the perfect breeding ground for parasites and bacteria that can lead to diarrhea, rashes, respiratory illnesses, and ear infections.

On average, you can bring 10 million microbes into the pool just from your hair alone. Your hands can carry an additional 5 million microbes, and a single drop of spit can add another 8 million.

While not all microbes are harmful, some can cause illnesses. The average adult will swallow about 1 tablespoon of pool water during a 45-minute swim session, which is “more than enough to make you sick,” according to the Centers for Disease Control and Prevention (CDC).

Pools and water parks are treated with chemicals, such as chlorine and bromine, to kill most of the germs. But it's a common misconception that swimming pools are a "sterile environment," according to Trisha Robinson, MPH, an epidemiologist supervisor with the waterborne diseases unit at the Minnesota Department of Health.

Chlorine often fails to kill parasites like Cryptosporidium (crypto), she said, which can survive in a filtered and disinfected pool for more than 10 days. While E. coli, giardia, and other germs can also contaminate the water, crypto is the most common cause of waterborne diarrhea.

A crypto infection can take anywhere from two to 10 days for symptoms to appear. People may experience watery diarrhea, nausea, dehydration, and stomach cramping. Those with weakened immune systems may have a more serious, or even fatal, infection.

How Can You Protect Yourself and Others?

If you're infected with crypto, the parasite can stay in your body for up to two weeks, so the CDC recommends staying out of the water for two weeks after your diarrhea has completely stopped.

However, a 2017 survey led by the Water Quality and Health Council found that 25% of adults "would swim within one hour of having diarrhea."

Kelly Johnson-Arbor, MD, a medical toxicologist and co-medical director at the National Capital Poison Center, told Verywell that people might assume it's fine to go swimming once their diarrhea stops, but they can still carry germs with them.

According to the CDC, most people always have a small amount of poop on their bodies. And even though this amount of poop is equivalent to just "a few grains of sand," this can wash off and contaminate the swimming water.

"If one person with Cryptosporidium gets into a pool, it can lead to days or even weeks of exposures for many, many people," Johnson-Arbor said.

Showering for 60 seconds before getting in a pool could remove most of the unwanted substances from your skin to avoid using up the chlorine or bromine that's meant to kill germs.However, according to a 2019 survey, only 31% of respondents follow the recommendation to shower before swimming.

How Can You Tell if the Water Is Clean?

Pools, waterparks, and hot tubs are mainly disinfected with chlorine. When chlorine comes in contact with dirt, sweat, urine, and other contaminants, a strong-smelling substance called chloramine is formed.

"If you smell a strong odor of chlorine, it doesn't mean that the pool is super clean, it means that there are a lot of germs coming off of peoples' bodies hanging out at the pool," Johnson-Arbor said.

Chloramines can lead to skin, eye, and nose irritations, but moving to an area with fresh air can clear up these irritations for most people.

Public pools in general are routinely inspected for chlorination and pH balance. You can often find the records of safety inspections on your local health department's website.

If you want to be extra cautious, you can use over-the-counter test strips to check the water's chlorine level before getting in.

If you have an open wound or recently got a tattoo, experts also say it's best to avoid swimming pools until it's healed.

Robinson reiterated that as long as you shower before going into a pool, stay out of the water if you're sick, and keep water out of your mouth as much as possible, you can minimize the risk of spreading germs.

"It's not to scare people to stay out of the water or anything like that," Robinson said. "Swimming is really great. People can have a lot of fun with it."