Cooling off in the pool or spending the day at a water park are classic summer activities, but government health officials warn that there are parasites lurking in communal water that even chlorine can't kill.

In a well-chlorinated pool, the Giardia parasite can survive for up to 45 minutes, and the hepatitis A virus can survive for about 16 minutes, according to the Centers for Disease Control and Prevention. A parasite called cryptosporidium, or crypto, can survive for more than a week even in a properly treated pool or water park, according to epidemiologist Michele Hlavsa.

Exposure to crypto in pools and water playgrounds caused 4,232 illnesses from 2009 to 2017, according to a report by the CDC. People, children in particular, who swim too soon after having a case of diarrhea can spread the parasite.

"Unlike maybe norovirus or E. coli, which cause diarrhea or vomiting for a couple days, you can have diarrhea caused by crypto for up to three weeks," said Hlavsa, one of the study's authors. "That's not fun."

The number of illnesses caused by crypto associated with "treated recreational water" peaks from June to August and increased 14.3% each year from 2009 to 2016, according to the CDC. The report noted that these findings probably underestimate the actual number of crypto outbreaks.

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"To protect ourselves from crypto, the best thing we can do is not swallow the water we swim in," Hlavsa said. "We want to keep crypto out of the pool in the first place, and the way we do that is not to swim or let our kid swim when we’re sick with diarrhea."

The CDC advises anyone who's had diarrhea to wait at least two weeks before getting into a pool, but a recent survey found that 24% of respondents would go in a pool within one hour of having diarrhea.

That survey, presented by the Water Quality and Health Council, found that 40% of Americans admitted they have urinated in a pool as an adult. Although you can't catch crypto from urine, it reacts with chlorine and reduces the amount of chemicals available to kill other pathogens.

The Water Quality and Health Council is a group sponsored by the Chlorine Chemistry Division of the American Chemistry Council, an industry trade association.

About eight in 10 pool inspections find a serious health code violation, according to the CDC. Of those eight, at least one inspection will result in the pool being shut down immediately.

Hlavsa recommended going online and checking inspection scores on the local or state health department's website before getting into a pool or swimmers doing their own inspection when they arrive.

"We’re calling this a mini-inspection, where you use test strips to check the chlorine level and the pH before getting in," she said. "We, as swimmers or parents of young swimmers, need to take a more active role to make sure we have a fun and healthy and safe time in the water this summer."

Contributing: Micah Walker, Detroit Free Press

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