Many sports-medicine experts encourage athletes to soak in a hot tub after working out to soothe aching muscles. Other people simply enjoy the relaxation of the warm water and water jets. But beneath the surface can lurk some unsavory microorganisms. One expert, Jonathan Yoder, an epidemiologist at the Centers for Disease Control and Prevention, and team lead for the agency’s waterborne-disease-prevention branch, describes the telltale signs when not to climb into a hot tub.

Heat Seekers

Hot-tub temperatures are typically 104 degrees or less—not high enough to kill the many bacteria that thrive in warm water, says Mr. Yoder. These can include the common Pseudomonas, which can cause an itchy and painful rash known as Hot Tub Rash. More dangerous is Legionella, which gets aerosolized in the hot, humid environment and can cause Legionnaires’ disease, an extreme form of pneumonia, or Pontiac fever, which is like a bad case of the flu, he says.

“Most of these organisms will be inactivated if you maintain the proper level of disinfection,” says Mr. Yoder. And despite rumors one might hear, “to my knowledge, there has never been a reported case of herpes confirmed to be contracted through hot tub use,” he says.

Know the Numbers

Public hot tubs are supposed to be monitored regularly by maintenance professionals for proper recirculation, filter cleanliness and scrubbed surfaces. “Hopefully at your gym, they are checking the water at least twice per day, and maybe even hourly when there are a lot of people using it,” says Mr. Yoder.

Tub information is often posted in plain view at gyms. “If they use chlorine, you should look for a value of between 2 and 4 parts per million, and if they are using bromine, which is a similar disinfectant but lasts longer in hot temperatures, you’d look for a value between 4 and 6 parts per million,” he says. Higher numbers can indicate too many residual chemicals, which can irritate the skin and throat. Private hot tubs often don’t get the maintenance they require, and he advises homeowners to follow the manufacturer’s guidelines or hire a maintenance company.

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