Whether you’re treating yourself to a spa day or you just finished skiing, hitting the hot tub is a surefire way to relax and soothe sore muscles. But did you ever think about what you’re literally getting yourself into? The water in approximately 60 percent of public hot tubs is tainted with bacteria, according to the Centers for Disease Control and Prevention (CDC). In a study of 5,000 hot tubs, the CDC found that 11 percent of public hot tubs had water quality so poor they were forced to close.

Hot tubs can be a breeding ground for certain heat-loving germs, says epidemiologist Michele Hlavsa, RN, MPH, chief of healthy swimming for the CDC. “Water temperatures are typically higher in hot tubs/spas than in pools, so it’s harder to maintain proper disinfectant levels,” she says. Germs multiply when disinfectant levels are too low.

Related: How to Stay Germ-Free at the Gym

Funky waters

According to the most recent statistics from the CDC, there were some 18 outbreaks of water-associated disease linked with public hot tubs or spas from 2009 to 2010. Some were caused by the pseudomonas aeruginosa, a bacteria that causes an itchy, bumpy and painful rash aptly named “hot tub rash.” About 40 percent of the outbreaks occurred in February or March, and many of the winter outbreaks happened in hotels.

Another germ of concern is legionella bacteria. Each year up to 18,000 people in the United States are hospitalized with Legionnaires’ disease, an extreme form of pneumonia. Breathing in the steam from a hot tub that contains legionellacan also infect you with Pontiac fever, which feels like a bad case of the flu.

While public hot tubs like the one at your gym or hotel are supposed to be monitored regularly for cleanliness, don’t assume they’d pass a test. In a study by Texas A & M University microbiologist Rita B. Moyes, 95 percent of tested private and public hot tubs contained bacteria from feces, and 81 percent had fungi. Both of these can cause skin, ear, eye and urinary tract infections as well as diarrhea. Thirty four percent contained staphylococcus, which can cause deadly staph infections.

Related: How Often Should You Wash Your Kitchen Towel?

5 signs that could signal trouble

You can try to gauge how well the facility cleans and maintains a hot tub by looking for these potential signs of trouble.

1. You don’t see info posted about how they maintain the tub. Hot tubs available for public use must meet certain sanitary and safety standards of operation, which includes passing regular health inspections. However, because individual states and local governments formulate the standards, when it comes to posting this information, “what’s required in one community can be very different from standards in another community,” says Hlavas.

Take matters into your own hands and test the tub yourself. “You can buy fifty test strips at the big box, hardware, and pool supply stores for about seven to fifteen dollars,” says Hlavas. You’ll need to know which disinfectant — bromine or chlorine — the facility uses, since the strips are designed to check for levels of one or the other. The strips also measure pH levels. Just dip them into the water and the results show up within 15 seconds or so.

If the facility is using chlorine to sanitize the water, look for a chlorine value between 2 and 4 parts per million. If they’re using bromine, which lasts longer in hot temperatures, look for a bromine value between 4 and 6 parts per million.) The pH level should range from 7.2 to 7.8.

While the goal is to use enough chemicals to discourage bacterial growth, chemical levels that are too high can cause skin and throat irritation. When the pH levels are too low, the acidity can irritate your eyes, skin and nose. When it’s too high, it can make the tub water feel gritty.

Don’t have any strips handy? “Hot tub/spa users can ask the pool operator or manager about the water quality before getting in,” says Hlavas. “They can also ask about the latest inspection score.” A public hot tub must be inspected by a city, county or state inspector, and scoring can include a letter or number grade. “If a hot tub/spa fails inspection, it will be immediately closed until the issues that threaten public health/safety are resolved,” she says.

2. You smell an odor. A healthy hot tub/spa has no or only a slight chemical smell. A strong chemical smell indicates a maintenance problem.

3. You don’t hear a filtration system running. If the hot tub equipment is working, the pumps and filtration system should be audible. Also, if the water looks murky, it could indicate a problem with the filter.

4. The water feels super hot. For personal safety reasons, the water temperature shouldn’t be above 104 degrees Fahrenheit (40 degrees Celsius).

5. The water feels slimy. The sides of the hot tub should be smooth and the tiles shouldn’t be slippery or sticky. Learn more from the CDC’s fact sheet on safe hot tub use.

More hot tub safety tips

Lower your risk of infection and other health problems by sticking to these smart strategies.

1. Don’t put your face in it. Putting your face in or getting hot tub water in your mouth makes it a whole lot easier for germs to enter.

2. Skip the group soak. The more people in the tub, the more the disinfectant levels drop, which increases your odds of getting an infection.

3. Pass if you smoke or have a weakened immune system. Legionella can be breathed in from the steam of contaminated hot tub. If you’re a smoker or your immune system is weak, you’re especially susceptible.

4. Keep little ones out. The CDC recommends that children under 5 not go in a hot tub since they don’t get the same cues that adults get when they’re overheating.

5. Stay out if you have a cut or scrape. Any opening on the skin allows an easy entryway for bacteria.

6. If you’re pregnant, ask your doctor first. Excess heat can harm a developing fetus, especially in the first trimester.

7. Don’t drink and soak. The combo of heat and alcohol can make you feel extra tipsy and also dehydrate you more quickly.

8. Keep the dip quick. You can get heatstroke or heat exhaustion from being in a hot tub too long. Experts recommended you soak for only about 10 to 15 minutes. 

Tara Rummell Berson is a health and wellness writer and editor. Her work has been featured in numerous media outlets, including Redbook, WebMD and The Huffington Post.