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Soaking in a hot tub is a great way to relax your tired body, but it also comes with some microbial risks.

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Maybe it’s been a long day and you want to kick back and spend some time in a Jacuzzi, letting those magic water jets get up all on your neck.

But as relaxing as hot tubs can be, they also come with some... microbial risks. Like any pool of water, hot tubs can incubate all kinds of germs. And because they’re smaller, and warmer, and have those bubbly jets, they actually pose more of a risk for certain diseases.

Take, for instance, the most common hot tub illness: pseudomonas folliculitis, an infection of your hair follicles with the bacteria Pseudomonas aeruginosa. Hair follicles are the little breaks in the skin that hairs grow out of, all over your body.

So if these bacteria are soaking in the water with you, they set up shop in your follicles. A day or so later, pustules will pop up all over your skin, usually where your swimsuit was. The good news is that unless your immune system isn’t working, well this infection is itchy and maybe embarrassing, but mostly harmless, and will usually go away on its own after about a week.

You can get pseudomonas folliculitis from any contaminated pool or lake, but it’s known as hot tub rash for a reason. Hot tubs are hard to keep clean, and the warm water makes this species of bacteria grow faster. You can, of course, disinfect pools and hot tubs by adding chlorine to water, which forms hypochlorous acid, which can tear apart bacteria and kill them.

But because chlorine also reacts with other stuff, like dirt, sweat, and lotions, it gets used up faster when there are more people in a small tub. Plus, chlorine is less effective at higher temperatures. Now, other bacteria like the wetness and warmth of hot tubs, too, including Legionella.

These bacteria aren’t so bad if they touch your skin, but hot tubs are bubbly. And those massaging jets of water also create a mist that you breathe in, so Legionella in the water can get into your lungs and cause an infection. If you’re otherwise healthy, this usually isn’t a huge problem. You might develop a few flu-like symptoms at worst.

But if you’re older, smoke, or already have a lung problem, it can cause a deadly type of pneumonia called Legionnaires’ disease. Certain types of mycobacteria can also lurk in hot tub water. These bacteria are inhaled like Legionella, but instead of creating an infection and multiplying the bacteria trigger a sort of allergic reaction called hypersensitivity pneumonitis, or hot tub lung.

Even if you aren’t old or already sick, your immune system can overreact to these bacteria so your lungs become inflamed, making it hard to breathe or making you very tired. And because not many doctors bother to ask their patients about their hot tub usage, people might not suspect their nightly soaks are causing the problem. If all this has you less psyched to hit the spa, remember that clean and properly chlorinated hot tubs are perfectly safe.

So, just check before you take a dip! Thanks to all of our Patreon patrons who keep these answers coming, and for more swimming science, we have a video where Michael explains why your fingers and toes get all wrinkly when you soak for too long.