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Is Your Water Bottle Making You Sick?

Here’s what germs could be lurking inside — plus, how to clean it properly

spinner image hand reaching for water bottle
Photo Illustration: AARP; (Source: Getty Images)

Reusable water bottles are all the rage these days — no doubt a boon for the environment and our hydration levels. But if you’re not cleaning your glass, stainless steel or plastic bottle, that must-have accessory could be a potential health hazard. 

“Just like any other surface where water accumulates, spores can drop and start forming mold,” says Benjamin Turner, an instructor in the Department of Biology at The University of Alabama at Birmingham.

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Mold, a type of fungus, spreads by way of microscopic reproductive cells, called spores, that waft through our indoor and outdoor air. They like to settle where there’s moisture. “That’s where the spores will incubate and start forming those black fuzzy or gray, white areas of mold patches,” Turner says.

Some molds are harmless, but others can cause allergic reactions and respiratory problems. Still, S. Wesley Long, M.D., medical director of microbiology at Houston Methodist, says you’ll likely be deterred by an off-putting smell or taste before drinking enough to make you sick. “But certainly for some people, especially the immunocompromised, they need to be more careful,” Long says.

Bacteria can also grow in your bottle. In fact, your mouth has one of the highest concentrations of microbes — tiny living organisms including bacteria, fungi and viruses — in the body, says Peter Iwen, a professor and microbiologist at the University of Nebraska Medical Center.  These various germs can transfer to the bottle and “produce a substance that allows them to clump together,” called a biofilm. “And then you have this slimy biofilm that forms on your bottles,” Iwen explains.

This biofilm likely won’t get you sick since the germs came from your mouth. But it could get someone else sick if they take a swig of your drink — especially if you’re harboring a virus like COVID-19, influenza or norovirus. “Sharing it with other people is a risk because your flora [the population of microbes in your body] is not necessarily applicable to that next person that might be drinking from that bottle,” Iwen says. 

Pouring anything with sugar into your bottle — a sports drink, for example — only adds fuel to the fire by supplying “a rich source to promote bacterial and fungal growth,” Long says.

spinner image washing water bottle
Getty Images

How to wash your water bottle

How do you prevent mold and other microbes from moving in and taking over? Cleaning your water bottle every day — or every couple of days — with hot water and soap is a good place to start. Iwen also recommends sanitizing your bottle every week to 10 days. You can either run it through the dishwasher or soak it in a vinegar solution.

“Take about one part vinegar to about three or four parts water, and just let the bottle soak in that for maybe five minutes,” Iwen says. Rinse it off with hot water and soap, and you’re good to go.


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If your bottle is equipped with seals, straws or silicone mouthpieces, you’ll want to disassemble those and give them a good wash and soak, too. “Rubber, for instance, is a good area for bacteria to form these biofilms. So you should be taking it apart,” Iwen says. Inexpensive, specially designed brushes are even sold to help clean the often-neglected inside of straws.  

“Pay attention to the nooks and crannies where substances can sort of get lodged and take hold and get missed in a superficial cleaning,” Long says. If a sugary drink was in your bottle, make sure it didn’t leave behind a dried, sticky residue. “That’s just a great medium for bacterial and fungal growth,” Long adds.

Just as important as washing the bottle is drying it properly. Think of what happens to your lawn after a good rain, Turner says. “Mushrooms form, and mold is a fungus, and so it’s just waiting for the right environment. It needs oxygen and water and a little warm temperature. So if you can dry things out, that’s probably the number one thing you can do to avoid mold and mildew growth,” he says.

A few other tips:

Buy easy-to-clean bottles. On the hunt for a new bottle? If you’re looking for a low-maintenance option, Iwen recommends a stainless steel bottle with a wide mouthpiece. They’re easier to clean, plus, he says, biofilms form easier on plastic. 

Keep your bottle with you. We’re all guilty of occasionally leaving our coffee mugs and water bottles in the car, but don’t make a habit out of it — especially if it’s warm outside. “Bacteria love to grow in that kind of environment,” Iwen says. The advice is the same for storing your bottle on your desk at work or in your gym bag for your next workout.

Don’t refill disposable bottles. “They weren’t designed to be used over and over again,” Iwen says. The plastic can break down quickly, making it easier for bacteria to take over.

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