That question led to a study, conducted by Behnke and a cohort of academics and scientists, into how reusable-bottle contamination levels are affected by usage and cleaning behaviors. If you’re regularly drinking water from a reusable bottle, their findings might prompt you to reconsider your own water-bottle handling practices.  

Change your habits

While carrying around a reusable water bottle all day is healthy for you—and good for the environment—neglecting its care could lead to some negative consequences. “Do you wash your dishes after dinner?” asks Behnke. “Yes. But with water bottles, we often take them all over the place and don’t properly clean them.”

You might rinse your bottle out, but if you don’t wash it properly as often as you should, you’d be surprised at what might be growing on the surface and on the inside. The study consisted of two sections of surveys filled out by water-bottle users. The first set of questions centered on the type and age of bottle they used, what they put inside it (water, energy drinks, etcetera), and their frequency of use. The second section focused on cleaning behaviors, from the method to the frequency.

Essentially, said Behnke, typical water-bottle use has all the ingredients to foster bacterial growth: moisture, contamination, and, often, warm temperatures. Dr. Yuriko Fukuta, assistant professor of medicine–infectious diseases at Baylor College of Medicine, agrees. “We’re constantly touching our water bottles with our mouths and hands, so it’s easy to transmit bacteria to them, and then it just grows,” she says. “In some cases this can make you sick, especially if you have a weaker immune system.”

Fukuta suggested that bacteria might include staph or strep. And there’s this, from the cohort study: “More than 20% of our samples had coliform bacteria, which is fecal matter,” says Behnke.

Proper care and cleaning for water bottles

If all that bacteria has you wanting to do right by your water bottle, there are a few dos and don’ts to follow:

• Stick to just water in your bottles to lessen bacterial growth. Energy drinks, tea, or other powders and add-ins will only increase the likelihood of bacterial growth. 
• Keep your water bottles out of spots where they might heat up. “Don’t leave your water bottle inside your car and then drink from it,” says Fukuta. “Warm temperatures and time accelerate the growth of germs.”
• Rinse it after every day’s use, recommends Behnke. 
• Give it a good washing once a week, preferably by hand and with a special bottle brush. When finished, leave it upside down on a drying rack to dry completely—don’t put your bottles away wet.
• Avoid sharing water bottles with anyone else. “If it’s your saliva and just water, it’s not a big deal,” says Behnke. “But if you share, you’re introducing other contaminants.”

The best type of water bottle for mitigating bacterial growth

According to Fukuta, your best bets are bottles with a wide mouth, which make them easy to clean and dry, and those with a built-in straw that keeps your hands away if possible.

If your goal is to keep your water bottle from turning into a germy breeding ground, the simplest approach is Behnke’s, which he changed after conducting the research. “I rinse my bottle once a day,” he says, “and wash it once a week, using good detergent, a bottle brush, and a spray of Clorox bleach.”

A version of this story originally published on Fortune.com on Aug. 1, 2024.

More health tips:

• It’s not 8 glasses a day anymore. Here’s how much water you should drink each day
• 5 tips for getting enough water if you struggle to stay hydrated
• Here’s how much water is too much water, according to experts