Environmentally friendly and convenient, reusable water bottles are on the rise. But on social media, people seem confused about how to clean them, and post horrifying pictures of mold growing inside.

Water sourced from a municipal water supply is likely very clean, explains Kelly Reynolds at the University of Arizona, who studies water quality and disease transmission.

But bacteria and viruses can make their way into a water bottle via our hands and saliva. “The water bottle itself is just the perfect environment for bacteria to grow,” says Reynolds. “It’s a little incubator.”

Keeping your water bottle clean comes down to hygienic practices – but these might not be as obvious as you think. For example, in a study of 30 people, 27 regularly replenished their water bottles without washing or rinsing them.

Here’s what experts recommend.

How to clean your reusable water bottle

Wash your hands – and wipe down your phone: Before you even get to the water bottle, you should wash your hands.

“The source of the water bottle’s contamination is probably your hands,” says Reynolds.

But frequent hand washing could also be moot if you don’t also wipe down your phone at least weekly, using a cloth slightly dampened with soap and water. Cell phones collect viruses and bacteria; some research suggests that at least 68% of phones are contaminated with at least some type of microorganism.

Wash the bottle with soap, bleach or vinegar: Rinsing your water bottle with just water is not enough to break down biofilm, explains Reynolds.

You have a few options. One is a diluted bleach rinse, either in the sink or in the dishwasher, says Reynolds. For the former, fill the sink with water, add a capful of bleach, and let the bottle sit in the mixture for a minute or two. Then rinse your bottle thoroughly with clean water. Note: use gloves, and never mix bleach with any other cleaners or disinfectants.

Basic dishwashing soap is another alternative. Use a brush to scrub the walls of the bottle. The more crevices, bumps and lumps, the more likely “bacteria can get a stronghold and multiply”, says Reynolds. But beware anti-microbial soap, which “does not have a place in a household, period”, says Kellogg Schwab, an environmental health and engineering professor at Johns Hopkins Bloomberg school of public health. These soaps can fuel antibiotic resistance.

Cleaning with vinegar can reduce biofilms too, says Schwab.

If you have a dishwasher, check the water bottle for the “dishwasher-safe” label first. Be aware that washing plastic in the dishwasher can release microplastics.

Hot water can be better than cold: The difference between using cold or hot water can be minimal, but hot is generally preferred because it is better for achieving a good soap lather, Schwab says. Lather time can positively influence washing efficacy.

Don’t forget the lid, mouthpiece or straw: These parts are frequently the most contaminated because they can trap moisture. Thus, it can be easier for microorganisms to build up, says Schwab, who recommends using a bottle brush for these areas.

Let the water bottle dry completely: Without moisture, bacteria have a harder time growing, Reynolds says.

Wash regularly and sensibly: It’s generally a good routine to wash and rinse daily.

“You’re not going to eat out of the same dirty cereal bowl every day,” says Reynolds. “So why would you drink out of a dirty water bottle every day?”

Certain conditions increase the risk of contamination, making it important to consider our habits when regularly cleaning your water bottle. For instance, leaving it in a hot environment such as your car or window ledge allows bacteria to grow more quickly.

If you can see or smell something, it’s too late: Bacterial growth won’t necessarily harm you immediately, but it can cause buildup of a slimy, gel-like matrix called biofilm, explains Schwab.

Biofilm can stick to surfaces like the sides of a water bottle. “Signs of biofilm could be taste and odor,” says Schwab. “There may be a kind of musty odor that’s just not enjoyable.”

If it grows for long enough, biofilm will become visible, much like mold growing in a shower. But if you can see it, you may be contending with millions to billions of microorganisms, says Reynolds. Generally, it could take as little as 100 micro-organisms to make you sick, she adds.

“If you can see or smell biofilm, you have way too much already,” she says. “That’s not a hygienic situation.”

Bacteria are the majority of microorganisms in a biofilm, but there’s also a chance of fungi, algae and viruses, says Schwab.

Choose your bottle wisely: Metal and glass water bottles are more inhibitory to bacterial growth and biofilm development than plastic, explains Schwab.

Regardless of the material, you must wash any water bottle regularly. “At a certain point, the bacteria can overcome a threshold of any material,” says Reynolds.

What happens if you don’t clean your water bottle?

Bacteria can easily grow in a moist microclimate, such as a water bottle. For example, under optimal conditions, E coli can double its population every 20 minutes.

Reynolds and her team recently conducted a survey at a water refill station, asking people if they would allow the water in their reusable bottles to be tested. The results are nott published yet, but early analysis suggests the water from the refill station itself was clean. The dregs of water left in bottles? Not so much. About half of the samples were contaminated with bacteria, and 10% contained E coli.

In Reynolds’s 2022 study analyzing different water sources, E coli was found in refillable water containers. Reusable bottles cleaned with soap and water had lower levels of E coli than those rinsed only with water.

Drinking water that contains E coli can cause symptoms such as stomach cramps, diarrhea and nausea. The risk of disease is not typically severe; still, “routine maintenance is simply good hygiene”, says Reynolds.

It’s also important to clean your water bottle if you have been sick, because you can reinfect yourself, says Reynolds. We can build immunity to some organisms, but it’s still best practice to clean the bottle before reuse.