Our reusable water bottles have become an everyday essential that we drop into our handbags just as frequently as our phones and house keys. In fact, 60% of people in the UK carried a reusable water bottle in 2023, according to eco-campaign Refill. This is all great news for the amount of plastic clogging up Earth’s oceans and our hydration levels, but, when you use a water bottle so frequently, it can be hard to keep tabs on when you last put it through the dishwasher.

While we may think reusable water bottles are clean because we’re constantly rinsing and replenishing them with water, their hidden nooks and crannies can harbour an alarming amount of bacteria and viruses. 

A recent independent study by WaterFilterGuru.com found that the average reusable water bottle had 40,000 times the microbes of a toilet seat and 14 times the bacteria of a pet bowl. Worryingly, it also found that a quarter of the study’s respondents wash their water bottles a few times per week and over 10% only clean them a few times per month.

The study, which swabbed a selection of different water bottles and analysed the results in a lab, found there were two types of bacteria present on the water bottles it tested: gram-negative rods and bacillus. Gram-negative bacteria are usually associated with hospital infections and also have a high resistance to antibiotics. Bacillus can cause gastrointestinal issues.

Another study conducted earlier this year and published in the National Library of Medicine, which swabbed 30 personal water bottles from a selection of first-year medical graduate students, found that bottles “can serve as reservoirs for a variety of harmful bacteria, fungus and other microbes” and suggested that while people have been encouraged to use reusable water bottles in recent years, there was an “unawareness of water bottle hygiene [that] needs to be addressed”. 

If you’re a constant water bottle user, you’ll be pleased to know that both studies found a simple wash in the kitchen sink can reduce the amount of bacteria on water bottles significantly.

All of the water bottles tested in the 2024 study, had a dramatic reduction in bacterial load after being washed, with the majority of the water bottles showing negligible amounts of bacteria post-wash. So how often should we be washing our water bottles if we want them to remain squeaky clean and bacteria-free?

Well, the study found that the longer water bottles were left before cleaning, the higher the bacterial load. So ideally, it suggests we should be washing our water bottles every day while we’re using them. If you’re drinking anything other than simply water from them, you should think about washing them after every use. 

Both the material a water bottle is made from and the kind of lid it has can affect how much bacteria it attracts.

The 2024 study found that stainless steel water bottles had a far lower bacterial load than those made of plastic, while the WaterFilterGuru study found that bottles with spout tops and screw-top lids contained the most bacteria of all the water bottles tested. 

Luckily, good old soap and water will do the trick when it comes to scrubbing off bacteria from our water bottles. “Empty your reusable bottle so no liquid is inside,” says John Girvan from kitchen equipment company Alliance Online. “If there are multiple parts to your bottle like a lid, straw or lid cover, make sure you disassemble it all to get a thorough clean. Clean the individual parts of the bottle and consider purchasing a bottle brush to ensure you clean the bottle and its edges properly. Ensure you rinse each part of the bottle thoroughly to remove any remaining soap or bubbles. Then gently pat each bottle part to dry or leave to air dry on a drying rack.”

If you notice that your water bottle is retaining unwanted flavours or smells, try using diluted vinegar and soaking it overnight. Making a cleaning paste from water and baking soda will also help give neglected water bottles a shine.

Girvan suggests avoiding steel, wool and harsh chemicals to clean water bottles as these can cause damage. Also be aware of not drying the bottles properly. “This can encourage mould or mildew to grow,“ says Girvan. When it comes to putting bottles in the dishwasher, Girvan recommends reading the manufacturer’s instructions and only putting it through a cycle if specified. 

So, if you’re as wedded to your trusty water bottle as I am, spare some time to give it a regular clean. Your stomach will thank you for it. 

Images: Getty