Every time you take a sip from your reusable water bottle, you are depositing bacteria inside, and over the course of a day these can multiply by the millions, according to scientists. A study by Purdue University food safety expert Carl Behnke found that many users have poor cleaning habits, leading to a significant bacterial build-up.
The global reusable water bottle market was worth around $10bn (£7.7bn) in 2024, with studies showing that between 50% and 81% of university students use them. However, while they help people stay hydrated, they may also pose health risks if not cleaned properly.
Primrose Freestone, associate professor in clinical microbiology at the University of Leicester, explains that bacteria thrive at room temperature and can multiply rapidly in water left in bottles. A study in Singapore found that bacteria populations in bottles used by adults grew from around 75,000 per millilitre in mid-morning to over 1-2 million per millilitre within 24 hours.
Most contamination comes from the drinker, with bacteria from the mouth and hands transferring into the bottle. Poor hand hygiene can introduce faecal bacteria such as E. coli. Sharing bottles can also spread viruses like norovirus. Drinks containing sugar or protein, such as protein shakes, further encourage bacterial growth.
Experts recommend washing bottles daily with hot soapy water and allowing them to dry completely. Storing bottles in the fridge between uses can slow bacterial growth but does not eliminate it entirely.
