Experts warn that improper maintenance of reusable water bottles can lead to rapid bacterial growth, potentially harming health.
Bacteria can multiply at an alarming rate if a bottle is left at room temperature for extended periods or reused without proper cleaning.
Carl Behnke, an Associate Professor at Purdue University’s College of Health and Human Sciences, stated in an interview with BBC on Tuesday that if the inside of a water bottle feels slippery when wiped with a paper towel, it is not due to the bottle’s material but rather a sign of bacterial accumulation.
Experts emphasize how people use and maintain water bottles, significantly impacting bacterial growth. According to Primrose Freestone, a clinical microbiology expert at the University of Leicester, bacteria capable of causing infections in the human body multiply rapidly at body temperature (around 37°C), but they can also thrive at room temperature (around 20°C).
Room-Temperature Water in Bottles Can Rapidly Accelerate Bacterial Growth
Freestone explained that the longer water is stored in a bottle at room temperature, the faster bacteria will proliferate. A Singaporean study found that boiled water stored in a bottle from morning until night increased its bacterial count from approximately 75,000 per milliliter to between 1 million and 2 million per milliliter after 24 hours—a staggering 2,500% increase. Additionally, a U.S. study conducted by Water Filter Guru revealed that unwashed tumblers contained 40,000 more bacteria than a toilet.
Every time a water bottle is used, microorganisms from the mouth and hands enter, further accelerating bacterial growth. Freestone warned that most bacteria inside a water bottle originate from the user’s mouth and hands. If a person fails to wash their hands properly after using the restroom, E. coli and other harmful bacteria can be transferred into the bottle and reintroduced into the body.
Moreover, filling water bottles with drinks other than water can exponentially increase bacterial growth. Freestone noted that sugar-containing beverages provide a food source for bacteria, while milk or protein shake residues create an ideal environment for bacterial overgrowth.
While most bacteria do not pose significant risks to healthy individuals, they can cause diarrhea and vomiting in people with weakened immune systems.
Proper Cleaning Methods: Use Soap, a Brush, and Hot Water
Experts stress the importance of regularly and correctly cleaning water bottles. Freestone explained that simply rinsing with cold water is insufficient to remove the slippery bacterial layer inside the bottle. To effectively prevent bacterial growth, she recommends rinsing with hot water (above 140°C), scrubbing the interior with a brush and dish soap, and rinsing again with hot water.
Additionally, it is crucial to ensure the bottle is completely dry after washing. Any residual moisture can encourage bacteria to multiply again. If daily washing is not feasible, experts advise thoroughly cleaning the bottle at least several times a week. If the bottle starts emitting an odor, it should be replaced immediately.
Furthermore, experts suggest that high-quality stainless steel bottles are generally safer than plastic bottles, as they do not contain chemical additives like BPA or phthalates. Since the interior of a water bottle remains damp, it provides an ideal environment for bacteria and mold to grow—especially if sweetened drinks are stored without proper cleaning. For hygiene and safety, experts recommend cleaning water bottles daily with hot water and soap, allowing them to dry completely before reuse, and storing beverages other than water in disposable cups or dedicated containers to prevent bacterial contamination.
