Most people don’t think twice about finishing a glass of water left out overnight, but experts say there’s a limit to how long leftover water remains safe to consume. The key factor isn’t necessarily airborne contaminants, but bacterial growth over time.
The 12-Hour Rule
Both registered dietitian Kristen Smith and microbiologist Jason Tetro (known as “The Germ Guy”) independently recommend against drinking water left standing for more than 12 hours. Smith follows this rule herself, discarding water after this point unless it’s been carefully covered to avoid contamination. Tetro explains that bacterial populations don’t significantly rise in the first 12 hours due to limited food sources, but beyond that, they begin to multiply rapidly.
Why Tap Water Matters
The risk isn’t just from the air; infrequently used taps can harbor bacteria. A study showed bacterial cell concentrations increase overnight in standing water. Even filtered water isn’t immune, as bacterial growth can still occur. Drinking water that’s been sitting for a full day increases the chance of gastrointestinal issues.
Sharing Is Risky
Drinking directly from a bottle or glass introduces bacteria from the mouth, accelerating contamination. Experts advise finishing the drink in one go or discarding it, especially if it’s been shared. This is because saliva contains microorganisms that can rapidly multiply in the remaining liquid.
Bottled Water & Staying Hydrated
Bottled water is designed to resist bacterial growth for longer periods, offering a slightly safer option. However, if you’re severely dehydrated, drinking stale water is still preferable to going without.
“If the only option you have is that water bottle that’s been sitting around for a day, it’s definitely better to drink than to stay dehydrated,” says Smith.
In conclusion, while a glass of water left out overnight is unlikely to cause harm, it’s best practice to discard it after 12 hours to avoid unnecessary bacterial exposure. Prioritizing fresh water minimizes the risk of illness while still ensuring proper hydration.
