This Microbiologist Is Sharing The Realities Of Drinking A Day-Old Glass Of Water

"Kristen Smith, a dietitian and spokeswoman for the Academy of Nutrition and Dietetics, said that she personally follows a 12-hour rule for a glass of water. After 12 hours, Smith will pour it out and drink a new glass. However, she said you can drink a day-old glass of water "as long as it hasn't been exposed to contaminants or left uncovered.""

"Microbiologist Jason Tetro, aka "The Germ Guy," also said 12 hours was the limit for drinking leftover water in glasses. To him, the potential bacterial problems wouldn't come from air contaminants but from the tap. One study found that bacterial cell concentrations in drinking water increase overnight."

"Infrequently used drinking water taps can also harbor high levels of bacteria. "For those first 12 hours, there's not going to be enough food for the bacteria numbers to rise," Tetro said. "After those 12 hours, there will be food for the bacteria to multiply.""

"Sharing water with someone else speeds up the timeline of when you should just dump it. "Once a person drinks directly from a bottle or glass, bacteria from their mouth can transfer to the remaining liquid and begin to multiply," Smith said. "So for that reason, after you put your mouth"

Experts recommend discarding leftover water after about 12 hours. A day-old glass can be acceptable if it has not been exposed to contaminants and has not been left uncovered. Microbiological concerns are linked to bacteria already present in tap water and the conditions that allow bacterial growth overnight. Infrequently used taps can contain higher bacterial levels, and after roughly 12 hours there is enough “food” for bacteria to multiply. Water left for a whole day can become a growing environment for opportunistic pathogens and may increase the risk of gastrointestinal issues. Sharing water accelerates bacterial transfer from the mouth into the remaining liquid, shortening the safe timeframe.

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