Wellness Health & Well-being Is Day-Old Water Safe to Drink? By Melissa Breyer Editorial Director Hunter College F.I.T., State University of New York Cornell University Melissa Breyer is Treehugger’s editorial director. She is a sustainability expert and author whose work has been published by the New York Times and National Geographic, among others. our editorial process Melissa Breyer Updated September 15, 2020 Aaron Amat / Shutterstock Share Twitter Pinterest Email Wellness Health & Well-being Clean Beauty And how about old bottled water or tap water that's been in storage? Do you drink water that’s been left sitting out overnight, or even for another day? Have you noticed it tastes different? What Makes It Taste Stale? Tap water that has been left to sit out slowly begins to acquire an off taste. Many assume that this is because of microorganisms. Treated water's added chlorine will take care of small communities of these little guys , but at room temperature they begin to multiply rapidly and can really get the party started. Even with clean water and a clean glass; one sip introduces a host of germs to the mix, in addition to whatever the water may have picked up by ambient dust. But that’s not what makes old water taste stale. For that we can thank carbon dioxide. After about 12 hours tap water starts to go flat as carbon dioxide in the air starts to mix with the water in the glass, lowering its pH and giving it an off taste. So Is It Safe to Drink? It's most likely safe to drink. However, back to those microorganisms. If you use a dirty glass day after day, there is more of a chance of unruly bacteria making themselves known; a risk that increases if you share the glass with another mouth as well. But assuming you use a fresh glass every few days, you likely won’t have a problem. Unless. Unless the rim of the glass has been touched by dirty fingers; and especially if those dirty fingers went unwashed after using the bathroom. With those kinds of germs left to multiply in a left-out glass of water, who needs enemies? What About Bottled Water? As for plastic water bottles that have been left out in the sun or the car, step away from the bottle, warns Dr. Kellogg Schwab, director of the Johns Hopkins Center for Water and Health, if it's a reusable water bottle that may contain BPA. “A chemical called bisphenol-A, or BPA, along with other things used to manufacture plastic can leach into your water if the bottle heats up or sits in the sun,” he explains. BPA, as you likely know, is a hormone disruptor that has been tentatively linked to everything from heart disease to cancer. Schwab also adds that plastic used for commercial bottled water isn’t meant to be washed or refilled, so use only one time and recycle. Or way better, don’t buy them at all; use a refillable water bottle instead. What About Old Water in Storage? If you’re concerned about drinking water that has been stored for a long time, it can also become unsafe to drink. Zane Satterfield, an engineer scientist with the National Environmental Services Center at West Virginia University, says most experts agree that tap water has a shelf life of six months. “After that point, the chlorine dissipates to the point that bacteria and algae start to grow.” And especially if you store water in a warm and/or sunny spot, or in a container that has not been properly cleaned and sealed.