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When in doubt, toss it.

Reviewed by Dietitian Mandy Enright, M.S., RDN, RYT

Key Points

• Water left sitting out doesn’t spoil but can become contaminated by environmental factors.
• Be wary if water looks cloudy, tastes off or has visible particles floating in it.
• While there is low risk in drinking day-old water, replace with a fresh glass if doubtful.

If you were to do a walk-through of your home right now, chances are you would find at least one abandoned glass of water sitting out somewhere—likely on the kitchen countertop, nightstand or coffee table. Maybe you took a sip before rushing out the door or filled up a glass before bed, only for it to go untouched while you were sleeping. When you finally do notice it, you might wonder, Is this still OK to drink?

We tapped an expert to learn more about the mysteries of “stale” water. The short answer: It’s probably OK to drink—with a few caveats to consider. Here’s what you need to know before taking a sip from yesterday’s glass.

What Happens When Water Sits Overnight

“Water itself doesn’t spoil—but once it’s been opened or handled, it starts to pick up microbes from its environment,” says David T. Dyjack, Dr.P.H., CIHexecutive director at the National Environmental Health Association. These microorganisms—including bacteria—are typically present in most water, including treated tap water, which means they’re already in your cup. But they can also come into contact with your water in a myriad of other ways.

Merely taking a sip or handling the rim of the cup can introduce bacteria from your body into the water. And anything floating around the air in your house can get into the water, too—think dust and dander and even carbon dioxide, which is inherently highly water soluble in ambient conditions.

Bacterial Growth & Contamination Risks

Dyjack says that the potential for and the speed of microbial growth are greatly dependent on the water itself, the drinking vessel and your home’s environment. He explained that water from local water treatment facilities can take anywhere from a few hours to a few days to arrive at your faucet—long enough for much of the added disinfectants, like chlorine and chloramine, to break down and escape as they travel through the pipes. “[Remaining] disinfectant generally evaporates within 8 to 24 hours after coming out of the faucet, at which time normal airborne bacteria and fungi can land in the water and possibly take up residence,” says Dyjack, noting that well water is usually not treated, a factor that can increase the rate at which germs grow.

Combine low levels of disinfectants with conditions such as a high indoor temperature or exposure to direct sunlight, and even cups with large openings, and Dyjack says the rate of off-gassing and microbial growth can quickly increase. While it’s unlikely that taking a sip of “stale” water will cause serious illness in a healthy adult, one study found an association between microbiologically contaminated drinking water and the risk of infectious diseases such as diarrhea.

Signs of Unsafe Drinking Water

According to Dyjack, harmful microbes often do not change the taste, smell or appearance of water, making it difficult to identify when it might be unsafe to drink. Still, he offered a few possible warning signs, along with these smart words of advice: “If it’s been sitting out for more than a day, it’s safer to refresh than to guess.”

• Off taste. Oxygen dissolves from water over time, leading to what some call a “flat” or “off” taste. Absorption of carbon dioxide can cause a bitter or metallic taste. And water that has been left in direct sunlight often forms a sweet flavor.
• Musty or unusual odor. Bacterial growth and organic waste, including food particles, can cause water to have a musty odor.
• Cloudiness or visible particles. Water that has floating or settled food and dust particles can increase the chance of bacterial growth.

Tips for Safer Water Consumption

Dyjack shares these tips that can help decrease your risk of drinking unsafe water, both at home and on the go.

• Drink treated or filtered water. When possible, choose clean drinking water that has been treated with disinfectants.
• Replace water regularly. A general rule of thumb is change your water glass every 24 hours for a healthy adult and every 8 to 12 hours for immunocompromised persons.
• Do not leave water in hot temperatures or direct sunlight. This is especially important for water stored in plastic bottles, as the hot conditions can release microplastics or other by-products associated with plastic into the water.

The Bottom Line

Water left sitting out overnight doesn’t spoil, but, according to an environmental health expert, it can become contaminated by airborne particles, bacteria from your mouth and environmental factors. Although the risk of illness is low for most healthy people, changes in taste, odor or appearance can signal contamination. When in doubt, it’s safer to replace the water—ideally within 24 hours or sooner for those with weakened immune systems. It’s also best stored covered and out of heat or direct sunlight.