How Long Can Water Sit Out, and Is It Safe to Drink?

We’ve all been there: cotton-mouthed after a night of sleep and reaching for a glass of water we left on the bedside table the night before. But just how long can water sit out?

You probably won’t get sick from last night’s H₂O, but what about that bottle of water that’s been sitting on your coffee table for a week? (Or has it been a month?) Before you gulp down your next glass of stale water, read on. We talked to Marc Leavey, MD, an internist with decades of medical experience, to get details on how long water can sit out, what may be lurking in it and what can happen if you drink old water.

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How long can water sit out?

The answer depends on where it’s been sitting and whether it was covered. Water is generally safe to drink for one to two days, provided it was not laying uncovered or contaminated. But the 12-hour mark is when it really starts to go downhill. After that point, the water begins to taste stale or can pick up funky odors or bacteria, depending on where it has been left to sit. No, it’s likely not harmful, but you may notice a difference in taste.

What happens as water sits out?

Water changes and reacts to its environment. According to the Environmental Protection Agency (EPA), stale water tastes “off” and lacks a refreshing quality because, once exposed to air, water begins to lose its oxygen content and absorbs CO₂, altering its pH and freshness. While it’s not particularly dangerous, it can be off-putting.

If your glass has been left uncovered or someone has taken a sip from the water, that’s when things start to get a little dicey. Even a closed container, like a clean bottle or pitcher, introduces problems, mainly because our skin is coated with sweat, dust, skin cells and even nasal discharge, and our saliva carries bacteria. Once we put the bottle in our mouth, these can all “wash back” into the remaining water, causing contamination.

Bacteria can begin to thrive after just a few hours in such a scenario. According to the Centers for Disease Control and Prevention (CDC), microorganisms thrive in environments with contamination, such as the backwash from a sip. 

“If it’s allowed to incubate for hours, that could potentially contaminate the water and make you ill by reintroducing that bacteria,” says Dr. Leavey. “Once you have put your lips to the bottle, you should consume that bottle in one sitting and then discard it.” 

Even if no one has taken a drink, the glass sitting near the kitchen sink could absorb all the odors from trash or cleaning products, making it unfit for drinking. 

What can get into the open glass?

Water in the uncovered glass on your nightstand should be thrown out in the morning, as drinking from a glass that’s been left out isn’t the most hygienic choice. Dust, airborne particles and even the odd passing mosquito can drop into the glass overnight, leaving an unhealthy surface scum.  

The right cup can be a nice barrier against contaminants. When it comes to storing water safely so it’s drinkable even when you wake up in the morning, covered containers are your best bet. 

Could you get sick from drinking stale water?

Let’s get real: Since it’s your own bacteria, it’s unlikely that you’ll actually get sick. Though no one brags about it, many people sip from used drinking glasses, mugs and bottles without any ill effects. But it’s certainly not advisable to share your bottle with someone else. People with reduced immune systems—such as transplant patients, those undergoing chemotherapy or anyone living with HIV/AIDS—shouldn’t be exposed to potentially contaminated water.

And it makes no difference whether it’s from the tap or bottled water. It’s a common myth that bottled water is cleaner than tap water. Both have to meet exacting hygiene standards, and up to 25% of bottled water is drawn from the main water supply anyway. 

OK, so how can I stay hydrated?

Of course, staying hydrated is good for our health, so it’s important to recognize the signs of dehydration. Dr. Leavey offers this advice to stay healthy: “Avoid putting your mouth to the bottle. Just pour it into a cup or pour it directly into your mouth.” To also keep yourself safe, don’t drink water that is past its expiration date. 

FAQs

Are some water glasses and bottles better options than others?

Closed containers with lids are your best bet.

Beyond that, some plastic bottles contain bisphenol A (BPA) or similar chemicals, which can leach into the water, especially when it’s exposed to sunlight. Some research suggests that BPA could be linked to health problems affecting the brain and behavior, although the Food and Drug Administration has stated that the level of BPA transference is within safe limits.

Using a BPA-free water bottle could eliminate this worry, but it won’t stop the growth of bacteria, especially if you’re using a metal bottle, which heats up quickly, encouraging the germs to multiply.

Does bottled water go bad if left in a hot car?

Water left in the sun will heat up, making it the perfect breeding ground for bacteria, especially if you’ve already drunk from it. Placing the bottle of water under your seat may reduce the heat a little, but bacteria will still grow.

Plus, heat may cause chemical leaching, even in BPA-free bottles.

Why does water have an expiration date?

It’s not the water that is to be blamed; it’s the bottle, especially if it’s a plastic bottle. If plastics are exposed to light or heat, they may leach chemicals, making the water taste stale and possibly harmful.

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