Is It OK to Drink Water that’s Been Sitting Out for a While?

Here’s what happens to the water you leave out on your bedside table—plus how to make your water last.

We’ve all been there—you leave your water bottle on your desk all weekend, have too many glasses of water around the house to count, or leave a tumbler on your bedside table semi-permanently. You may wonder whether it’s safe to drink the water that’s been left behind, scattered across your house and/or office.

Does it make a difference if the glasses or bottles were covered or not? What about the material of the container they’ve been sitting in—is that important? While many single-use plastic water bottles have expiration dates, it can be a lot less clear what to do with water left in other containers. Let's take a deep dive on old water: Here’s everything you need to know about your bedside water stash.

BHG / Sarah Crowley

As you’re sound asleep, what’s happening in that glass of water sitting on your nightstand? Well, a few things.

First off, as your water sits, especially if it’s uncovered for eight or more hours, the carbon dioxide in the air starts to mix with it. This chemical reaction will result in the formation of weak acid, reducing the pH level of your water. While this water is still totally safe to drink, its pH is the first culprit of why that water tastes a little off the next morning.

Something else to keep in mind is that with every sip taken from your glass or water bottle, new bacteria is introduced. Most of the time this bacteria is coming from your mouth, but other times it could be from a loved one. Dirty fingers are also at play here—it’s so easy to grab the lip of a glass or the straw found on many reusable water bottles these days with unwashed hands.

The bacteria from our mouth is, of course, ours, and therefore unlikely to cause illness. However, sharing drinks with others and introducing the bacteria found on your hands—especially if they haven’t been washed recently—can be problematic, and can potentially introduce bacteria into your system that might make you sick. This risk becomes greater when our water is exposed to heat (especially the extreme temperatures of a car’s interior, for example), as heat accelerates bacteria growth.

If your water vessel is uncovered overnight or for a longer period of time, the water can also accumulate dust, debris, or other small particles from the air that circulates your home, office, car, or wherever else you keep it. While it’s very unlikely that these will cause you any harm, they may alter the flavor of your water the next day.

Tap water has chlorine in it for exactly this reason—it kills bacteria, parasites, and viruses. However, chlorine dissipates when exposed to air, and can be mostly absent from water after anywhere from one to five days. Depending on how long your water is sitting out for, it may have lost that built-in bacterial defense system by the time you pick it up again.

When it comes to the containers we store our water in, material matters. You’ve probably heard that plastic water bottles aren’t the best for your health—remember the BPA-free movement in the late 2000s? BPA, or bisphenol A, which was previously found readily in plastic water bottles, is a known endocrine system disruptor that interferes with our natural hormonal processes. Research has also shown that it’s highly likely that BPA is linked to several issues, including heart disease and reproductive concerns.

Even with the BPA-free plastic bottles that slid into that market gap, there is some risk. Polyethylene terephthalate (PET)—what most plastic water bottles are made of these days—can also leach harmful chemicals, particularly in warm conditions or in direct sunlight. Microplastics are also a concern here as they leach even more readily into water from plastic bottles and can accumulate in our organs, leading to a whole host of health concerns when consumed in large amounts.

Also of note here: If you do use single-use plastic bottles, avoid washing and reusing them. Despite the environmentally friendly mindset behind this move, doing so can increase the odds of microplastic and chemical leaching. The expiration dates found on these single-use bottles also indicate the time when more chemical leaching will occur, so it’s important to pay attention.

With all this information, there’s no better time to invest in a glass or metal reusable water bottle—that is, if you haven’t already! The only thing to be cautious of with these options is that metal reusable bottles can heat up rapidly, accelerating the growth of any bacteria that may be hanging out in your water.

When it comes down to it, day-old water isn’t topping the list of public health concerns. That old water on your countertop, water left overnight next to your bed, or even days-old water on your office desk from Friday are all okay and safe to drink. It’s very unlikely that they’ll cause you to be ill, but they just might not taste very good—especially if they’ve been left uncovered.

Here are a few a tips to follow if you want to be extra safe:

Basically, as long as you’re not sharing your water with everyone at the gym and then leaving it for a week, you should probably be totally safe to enjoy water that’s a day (or even a few days) old. If you want to play it super safe, strive to get a fresh glass or reusable bottle of water everyday—not only will it keep your water tasting delicious, but it will also encourage you to drink more, finishing that water instead of just leaving it on the counter. Happy hydrating!