Does a glass of water ever go bad? Experts weigh in.

Chances are, you have a glass of water or a water bottle on your nightstand to drink with medicine or just in case you get thirsty before heading to bed. After all, as water expert and America’s first water sommelier Martin Riese, tells Yahoo Life: “Hydration is key for everything, and it’s completely underrated.”

We know we need to drink a lot of water, but is that water you’ve left out overnight actually OK to drink the next morning or even a couple of days later? And why does the taste of water seem to change overnight? Here’s what experts have to say.

Why does water taste different after it’s been sitting out for 24 hours?

Chuanwu Xi, molecular microbiologist and microbial ecologist at University of Michigan School of Public Health, tells Yahoo Life that many factors can result in that stale, unpleasant taste of water that’s been sitting out overnight. Over time, chlorine that has been added to tap water to help disinfect it evaporates and changes the taste profile. Carbon dioxide dissolves in water, lowering the pH level and making it more acidic. There may also be changes in mineral content.

Water will always react to everything around it because it is a universal solvent, explains Riese. “It takes odors and it takes flavors,” he says. For example, if you leave an open glass of water next to coffee, the water will smell a bit like coffee the next day, even if it doesn’t make any contact. It’s one of the reasons the CDC advises against storing water near any cleaning products or chemicals, especially if it’s in a plastic container.

To keep your water tasting fresh, make sure to close the lid or cover your glass to help keep out additional bacteria and reduce the amount of carbon dioxide dissolving in your water.

Does water ever go bad or expire?

As a general rule for tap water, it’s not good to keep it in the pipes for more than one week, notes Xi. Tap water is not sterile and may contain waterborne germs, such as bacteria, fungi and amebas, which form a biofilm barrier to water treatment chemicals — mainly chlorine and chloramine — making them more likely to grow when water is stagnant in pipes.

Keeping water in a container for a long time can also lead to taste changes and regrowth of microbes in water, says Xi. But storing water in a fridge helps prolong its freshness, compared to letting it stay at room temperature or near heat, which increases the chances of bacteria growing in your water.

The Centers for Disease Control and Prevention (CDC) advises replacing any stored tap water, i.e. non-store-bought water, every six months — even under ideal conditions of 50 to 70 degrees Fahrenheit and out of direct sunlight.

For bottled water, Riese explains that most American water brands will have an expiration date that is one to two years away. But that’s more for the container, particularly those made of plastic, than for the water itself. That’s because when a plastic bottle starts to break down, chemicals can leach from the bottle into the water.

Can old water make you sick?

Experts agree that old water contains more bacteria and could potentially make you sick, though it does depend on what type of bacteria is present and the health status of the person drinking it. While your own bacteria won’t likely cause any serious health problems, bacteria from sharing a water bottle or glass or from touching other items like keyboards and door handles is the real concern.

Finally, it goes without saying that “if a strange color, odor or taste of the water is observed, better not drink those,” warns Xi.

Does water last longer or taste better when it’s kept in aluminum water bottles?

Aluminum bottles don’t necessarily make water last longer or taste better, says Xi.

Riese also explains that “an aluminum bottle is not just aluminum.” Water reacts with aluminum, so aluminum bottles typically have a thin layer of plastic or paint in the bottle too.

The main concern here is that the epoxy resin inner layers in some aluminum water bottles can contain Bisphenol A (BPA) that can leach into the water. The FDA continues to deem these bottles safe, despite many studies showing BPA might cause health concerns for fertility, birth defects, type 2 diabetes, heart disease and cancer.

So what's the best reusable bottle for drinking water?

Both experts agree it’s best to store water in glass bottles with closed caps. Riese is a strong believer in glass bottles, “as glass does not give anything to water or of water, so it’s the perfect container for water.”

If you’re nervous about breaking a glass bottle, stainless steel is another good option because it’s corrosion resistant and does not leach chemicals like plastic.

How often do you need to wash a reusable water bottle?

As a best practice, Xi recommends washing your water bottles at least once a day.

After drinking from the same bottle all day, “you’ll feel like something is off, that it smells or it doesn’t taste right anymore,” says Riese. He even advises washing your water bottle a few times throughout the day. That may sound like a bit much, but compare that to using the same fork all day long without washing it — you probably wouldn’t.

Now how much water do we actually need on a daily basis?

You’ll often hear that we need to drink 8 cups of water daily. However, the CDC says there are no exact recommendations for how much water a person needs. An individual’s water needs depend on many factors, including age, sex, weight, if you’re pregnant or breastfeeding, how much activity you do each day and your overall health status.

But in general, the U.S. National Academies of Sciences, Engineering, and Medicine states on average, women need about 11.5 cups of water per day and men about 15.5 cups. It’s also important to note that about 20% of your water intake comes from food, such as fruits and vegetables, as long as you eat a varied diet.

The best way to tell if you’re hydrated, though, is to check your urine. If it’s a pale yellow color, you’re likely getting enough water. If it’s dark yellow and has a strong odor, you might want to drink more water. Just make sure you use a clean drinking glass or bottle and keep it covered If you leave it out!

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