Should You Still Drink Water That’s Been Sitting Out?

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

Published on December 29, 2025

Key Takeaways

Water that sits uncovered can develop a slightly off taste due to a change in pH from carbon dioxide exposure.
Using plastic bottles, especially in hot conditions, can cause chemicals to leach into the water.
For safety and taste, it is recommended to store water in covered glass or metal containers and refresh it daily.

Forgotten water in a glass or bottle at home or the office can still be safe to drink, but several factors play a role. The material of the container and whether it has a cover affect the water's quality. Understanding how these elements interact can help you decide if the water is still good to consume after sitting out.

Credit:

BHG / Sarah Crowley

What Happens to Water as It Sits?

Ever wonder what's happening in that glass of water on your nightstand while you sleep? A few interesting things occur.

First, if the water sits uncovered for eight hours or more, carbon dioxide from the air interacts with it. This reaction creates a weak acid, reducing the water's pH level. It's still safe to drink but the change in pH might explain why it tastes a bit off by morning.

With each sip from a glass or bottle, you introduce new bacteria, primarily from your mouth. While this usually isn't a cause for concern, sharing drinks introduces additional bacteria, and touching the glass or straw with unwashed hands can introduce more. Heat, such as that found in a car, can accelerate the growth of these bacteria.

If your open water vessel sits out, it can collect dust or small particles from the air. While these bits likely won't harm you, they might affect the taste.

Tap water contains chlorine to kill bacteria and viruses. However, chlorine evaporates when exposed to air, often dissipating in one to five days. If your water sits out too long, it will be susceptible to bacteria.

Does the Container Matter?

When choosing water containers, the material makes a difference. Remember the BPA-free movement from the late 2000s? BPA, found in some plastics, can disrupt hormones and is linked to health concerns like heart disease and reproductive issues. It's best to avoid it.

Many plastic bottles today are BPA-free, but they still come with risks. Polyethylene terephthalate (PET), the material in most plastic bottles, can release chemicals like antimony and phthalates when exposed to heat or sunlight. These can impact your endocrine system. While the ATSDR states that drinking from plastic bottles isn't known to cause health issues, it's wise to stay cautious.

Microplastics also pose concerns, as they can leach into your water, especially when bottles are heated or left for a long time. These tiny particles can build up in our bodies, but the health effects are still under study.

It might seem eco-friendly to wash and reuse disposable plastic bottles, but it raises the risk of chemicals leaching into your water. Keep track of the expiration dates on bottles, which signal the plastic's deterioration.

Considering all this, a glass or metal reusable bottle is a sound investment. Just note that metal bottles can warm quickly, speeding up any bacterial growth if left in hot places.

Tips for Keeping Your Water Safe

Day-old water is generally safe to drink as long as it wasn't exposed to extreme conditions or heat. However, it may not taste the best, especially if it was left uncovered. Here are some tips to keep your water safe:

Use a cover for your water and store it in glass or metal containers.
Choose reusable bottles with screw tops to keep germs from your hands or bag away.
If your water bottle contains old water, give the last bit to your houseplant instead.
Keep plastic water bottles in a cool, dark area and check their expiration dates.
Don't drink water left in a hot car.
If you're immunocompromised, it's best to fill a clean glass or fresh bottle of water each day.
Avoid sharing water. This can prevent spreading colds or cold sores.
For larger bottles, divide servings into a different vessel to minimize introducing bacteria.

If you’ve avoided sharing your water and it wasn’t in the sun for too long, your day-old (or even a bit older) water should be fine. Regardless, refill a fresh glass or bottle each day if you want the freshest-tasting water. This not only improves flavor; it also encourages you to drink more throughout the day. Happy hydrating!

Frequently Asked Questions

Wash your reusable water bottles daily to prevent bacteria buildup. If your bottle has a straw, it should be cleaned regularly using a brush or pipe cleaner for thorough cleaning.
Boiling your drinking water kills bacteria, but it doesn't make it last longer. New bacteria grow once you start drinking.
Yes, unsweetened coffee or tea left out for less than 8 hours is typically safe to drink, though the taste might not be as good. Brewed drinks can grow bacteria, so if your tea or coffee has milk or sweeteners, or if it's been sitting out for more than 8 hours, it's wise to throw it away.