Is It Safe to Drink Water Left Out Overnight? Experts Reveal the Risks

Are you the type of person who notices a glass of water left on the nightstand or kitchen counter and drinks it without a second thought? Or do you eye it suspiciously and immediately pour it down the drain? If you belong to the first group, you may want to reconsider.

In an article published by HuffPost, nutrition and microbiology experts explained where to draw the line between water that is still safe to drink and water that is better left untouched.

What Happens to Water Left Sitting in a Glass?

If you've always assumed that the greatest danger lurking in a glass of water left out overnight is dust or flying insects, microbiologists suggest that the real concern may already be present in the water itself.

Microbiologist Jason Tetro explains that drinking water naturally contains low concentrations of bacterial cells. When water flows from the tap, these concentrations are extremely low and generally considered safe for consumption. However, when water is left standing in an uncovered glass, conditions can gradually change.

"After 12 hours, it's better to simply pour out the water and fill a fresh glass," advises Christine Smith, a registered dietitian and spokesperson for the Academy of Nutrition and Dietetics. According to Smith, water remains relatively safe within that period, provided it has not been exposed to external contaminants or left completely uncovered.

Tetro supports this recommendation, explaining the biological process that takes place inside the glass. "During the first 12 hours, there isn't enough organic material in the glass—such as dust, tiny skin particles, or other microscopic residue—for bacterial populations to increase significantly. But once that period passes, conditions can develop that allow bacteria to multiply more rapidly."

Why Even Filtered Water May Become a Concern

Bacterial concentrations in drinking water can naturally increase over time, particularly in water that has been sitting overnight or in plumbing fixtures that are rarely used.

According to Tetro, water left in a glass for an entire day can become a breeding ground for potentially harmful microorganisms. Drinking it could lead to digestive discomfort or stomach pain, even if the water originally passed through an advanced filtration system.

The takeaway is simple: If a glass of water has been sitting out for many hours, particularly uncovered, pouring yourself a fresh glass is a sensible precaution.