What Happens if You Leave Tap Water Out?

January 9, 2026 •

4 min read

According to water experts, open tap water should not be left out for more than 24 hours due to an increased risk of contamination. So, what happens if you leave tap water out, and is it still safe to drink? From the evaporation of chlorine to the accumulation of airborne particles, several changes occur when a glass of water sits for an extended period.

Quick Summary

When left out, tap water changes in taste as protective chlorine dissipates, allowing airborne bacteria and dust to accumulate. The water's pH level also shifts slightly due to dissolved carbon dioxide, contributing to a stale taste. While unlikely to cause illness in most healthy individuals, proper storage is recommended for hygiene, taste, and safety.

Key Points

Taste Changes: Tap water left out will taste stale because protective chlorine evaporates and carbon dioxide from the air is absorbed.
Bacteria Accumulation: Once chlorine dissipates, bacteria and other microorganisms can grow, especially if the container is left open.
Health Risks (Generally Low): For most healthy individuals, drinking overnight-stale water is not dangerous, but those with weakened immune systems should be cautious.
Storage Matters: To keep water fresh and safe, store it in a clean, covered, and preferably cool environment like the refrigerator.
Reusing Bottles: Sipping from and reusing the same unwashed water bottle introduces mouth bacteria, which can multiply and pose a risk over time.
Container Material: Use glass or stainless steel instead of plastic, which can leach chemicals, particularly when exposed to heat.
Duration: As a general rule, it is best to drink water within 24 hours if left uncovered, or 3-5 days if refrigerated and sealed.

The Science Behind Stale Water

When you pour a fresh glass of tap water, it contains a few key components that make it taste crisp and clean. However, the moment it is exposed to the open air, these components begin to change. Understanding these scientific processes helps explain why that forgotten glass on your nightstand tastes different by morning.

1. Chlorine Dissipation Most municipal tap water is treated with chlorine to kill bacteria and other microorganisms, making it safe to drink. However, chlorine is a volatile compound, meaning it readily evaporates into the air. When left uncovered, a significant portion of this chlorine can dissipate in just a few hours. This loss of chlorine is one of the main reasons for the altered taste, as the water's protective barrier against bacterial growth is removed.

2. Absorption of Carbon Dioxide As the protective chlorine fades, another process begins: the absorption of carbon dioxide ($CO_2$) from the atmosphere. Water is a universal solvent and naturally absorbs surrounding gases. When $CO_2$ dissolves in water, it reacts to form carbonic acid ($H_2CO_3$), which slightly lowers the water's pH level. This slight increase in acidity is a major contributor to the flat or stale taste you notice.

3. Airborne Particles and Dust Accumulation An uncovered glass of water is also a magnet for airborne contaminants. This can include dust, skin cells, and other microscopic particles floating in the air of your home or office. While these particles are unlikely to make a healthy person sick, they can certainly affect the taste and overall hygiene of the water. For individuals with compromised immune systems, this accumulation of foreign particles presents a greater risk.

Potential Health Risks and Bacterial Growth

For most healthy adults, drinking tap water that has been left out overnight poses minimal risk, as any bacteria introduced are likely from your own environment. The key risks arise from extended exposure and potential external contamination. In warmer conditions, bacteria multiply much faster, making a glass of water left in a hot car, for example, a much riskier proposition.

Factors Influencing Risk:

Container Contamination: Drinking directly from a bottle can introduce bacteria from your mouth, which can then grow in the water over time.
External Contamination: An uncovered glass is susceptible to contaminants from its surroundings, including nearby household cleaners or microorganisms.
Immune System: People with weakened immune systems, such as those undergoing chemotherapy, should be more cautious about drinking stagnant water.

How to Keep Your Drinking Water Fresh and Safe

Preventing your water from going stale or becoming contaminated is simple. By following a few easy steps, you can ensure your water is always fresh and hygienic.

Use a Lid: Covering your glass or using a water bottle with a screw top is the most effective way to prevent dust and other particles from entering.
Clean Containers: Wash reusable bottles and glasses daily to remove bacteria and biofilms that can build up.
Refrigerate: Storing water in the fridge slows down bacterial growth, especially after the protective chlorine has evaporated.
Drink Quickly: The best way to enjoy fresh-tasting water is to drink it within a few hours of pouring. For overnight storage, a covered container is best.

Stored Water Comparison: Tap Water vs. Filtered vs. Bottled

Understanding the differences between types of stored water is crucial for safety and taste. The table below compares the characteristics and safety of water based on its source and how it's stored.


Feature	Unsealed Tap Water (Left Out)	Filtered Water (Fridge, Sealed)	Opened Bottled Water (Left Out)
Chlorine Content	Evaporates completely within 1-5 days.	Usually minimal to none, depending on the filter.	None. Contains no protective disinfectant.
Bacterial Growth	Increases as chlorine dissipates, especially in warmer conditions.	Minimal in a cool, sealed environment.	High risk due to no chlorine and mouth bacteria introduction.
Taste	Becomes stale or flat due to chlorine loss and CO2 absorption.	Maintained and fresh for longer due to sealed, cool storage.	Can taste stale or of plastic from the bottle.
Contaminants	Open to airborne dust and particles.	Generally protected if sealed and container is clean.	Risk of leaching from plastic, especially if exposed to heat.
Duration of Safety	Best consumed within 24 hours.	Up to 3-5 days when sealed and refrigerated.	Best consumed within hours of opening, especially if sipped directly.

Conclusion

While a glass of tap water left out for a few hours is highly unlikely to be harmful for most people, the taste will change and hygienic quality will diminish. The evaporation of protective chlorine, absorption of ambient carbon dioxide, and accumulation of airborne particles all contribute to the stale flavor and potential for bacterial growth. By taking simple precautions like using a covered container and keeping your water cool, you can ensure that the water you drink is not only safe but also refreshing. For those with compromised immune systems or those reusing the same bottle multiple times, extra vigilance is advised. For a definitive guide to water safety standards, consult the World Health Organization (WHO) guidelines.

WHO Guidelines on Drinking-water Quality

WHO is the leading authority on global health, including drinking water safety. You can access their comprehensive guidelines for more in-depth information on water quality, monitoring, and treatment. WHO Guidelines

Frequently Asked Questions

For most healthy individuals, it is generally safe, as bacteria from your own environment are unlikely to cause illness. However, the taste will be different due to the evaporation of chlorine and absorption of carbon dioxide, and it is less hygienic due to the accumulation of dust.

The change in taste is primarily caused by the evaporation of chlorine, a disinfectant in tap water, and the absorption of ambient carbon dioxide, which slightly lowers the water's pH and creates a flat or stale flavor.

While it won't go 'bad' in the same way food does, experts suggest drinking uncovered tap water within 24 hours. After that, the lack of chlorine allows bacteria to multiply, and the risk of contamination from airborne particles increases.

Yes. While a glass is more exposed to dust, drinking from a reusable bottle can introduce bacteria from your mouth, which will then incubate in the warm, stagnant water. Reusing unwashed bottles repeatedly increases this risk.

Yes. Storing water in a sealed container in the refrigerator significantly slows down the growth of bacteria. Refrigerated, sealed filtered water can be safe for 3-5 days.

You should avoid drinking this water. Heat accelerates bacterial growth in the water, especially if you have sipped from it before. It also increases the risk of chemicals from the plastic bottle leaching into the water.

Yes, 'stale water' refers to the phenomenon where water tastes flat or unpleasant after being left out. It is a real effect caused by the dissipation of chlorine and the absorption of atmospheric gases, resulting in a slightly more acidic and less clean-tasting liquid.