Most people have a glass of water on their nightstand or a reusable bottle on their desk that sits for hours, or even overnight. It looks and smells fine, but is it actually safe to drink? The science behind stale water involves a few key processes that can impact both its flavor and, in some rare cases, its safety. While day-old water isn't typically a major health risk, certain conditions can make it a concern, especially for vulnerable individuals.
Why Does Sitting Water Taste Different?
The most noticeable change in sitting water is often a stale or flat taste. This isn't because the water has gone 'bad', but rather because of a simple chemical reaction with the air.
- Carbon Dioxide Absorption: When water is exposed to the air, it absorbs carbon dioxide ($CO_2$). This reacts with the water molecules ($H_2O$) to form carbonic acid ($H_2CO_3$), which slightly lowers the water's pH. This mild acidity is what gives the water a slightly different, and often less pleasant, taste. This is the same principle used to create carbonated soda water, just on a much smaller, slower scale.
- Chlorine Dissipation: Tap water is treated with chlorine to kill bacteria and make it safe for drinking. When left exposed, this chlorine evaporates over time. While this removes the chlorine smell, it also removes the disinfectant, making the water more susceptible to bacterial growth.
Is Stale Water Dangerous to Drink?
For most healthy adults, drinking water that has been sitting out overnight or for a day or two poses very little risk. However, the safety can quickly diminish depending on the circumstances, particularly if bacterial growth is encouraged.
The Impact of Backwash and Contamination
If you drink directly from a bottle or glass and then let it sit, you introduce bacteria from your mouth into the water. This provides a nutrient-rich environment for microbes to grow. While the bacteria are your own and unlikely to cause illness, the population can multiply rapidly, especially in warm conditions. Sharing this water with others, however, introduces foreign bacteria and increases the risk of spreading illness.
Heat and UV Exposure
Leaving a plastic water bottle in a hot car is particularly risky. The combination of warmth and bacteria from your mouth creates a perfect breeding ground. Additionally, heat and UV light can accelerate the leaching of chemicals like antimony from plastic bottles into the water. While most modern bottles are BPA-free, other chemicals can still transfer, though health impacts are still under study.
Dust and Airborne Particles
An uncovered glass of water can collect dust, airborne microbes, and other debris from its surroundings. While usually not harmful in small amounts, it's not the most hygienic choice, especially in areas with a lot of dust or pollen.
Comparison: Risks in Different Water Storage Methods
| Storage Method | Bacterial Risk | Chemical Leaching Risk | Taste Changes | Recommended Duration |
|---|---|---|---|---|
| Uncovered Glass | Moderate (from air and backwash). | None (glass is inert). | Yes, can taste stale from $CO_2$ and absorbed odors. | Best to discard and refill daily. |
| Reusable Bottle (Glass/Stainless Steel) | Low (if kept clean). High (if not washed daily). | None. | Little to none. | Drink within a day; wash daily. |
| Sealed Plastic Bottle (Reusable) | Low (if kept clean). High (if not washed daily). | Low (but possible with heat/UV exposure). | Possible (gas permeation). | Drink within a day; wash daily. |
| Sealed Plastic Bottle (Disposable, Hot Car) | Very High (warmth accelerates growth). | High (heat accelerates leaching). | Yes, often tastes like plastic. | Discard immediately. |
| Sealed Tap Water (Fridge) | Very Low (cool temps inhibit growth). | Varies by container (use glass or steel). | Low (sealed prevents gas exchange). | Up to 24 hours. |
Best Practices for Safe Hydration
- Cover Your Water: Always keep your glass or container covered to protect it from dust and airborne contaminants.
- Wash Reusable Bottles Daily: Regularly and thoroughly wash reusable bottles, especially those with narrow openings or straws, to prevent bacteria and biofilm from building up.
- Avoid Heat Exposure: Never drink water from a plastic bottle that has been left in a hot car, as the heat can promote bacterial growth and chemical leaching.
- Don't Share: Avoid sharing water bottles with others to prevent the exchange of bacteria, particularly if someone is sick.
- Pour, Don't Sip: If you are drinking from a larger container, pour water into a fresh glass instead of drinking directly from the source to prevent backwash contamination.
The Final Sip: When to Trust and When to Toss
When it comes to drinking water that has been sitting out, your senses are a reliable guide. If the water looks cloudy, has floating particles, or smells or tastes off, it is best to discard it and get fresh water. While a glass of water from your bedside table is likely safe to finish in the morning, leaving it for several days, or in unfavorable conditions, significantly increases the risk of contamination. For optimal health and taste, prioritize freshness. Use clean, sealed glass or stainless steel containers, and when in doubt, pour it out.
Conclusion
Drinking a glass of water left overnight is generally safe for most people, though it may have a slightly altered taste due to absorbing atmospheric carbon dioxide. However, the risk of contamination increases significantly with time, heat exposure, and direct mouth-to-bottle contact, which encourages bacterial growth. For maximum safety and optimal taste, use covered containers, wash reusable bottles frequently, and when unsure, opt for a fresh, clean serving. Ultimately, the freshest water is the safest and most enjoyable. For more detailed information on safe water storage and handling, consult the Centers for Disease Control and Prevention guidelines.
