What Happens to Water When It Sits Out?
When a glass of water sits out, several things happen that affect its taste and hygiene. While water itself does not 'spoil,' external factors begin to change its composition over time. The primary culprits are exposure to air, temperature fluctuations, and contact with airborne particles or microbes from a person's mouth.
Airborne Contamination and Chemical Changes
Water is an excellent solvent, meaning it absorbs substances from its environment. Left uncovered, it can absorb odors and dust particles from the surrounding air. This is why a glass left near cleaning supplies might acquire a chemical scent.
Furthermore, water absorbs carbon dioxide from the air. This chemical reaction creates a weak acid that slightly lowers the water's pH, which can explain the distinct, slightly flat taste of water left out overnight. While this change is harmless, it significantly affects the flavor profile.
Bacterial Growth and Biofilm Formation
One of the most significant concerns with stagnant water is bacterial growth. Even the cleanest glass can become a breeding ground for microorganisms, especially once someone has taken a sip. Saliva introduces bacteria from your mouth into the water, and if the cup is left for hours, particularly at room temperature, these microbes can multiply. This is exacerbated in warmer conditions where bacteria thrive, and is why leaving a water bottle in a hot car is strongly advised against.
In some plumbing systems, stagnant water can also lead to the formation of biofilms—slimy layers of bacteria that grow on surfaces. While this is more common in pipes, it illustrates how easily bacteria can flourish in still water. For this reason, refilling a fresh cup daily is the most hygienic practice.
Water Safety Based on Type and Storage
The safety and longevity of your water also depend heavily on its source and how it is stored.
- Unsealed Tap Water: Tap water contains small amounts of chlorine to keep it disinfected. When left in an open cup, this chlorine evaporates, typically within 12 to 24 hours, leaving the water more vulnerable to bacterial growth. Discard water that has been sitting uncovered for more than a day, or even sooner if it's warm.
- Sealed Tap Water: When stored correctly in a sealed, food-grade container and kept in a cool, dark place, tap water can be safely stored for up to six months.
- Filtered Water: Since filtration often removes chlorine, filtered water lacks a residual disinfectant and can be more susceptible to bacterial contamination than regular tap water. It should be consumed within a few hours if left at room temperature and ideally stored in a sealed glass container in the fridge for up to 3–5 days.
- Store-Bought Bottled Water (Plastic): The expiration date on bottled water refers to the plastic bottle itself, not the water. Over time, chemicals from the plastic can leach into the water, a process accelerated by heat and sunlight. An opened plastic bottle is prone to bacterial contamination, especially if you drink directly from it, and should be consumed within 24 hours.
Comparison of Water Storage Longevity
| Storage Method | Typical Longevity | Key Considerations |
|---|---|---|
| Uncovered Cup/Glass | Overnight (8-12 hours) | Taste can become stale; potential for airborne dust or bacteria. |
| Covered Cup/Glass | Up to 24 hours | Protects from dust, but bacterial growth can still occur, especially if warm. |
| Sealed Food-Grade Container (Cool/Dark) | 6 months+ (with rotation) | Requires initial sanitation and proper storage conditions. |
| Sealed Filtered Water (Refrigerated) | 3-5 days | Chlorine has been removed, so refrigeration is key for hygiene. |
| Opened Plastic Bottle | Finish within 24 hours | Mouth contact introduces bacteria; avoid heat and reusing disposable bottles. |
Conclusion
While a glass of water left out overnight is unlikely to cause serious harm, especially if you are not immunocompromised, its quality degrades quickly. The presence of airborne dust, evaporating chlorine, and potential bacterial growth from sipping all contribute to a stale taste and diminished hygiene over time. For optimal freshness and safety, it's best to start with a fresh glass each day. If you choose to drink water that has been sitting out, covering the cup can reduce contamination. Those with weakened immune systems should be particularly vigilant about drinking fresh water from a clean vessel.
Actionable advice for optimal water quality
- Cover Your Cup: To prevent dust and other particles from settling, always cover a glass of water that you intend to drink later.
- Refill Daily: For the freshest taste and minimal risk of bacteria, make it a habit to pour out and refill water daily.
- Use Stainless Steel or Glass: Opt for glass or stainless steel containers for storing water, as these materials do not leach chemicals and are less porous than plastic.
- Avoid Contamination: Never share an unwashed bottle or cup, as backwash from saliva is a major source of bacterial contamination.
- Keep It Cool: Store water containers in a cool, dark place to inhibit bacterial growth and prevent chemical leaching from plastic.
- Clean Reusable Bottles: Wash reusable water bottles daily with hot, soapy water to prevent bacterial buildup.
These simple habits can significantly improve the quality and safety of your drinking water, ensuring you stay properly hydrated with a clean and fresh supply.
