What Happens to Water That Sits Out?
When a glass of water is left uncovered, it is exposed to the surrounding environment, which initiates a number of subtle but significant changes.
Airborne Contamination
Dust particles, dirt, and microscopic debris floating in the air can settle into an uncovered cup. While these are unlikely to cause serious harm, they compromise the water's hygiene. In some cases, airborne mold spores or bacteria can also find their way into the water.
Chlorine Dissipation
Tap water is treated with chlorine to kill bacteria and other microorganisms. When left uncovered, this chlorine dissipates into the air over time. This leaves the water without its built-in disinfectant, allowing bacteria that enter the cup to multiply more easily. The rate of dissipation varies, but tap water's protective barrier significantly weakens within 12 to 24 hours.
Chemical Absorption
Water is an excellent solvent and can absorb gases from its surroundings. Most notably, it absorbs carbon dioxide from the air, which lowers its pH and makes it slightly more acidic. This is the primary reason that water left out for a while develops a 'stale' or 'flat' taste. While not harmful, it is unappealing.
The Role of Bacteria and Personal Hygiene
Bacteria are a primary concern when it comes to drinking from a used cup. The moment you take a sip, you introduce microbes from your mouth into the water.
Biofilm Formation
If the cup is reused multiple times, or left out for an extended period, these bacteria can grow and form a sticky layer known as biofilm. Biofilms are hard to remove and can harbor harmful microbes that may cause gastrointestinal discomfort or other illnesses. This is especially true for reusable bottles with tight lids, which can be perfect breeding grounds for bacteria if not washed daily.
Health Risks
For a healthy individual, drinking from their own cup after a few hours is low-risk, as their body is adapted to their own bacteria. However, the risk increases if the water is shared with others, or if the individual has a compromised immune system. In such cases, introducing foreign bacteria could lead to illness.
How the Container and Environment Matter
Not all containers are created equal, and the storage environment plays a crucial role in water safety.
Comparison: Glass vs. Plastic vs. Metal
| Feature | Glass Cup | Plastic Cup (Reusable) | Plastic Bottle (Single-Use) |
|---|---|---|---|
| Chemical Leaching | Minimal to none | Can leach BPA/microplastics over time, especially with heat. | Designed for single use; leaching risk increases over time and with heat. |
| Bacterial Growth | Non-porous; easy to clean, resisting bacterial adhesion. | Can harbor bacteria in scratches and crevices if not cleaned properly. | Not designed for reuse; scratches can trap bacteria. |
| Odor/Taste | Non-absorbent, preserving water's neutral taste. | Can absorb odors and tastes over time. | Can absorb flavors from the surrounding environment. |
| Best Practice | Ideal for overnight storage, especially when covered. | Needs daily, thorough cleaning. | Avoid reusing; discard after a few days. |
The Temperature Effect
Temperature directly influences bacterial growth. Water left in a warm environment, such as a hot car or a sunlit windowsill, will encourage bacteria to multiply rapidly. Conversely, refrigerating water significantly slows this process, keeping it safer for longer.
Practical Steps to Ensure Your Water Is Safe
To minimize the risks associated with sitting water, follow these simple guidelines:
- Keep it covered: A simple coaster or lid can prevent airborne dust and debris from settling in your cup, protecting the water's hygiene.
- Wash thoroughly: Always wash your cup or reusable bottle daily with hot, soapy water to eliminate bacterial build-up, especially if you've been drinking directly from it.
- Choose the right container: Opt for glass or stainless steel for reusable containers, as they are non-porous and do not leach chemicals.
- Use your senses: If the water looks cloudy, has an unusual smell, or tastes off, it is best to pour it out and get a fresh glass.
- When in doubt, throw it out: This is especially true for water that has been sitting out for more than 24 hours, or if it was left in a hot environment.
Conclusion
For most people, drinking a glass of tap water left out overnight is unlikely to cause serious illness, although the taste may be affected. However, the safety of sitting water decreases over time, particularly in warm, uncovered, or unhygienic conditions. The key factors to consider are the duration, temperature, and container material. By adopting simple practices like covering your cup, choosing safe materials, and refreshing your water daily, you can enjoy clean and fresh hydration with peace of mind. For ongoing water safety, regular washing of all drinking vessels is the most effective prevention. The ultimate takeaway is that while a few hours is usually fine, opting for a fresh pour is always the safest and most refreshing choice.
For more information on waterborne pathogens and guidelines for drinking water quality, you can consult the World Health Organization's (WHO) fact sheets on drinking-water.
