Why a week is too long for stagnant water
While water itself does not technically expire, the environment it is left in for an extended period, like a week, causes significant degradation in quality. The primary issue with stagnant water is bacterial growth. Once water is exposed to the air or an unsterile container, microorganisms from the air and any previous sips are introduced. Over several days, these microbes can multiply, forming biofilms—sticky, slimy layers of bacteria that cling to the inside of containers. These biofilms can contain pathogens that cause stomach upset, diarrhea, and other infections.
Tap water initially contains chlorine, which acts as a disinfectant. However, when left uncovered, this chlorine evaporates within one to five days, removing the built-in protection against microbial invaders. Without this defense, stagnant water at room temperature becomes an ideal breeding ground for bacteria. Warm temperatures, such as those found in a hot car, further accelerate this growth exponentially, turning a bottle of water into a bacterial soup in a matter of days.
The dangers of plastic containers
The type of container used to store water for a week is a critical factor in determining its safety. Single-use plastic bottles, typically made from polyethylene terephthalate (PET), are not designed for repeated use or long-term storage. These plastics are slightly permeable, allowing ambient gases and odors to affect the water's taste. More concerning, exposure to heat and UV light can cause the plastic to degrade, leading to the leaching of harmful chemicals like Bisphenol A (BPA) and microplastics into the water. Some research has linked BPA exposure to hormonal disruptions and other health issues. For this reason, leaving water in a plastic bottle in a hot car for a week is especially dangerous. Safer alternatives for storage include glass or food-grade stainless steel containers, which do not leach chemicals.
Risks of uncovered water
Leaving a glass or bottle uncovered for a week exposes the water to environmental contaminants. Dust, dirt, insect larvae, and airborne particles can all settle into the liquid over time. While a single dust particle is unlikely to cause harm, the accumulation of microscopic debris can introduce pathogens and affect the water's cleanliness and taste. A sealed container, therefore, is a necessary first step for any storage beyond a few hours.
Safe practices for drinking water
To ensure your drinking water is always safe, fresh, and free from contaminants, follow these simple but crucial guidelines:
- Use clean, non-toxic containers. Opt for BPA-free plastic, glass, or stainless steel bottles designed for reuse. Wash reusable bottles daily with hot water and soap to prevent biofilm buildup.
- Cover your water. Always use a lid or cap to protect your water from airborne particles, especially overnight or for short-term storage.
- Avoid heat and sunlight. Store water containers in a cool, dark place, away from direct sunlight or sources of heat, to inhibit bacterial growth and prevent chemical leaching from plastic.
- Pour, don't sip. If you are drinking from a large bottle that will last more than a few hours, pour the water into a separate glass. This prevents backwash from introducing oral bacteria into the main supply.
- Replace stored water regularly. For emergency preparedness or home storage, replace tap water every six months. Commercially bottled water should be consumed within two years, though it will remain safe if sealed and stored correctly.
- When in doubt, throw it out. If week-old water has an off taste, smells funny, or has particles, it is not worth the risk. Pour it out and get a fresh, clean glass.
Stale water vs. contaminated water: a comparison
| Feature | Freshly Poured Water | Water Left Out Overnight | Water Left Out For a Week |
|---|---|---|---|
| Appearance | Clear and fresh | Clear (unless uncovered) | Potentially cloudy, with particles or biofilm |
| Taste | Neutral, clean | Slightly flat or stale | Off-tasting, metallic, or musty |
| Smell | Odorless | Odorless | Potentially musty or unpleasant |
| Bacteria Content | Minimal | Low (mostly own mouth bacteria if sipped) | High (significant microbial growth) |
| Chlorine Content | Full concentration | Reduced (evaporation starts) | Completely evaporated |
| Safety Level | High | Low risk (for a healthy person) | High risk (not recommended) |
Conclusion
While a single night may not pose a serious threat to a healthy individual, drinking water left out for a week is unequivocally unsafe. Over seven days, water loses its protective chlorine, allowing bacteria to multiply exponentially, especially when introduced through sipping. Plastic containers further compromise safety by leaching harmful chemicals, a process accelerated by heat. To avoid the risk of illness, from simple stomach upset to more serious infections, it is best to discard any water that has been sitting for more than 24 hours. Proper storage in clean, covered, non-plastic containers and regularly refreshing your supply are the best practices for ensuring safe and healthy hydration. For more detailed information on emergency water storage, consult resources like those from the Centers for Disease Control and Prevention.
