When water sits out overnight or is discovered forgotten in a car, the immediate question is whether it is safe to drink. The answer depends on distinguishing between aesthetic quality and actual safety concerns. Water that has been sitting out undergoes subtle chemical shifts that alter its taste, while the true safety risk comes from potential bacterial contamination and the container material itself.
The Immediate Changes in Uncovered Water
The “stale” or “flat” taste often noticed in water left uncovered is primarily due to two non-biological chemical processes. First, exposed water readily absorbs atmospheric carbon dioxide (\(\text{CO}_2\)). This dissolves and reacts with water molecules, forming carbonic acid (\(\text{H}_2\text{CO}_3\)), which slightly lowers the water’s pH. This minor increase in acidity gives the water a noticeably different, less crisp flavor compared to fresh tap water.
A second factor affecting the taste of tap water is the dissipation of chlorine, a disinfectant added by municipal water treatment facilities. Chlorine is volatile and will gradually evaporate, or “gas off,” when water is exposed to air. This chemical can largely dissipate within 12 to 48 hours, depending on the initial concentration and the container’s surface area. The absence of the familiar chlorine scent and taste further changes the water’s flavor profile.
Biological Safety Concerns and Contamination
The primary safety concern with old water involves microbial contamination. This can occur when airborne particles like dust and environmental microbes settle into an open container. More importantly, drinking directly from a container introduces bacteria from the mouth, known as backwash, which can multiply rapidly, especially in a warm environment.
For an individual, drinking their own backwash-contaminated water is generally low risk, as the bacteria are already part of their normal microbial flora. However, if the water has been shared, or if it has been exposed to extreme contamination or warmth for an extended period, the risk increases. Purified water stored in an open container at room temperature remains safe overnight, but its quality begins to decline significantly after 12 to 24 hours of being left open, especially if it was drunk from. The loss of protective chlorine and the introduction of organic material from saliva create an environment where these microbes can thrive.
Storage Considerations: Bottles and Containers
Long-term storage and exposure to heat introduce chemical concerns related to the container material, particularly plastic. Most single-use plastic water bottles are made from polyethylene terephthalate (PET). When exposed to high temperatures, such as inside a hot car, the plastic begins to break down. This degradation accelerates the leaching of chemicals, including antimony and bisphenol A (BPA), into the water. Heat and ultraviolet (UV) exposure also cause the plastic to shed microscopic fragments, releasing microplastics directly into the drinking water.
The “best by” date on commercially bottled water does not signify that the water spoils, but rather that the plastic container may degrade over time. This deterioration can affect the taste and increase the risk of external contaminants entering the sealed bottle. To mitigate these risks, use containers made of glass or food-grade stainless steel for stored water. Water should always be stored in a cool, dark location to limit chemical leaching and inhibit microbial proliferation.