Leaving a glass or bottle of water at room temperature, often overnight, raises questions about quality and safety. Generally, drinking water that has been sitting out is safe, provided it was clean initially and the storage time is relatively short. However, the water’s quality, including its taste and chemical composition, changes almost immediately upon exposure to the air. Concerns shift from a decline in flavor to a potential increase in microbial contamination the longer the water remains exposed.
The Chemical Changes That Affect Taste
The perceived “staleness” of water left out is primarily a chemical phenomenon resulting from interaction with the surrounding atmosphere. Water naturally absorbs gases from the air, most significantly carbon dioxide (\(\text{CO}_2\)). When carbon dioxide dissolves, it reacts to form a small amount of carbonic acid. This chemical change lowers the water’s pH, making it slightly more acidic, which is perceived as a flat or odd flavor.
If the water is from a municipal source, another factor contributing to flavor change is the dissipation of residual chlorine. Chlorine is added to public water supplies as a disinfectant, and its evaporation removes the familiar chlorinated smell and taste over several hours. Since the water has warmed to room temperature, the absence of coldness also affects the taste buds, making the altered flavor more noticeable. These non-biological changes affect palatability but do not pose a health risk.
Understanding Microbial Contamination Risks
While chemical changes affect the taste, the actual safety risk comes from potential biological contamination. Uncovered water is susceptible to airborne particles, including dust, pollen, and microscopic spores, which settle on the surface. Once contaminants enter the water, the room temperature environment provides suitable conditions for bacterial growth. This growth becomes a concern, especially if the water is left out for more than a day.
A more significant source of contamination occurs when a person drinks directly from a bottle or glass and then leaves it out. This action introduces oral bacteria into the water via “backwash.” These microorganisms, including those from the mouth and saliva, can multiply rapidly, especially since the chlorine initially present in tap water dissipates. Although the bacteria introduced by one’s own mouth are generally not pathogenic to that individual, their concentration increases over time, making the water less hygienic. Sharing a backwashed container is a greater concern, as it introduces foreign bacteria that the other person’s body is not accustomed to.
Factors Influencing Water Safety Over Time
The degree of risk associated with drinking water that has been sitting out depends on environmental and behavioral factors. The most defining factor is whether the container is covered, as a lid or cap prevents the settling of dust and airborne microbes. Covered water in a clean container generally remains safe to drink for up to 24 to 48 hours at room temperature. After this period, bacterial growth becomes a greater concern.
The ambient temperature of the room is also a major accelerator of bacterial multiplication. Warmer temperatures, particularly between 70°F and 85°F, create an optimal breeding ground for any introduced bacteria. Finally, the container material and cleanliness play a role in safety over time. Reusing the same unwashed vessel repeatedly allows bacteria to form a biofilm on the container walls.