The question of whether water is safe to drink after sitting out, often overnight, is common. Water left out for a short period, such as on a bedside table, is generally safe to consume. However, chemical, physical, and biological changes begin to occur immediately, affecting both the taste and the long-term safety of the water. These changes cause the water to taste noticeably different and can eventually lead to contamination issues.
Why Water Tastes Different
The “stale” taste of water that has been sitting out is primarily due to chemical interactions with the surrounding air. Water naturally absorbs carbon dioxide from the atmosphere once it is exposed in an open container. This absorbed carbon dioxide reacts with the water to form carbonic acid, which slightly lowers the water’s pH. This minor change in acidity is perceived as a “flat” or less refreshing taste compared to freshly poured water.
If the water is from a municipal source, another significant change is the dissipation of chlorine. Tap water is treated with chlorine or chloramine to kill harmful bacteria and viruses. Over several hours, this volatile chemical evaporates into the air, removing the familiar disinfected taste and leaving the water more susceptible to microbial growth. Also, the water temperature adjusts to room temperature, which influences taste perception, as colder water tends to dull the taste buds.
Understanding Microbial Contamination
While the taste change is chemical, the main safety concern is the potential for biological contamination. When water is left uncovered, it is exposed to airborne particulates, including dust, fungal spores, and bacteria. These household bacteria can settle into the water, and their concentration will increase over time, even in a nutrient-poor environment.
A significant source of contamination is the transfer of oral flora if a person has already taken a sip from the container. Saliva contains a high concentration of bacteria, which are introduced into the water. The water becomes an environment where these microbes can begin to multiply. For immunocompromised individuals, reintroducing a high concentration of their own bacteria after it has multiplied can pose a slight risk.
The prolonged use of the same container can also lead to the formation of a biofilm. A biofilm is a complex community of microorganisms encased in a slimy matrix that adheres to a surface, such as the inside of a glass or bottle. Bacteria within this protected structure are much more resistant to disinfectants and can be a continuous source of contamination. The lack of continuous flow and the introduction of organic material from the mouth create conditions where these microscopic structures can develop.
Environmental Factors That Increase Risk
The safety of water left out depends heavily on the surrounding environment and the duration of exposure. Water is generally considered safe to drink for up to 24 to 48 hours, provided it has not been contaminated by a person drinking from it. This window shrinks rapidly when certain environmental factors are introduced.
Temperature is the primary factor that accelerates bacterial proliferation. Warmer temperatures, such as those found in a hot car or a sunlit room, create an ideal breeding ground for bacteria. Conversely, water left in a cool, covered environment will maintain its quality longer. The type of container also plays a substantial role in mitigating risk.
An open glass exposes the water to maximum dust and airborne contaminants, while a clean, covered bottle or pitcher limits this exposure. The source of the water matters, as tap water typically contains residual chlorine that offers some initial protection against microbial growth, unlike purified or bottled water. Water safety is a balance between the time the water sits, the room temperature, and how well the container is protected from outside contamination.