Drinking water that has been left sitting out, often referred to as “old water,” raises a common question regarding its taste and safety. This concept typically refers to water exposed to the surrounding environment in a glass, pitcher, or opened bottle, rather than water from a compromised supply. While the flavor may change noticeably after just a few hours, the presence of contaminants that could cause illness depends entirely on the water’s source and how it was stored.
The Risk of Microbial Contamination
The primary health concern with drinking old water comes from biological contaminants that settle in or multiply over time. Water left uncovered acts as a collection point for airborne particles, including dust, mold spores, and bacteria, which naturally fall from the surrounding air. While these environmental microbes are generally harmless, their presence allows colonization to begin.
A more significant source of contamination arises when water is partially consumed and then left sitting, such as in a reusable bottle used throughout the day. Backwash from the mouth introduces a high concentration of oral bacteria, skin cells, and other microorganisms into the water. If the water contains residual organic matter or is kept in a warm environment, these introduced bacteria can multiply rapidly.
Over a longer period, often starting after 12 to 24 hours, bacteria establish a persistent community on the container’s interior surfaces. This sticky layer is known as a biofilm, which can harbor various organisms, including potentially pathogenic bacteria like Pseudomonas aeruginosa. While galvanized steel and stainless steel containers show the lowest levels of biofilm formation, plastic and glass provide surfaces for microorganisms to adhere to. Pathogenic bacteria that enter the water can survive within these biofilms for several weeks, posing a risk to the consumer.
Chemical Changes from Storage and Containers
Beyond microbial growth, water left exposed undergoes chemical interactions that mainly affect its taste and quality. The “stale” flavor that develops in uncovered water is due to the absorption of carbon dioxide (\(\text{CO}_2\)) from the air. When \(\text{CO}_2\) dissolves into the water, it reacts to form carbonic acid, which lowers the water’s pH slightly. This chemical change creates a flatter, less appealing taste.
If the water is from a municipal source, the disinfectant added during treatment, typically chlorine, will evaporate over time when left in an open container. This evaporation removes the water’s residual protection against bacterial growth, making it more susceptible to airborne or environmental contamination. The loss of this antibacterial barrier means tap water becomes less safe to drink after extended periods of exposure.
The container material itself can introduce chemicals into the water, a process known as leaching, which is mainly a concern with plastic bottles. Polyethylene terephthalate (PET) plastic, commonly used for single-use water bottles, contains antimony trioxide as a catalyst. Antimony, a regulated contaminant, can leach into the water. Concentrations typically remain below the U.S. Environmental Protection Agency’s maximum contaminant level (MCL) of 6 parts per billion (ppb) during normal storage. However, this leaching rate increases significantly with temperature; storing a PET bottle in an environment exceeding 65 degrees Celsius can cause the antimony concentration to exceed the MCL within 38 days.
Practical Safety: When to Discard Sitting Water
The decision to drink or discard old water depends primarily on the level of exposure and whether the water has been drunk from previously. For water left in an open glass overnight, it is generally best to discard it after 8 to 12 hours, mostly due to the accumulation of dust and other airborne particles. The concentration of environmental microbes is usually too low to cause illness, but the quality has diminished.
Water stored in a clean, covered container, such as a pitcher with a lid or a tightly sealed bottle, is protected from airborne contamination and the environment. If the source water was initially safe, this water is typically safe to consume for up to 48 hours, though it may still taste flat due to \(\text{CO}_2\) absorption. This time frame assumes the container was sterile before filling and has not been opened frequently.
A significant risk factor is water that has been partially consumed directly from the container, which is often the case with reusable bottles. The introduction of oral bacteria from backwash means the water should ideally be finished or discarded within a few hours, especially if the bottle is kept in a warm place like a gym bag or car. Consistently refilling a bottle without daily washing should be avoided, as the developing biofilm provides a persistent source of bacteria that can contaminate the fresh water. Instead, containers should be washed regularly with soap and thoroughly dried to prevent the attachment and growth of microbial communities.