Leaving a glass of water on a nightstand overnight is a common habit. Consuming water that has been left out is almost always safe for a healthy person. The water may undergo minor shifts in chemical composition and cleanliness, resulting in a noticeable change in flavor or a slight risk of contamination. These changes are not significant enough in a typical household environment to pose a serious health risk.
How Contaminants Enter the Water
The primary safety consideration for water left uncovered is the introduction of non-chemical contaminants from the environment. An open glass allows dust, airborne particles, and microscopic spores to settle on the water’s surface. While these particles are not inherently harmful, they can affect the water’s cleanliness and introduce a slight film or off-flavor.
A more significant source of microbial contamination comes from the person who last drank from the container. If you took a sip, bacteria from your mouth, known as “backwash,” are introduced into the water. These oral microbes can incubate and multiply in the static water, especially at room temperature. For an individual with a healthy immune system, this is rarely a concern, but sharing the glass after it has been sitting is not recommended.
Why the Taste Changes
The “stale” or “flat” taste of water left out overnight is due to chemical interactions with the surrounding air. Water naturally absorbs gases from the atmosphere, primarily carbon dioxide (CO2). This absorbed carbon dioxide reacts with water molecules to form a small amount of carbonic acid (H2CO3).
This process slightly lowers the water’s pH level, making it more acidic than when it was first poured. The resulting change in acidity is what the human palate perceives as a different, less crisp flavor. If the water was sourced from a municipal tap, residual chlorine also dissipates into the air. Chlorine is added as a disinfectant, and its evaporation removes the subtle chemical taste, further contributing to the altered flavor profile.
Storage Conditions and Water Source
The degree of change and potential contamination is directly influenced by the storage conditions and the water’s initial source. Using a covered container, such as a bottle with a cap or a glass topped with a coaster, significantly reduces the ingress of airborne particles and limits carbon dioxide absorption. This simple action helps the water retain a cleaner, fresher taste for longer.
Ambient temperature also plays a role, as warmer conditions accelerate the multiplication rate of any bacteria introduced from backwash or the air. A glass left in a hot room will experience faster microbial growth than one in a cool environment. Tap water contains residual chlorine, which acts as a barrier against microbial growth, offering an advantage over commercial bottled water. If using a plastic bottle, especially in a warm location, there is a risk of chemicals like Bisphenol A (BPA) leaching from the plastic into the water.