It is a common habit to keep an opened bottle of commercially packaged water on a nightstand or desk, intending to finish it the next morning. This convenience raises a simple but important question about safety, especially regarding the quality of the water after several hours at room temperature. The concern centers on whether the water remains potable once the seal is broken and it is exposed to the environment.
The Primary Concern: Microbial Growth
The most significant and immediate risk factor for opened bottled water left out is biological contamination. When you take a sip directly from the bottle, you introduce the natural microbes from your mouth, known as oral flora, into the remaining water. This “backwash” deposits bacteria, which then find a suitable environment in the still, room-temperature water.
Room temperature, typically between 68°F and 77°F (20°C and 25°C), is ideal for most bacteria to multiply rapidly. Studies have shown that the bacterial count in bottled water can increase dramatically over 48 hours of storage at warmer temperatures. This exponential growth is why water that was safe at bedtime may contain a substantial microbial population by morning.
While the bacteria are initially your own, and unlikely to cause illness in a healthy individual, the risk increases if the bottle is shared with another person. Furthermore, an open bottle acts as a collection point for dust, airborne debris, and other microscopic particles that settle on the water surface overnight.
Chemical Considerations in Plastic Containers
Public concern often focuses on the chemical safety of the plastic container itself, particularly when the water is left out for a period. Most single-use commercial water bottles are made from polyethylene terephthalate (PET), identified by the recycling code #1. While PET is considered safe for food and beverage storage, it does contain chemical compounds that can migrate into the water.
Two chemicals frequently discussed in this context are antimony, used as a catalyst in PET manufacturing, and phthalates. At typical room temperatures, the leaching of these substances into the water over an overnight period is generally negligible and remains well below regulatory limits. For instance, one study found no statistical difference in antimony concentrations in bottled water stored at 72°F (22°C) over three months.
The risk of chemical migration accelerates significantly with heat, not time alone. If a bottle is left in a hot car or direct sunlight, where temperatures can exceed 140°F (60°C), the release of antimony and other compounds increases rapidly, potentially exceeding safety standards. This thermal effect is the primary reason for the chemical safety concerns associated with plastic water bottles, not the simple act of leaving a bottle at room temperature overnight.
Safety Verdict and Storage Guidelines
Drinking bottled water that has been left overnight is generally considered a low risk for most healthy adults, provided it was only consumed from by one person. The main issue is the slight increase in microbial load from oral backwash. The trace amounts of chemicals that may leach from a PET bottle at room temperature over a short period are not a significant health concern.
Storage Guidelines
To maintain the highest water quality, a few simple guidelines can be followed.
- If you must use a single-use plastic bottle, finish the water within 24 to 48 hours of opening and avoid reusing the bottle repeatedly, as scratches can harbor more bacteria.
- If the bottle was left in a hot environment, such as a car, it should be discarded due to the accelerated chemical leaching.
- For those seeking to avoid both microbial accumulation and potential chemical migration, switching to a reusable container made of glass or stainless steel is a better option.
- Always store opened water with the cap securely closed to prevent airborne dust and particles from entering the bottle.