A plastic water bottle forgotten in a car’s cup holder on a hot day is a common sight. Before taking a sip of that warm water, it is important to understand the two primary safety concerns: chemical leaching from the plastic and the potential for accelerated microbial growth. The overall safety of the water is conditional, depending on factors like how hot the car became, the duration of the heat exposure, and whether the bottle had already been opened and partially consumed. A clear understanding of these risks allows for better decisions about hydration.
How Heat Affects PET Plastic
The single-use water bottles typically found in stores are made from Polyethylene Terephthalate (PET), designated with the recycling code 1. This plastic is generally stable at room temperature, but heat breaks down its molecular structure, causing certain compounds to migrate into the water. This process, known as leaching, accelerates significantly when temperatures rise above 122°F (50°C), a temperature easily reached inside a parked car.
One of the primary chemicals of concern is antimony trioxide, which is used as a catalyst in the manufacturing of PET plastic. Studies have shown that antimony concentrations in the water can increase sharply when the bottle is exposed to temperatures around 122°F (50°C). After a day or more of exposure, concentrations can sometimes exceed the United States Environmental Protection Agency (USEPA) maximum contaminant level of 6 parts per billion (ppb). The increase is a direct result of the thermal breakdown of the plastic.
Another compound that can migrate is Bisphenol A (BPA), although it is not intentionally used in PET bottles. The migration of trace amounts of BPA and similar compounds, such as phthalates, also increases with temperature. Furthermore, the heat facilitates the release of degradation products like acetaldehyde, which can alter the water’s quality.
The Risk of Microbial Contamination
Separate from the chemical risks, a major concern for bottled water left in a hot car is the rapid proliferation of bacteria. The temperature range between 40°F and 140°F (4°C and 60°C) is often referred to as the “danger zone,” where bacteria can multiply quickly. The interior of a car on a hot day, which can easily exceed 100°F (38°C), provides an ideal, warm incubation environment for microorganisms.
The primary source of this contamination is not the sealed water itself, but rather the bacteria introduced when the bottle is opened and consumed. Taking a sip transfers microbes from the mouth into the water. When this partially consumed bottle is resealed and left in the heat, the bacteria, including common types like E. coli and Staphylococcus aureus, multiply rapidly.
An unopened, sealed bottle of water is generally safe from microbial contamination because it is packaged under sterile conditions. However, once the seal is broken, the combination of bacterial introduction and the car’s high temperature creates a biological risk within hours. The multiplication of these organisms is a much more immediate health concern than low-level chemical leaching, potentially leading to gastrointestinal distress.
Changes in Water Flavor and Odor
Even when a bottle of water is deemed chemically and biologically safe, exposure to heat often results in an unpleasant change to the taste and smell. This is primarily an aesthetic issue, but the plastic-like or stale flavor is directly linked to the increased migration of compounds from the PET bottle into the water.
One of the main culprits for this altered taste is acetaldehyde, which can impart a fruity or plastic-like flavor. High temperatures accelerate the movement of this compound from the bottle walls, making the off-flavor more noticeable. Additionally, the warmth itself reduces the water’s ability to hold dissolved gases, making the flavor of any residual chemicals more pronounced. While the change in flavor is generally not a health hazard, it serves as a clear indication that the plastic has degraded due to the heat exposure.
Safe Storage Practices and Recommendations
To minimize the risks associated with storing bottled water in a vehicle, simple and effective storage practices should be followed. The best solution is to avoid leaving single-use plastic bottles in the car for any extended period, especially on hot days. If water must be stored in a vehicle, using insulated containers or coolers will help maintain a lower temperature and prevent the accelerated leaching of chemicals and growth of bacteria.
Selecting alternative materials for hydration is also a proactive step to reduce chemical exposure. Bottles made from stainless steel or glass do not leach chemicals when exposed to heat and are easily cleaned to prevent bacterial buildup. If using a disposable bottle, it should be placed in a location away from direct sunlight, such as under a seat or in the trunk, to reduce the heat and UV exposure.
If a single-use bottle has been opened and left in a hot car, the safest recommendation is to discard it to avoid the risk of bacterial illness. For unopened bottles that have endured prolonged, extreme heat, it is prudent to dispose of them due to the increased concentration of leached chemicals. Regularly rotating any stored water and ensuring that reusable containers are thoroughly cleaned after each use are simple actions that promote safe hydration.