The convenience of a bottle of water ready for a commute or a quick stop is a common practice for drivers. Because a parked vehicle can become an oven in warm weather, often reaching internal temperatures far exceeding the outside air, the safety of that forgotten bottle is frequently questioned. Understanding the potential changes that occur when water is stored in plastic and exposed to high heat is important. The primary concerns revolve around chemical migration from the plastic, microbial growth, and the physical integrity of the container itself.
Chemical Safety: Heat, Plastics, and Leaching
Most single-use bottled water containers are made from Polyethylene Terephthalate (PET). This material is generally considered safe for food and beverage contact under normal use conditions, but heat significantly alters its stability. When the interior of a parked car climbs past 110°F, the thermal energy increases the movement of molecules within the plastic structure. This increased molecular movement accelerates the process of chemical leaching into the water.
Antimony and Other Compounds
A substance of particular concern is antimony, a metalloid compound used as a catalyst in PET production. Studies show that the rate of antimony leaching rises sharply when temperatures exceed 45°C (113°F). In some experiments, bottled water stored at 50°C (122°F) saw antimony concentrations increase to levels that exceeded the U.S. Environmental Protection Agency’s Maximum Contaminant Level after prolonged exposure.
Other compounds, such as Bisphenol A (BPA) and phthalates, can also migrate into the water. While the trace amounts found in a single, short exposure are considered low risk, these chemicals are recognized as potential endocrine disruptors. Repeatedly consuming water from bottles that have been heated in a car means an accumulation of these compounds over time, which is why experts advise against the practice. The noticeable “plastic” or “stale” taste many people report when drinking hot bottled water is often an early sensory indication of this degradation.
Water Quality: Bacterial Growth and Taste Alteration
Beyond chemical risks, a hot car creates an environment highly conducive to the growth of microorganisms, especially in bottles that have been previously opened. The “danger zone” for rapid bacterial multiplication is between 40°F and 140°F, a temperature range that the water inside a car can quickly reach. Once an individual drinks from a bottle, bacteria from the mouth are transferred into the water. The combination of warm water and introduced bacteria allows the microbes to flourish rapidly.
For an unopened, sealed bottle, the risk of bacterial contamination is minimal because the water is commercially sterilized. However, any bottle that has been sipped from and left in a hot vehicle should be discarded to avoid ingesting potentially harmful pathogens.
Heat also influences the sensory characteristics of the water, contributing to a less pleasant drinking experience. Even if the chemical leaching is minimal, the elevated temperature can cause the water to acquire a distinct, flat, or off-putting flavor. This alteration is related to the subtle breakdown of the plastic polymers and the resulting change in the water’s chemical makeup.
Practical Risks: Pressure Buildup and Bottle Damage
Leaving a sealed plastic bottle in a hot car can lead to visible physical damage and potential cleanup issues. The intense heat causes the air and water inside the bottle to expand. Because the bottle is sealed, this thermal expansion causes pressure to build up against the container walls. This pressure can lead to the plastic visibly deforming or bulging. In some instances, the pressure buildup can cause the cap seal to fail or the bottle to crack, resulting in a messy leak in the vehicle’s interior.
Furthermore, a plastic water bottle can pose a fire risk under direct sunlight. When positioned correctly in the path of the sun, the curved plastic and the water act as a lens, focusing solar energy onto a small, concentrated point. This focused beam of light can generate enough heat to ignite materials inside the car, such as upholstery or floor mats.