The question of how long a plastic water bottle remains “good” concerns both the quality of the water inside and the integrity of the container itself. The standard single-use bottle is made from polyethylene terephthalate (PET or PETE plastic), chosen for its lightweight, durable, and clear characteristics. However, this material is not completely inert. The effective lifespan of bottled water is determined by the slow chemical and physical changes that occur in the plastic packaging, which affects both taste and safety.
The Shelf Life of Bottled Water
Water itself does not technically expire or spoil like food. The “best by” date printed on a bottle refers to the expected commercial shelf life and quality of the product within its plastic packaging. This date is typically set at one to two years from the manufacturing date, and the primary concern is taste, not acute safety.
Manufacturers use these dates because the plastic can slowly impart a stale or “plasticky” flavor over extended storage periods. Federal regulators do not require an expiration date on bottled water, as the liquid is considered safe indefinitely if properly sealed and stored. The industry adopted the practice of including a date largely due to a now-repealed 1987 New Jersey state law that mandated a two-year limit.
Understanding Plastic Degradation and Chemical Leaching
Although PET is generally stable, it is not chemically inert. Over time, the plastic can undergo chemical leaching, where small amounts of compounds migrate into the water. One studied leachate is antimony, a heavy metal used as a catalyst in PET production. Studies show that antimony concentrations can increase slightly during storage, even at room temperature.
Other compounds, such as phthalates, can also migrate from the bottle into the liquid. Furthermore, the plastic matrix slowly breaks down into microscopic particles over the long term. These microplastics and nanoplastics have been detected in bottled water, though their health effects are still being studied. For bottles stored correctly, the levels of these chemicals are considered to be below regulatory safety limits.
Factors That Accelerate Bottle Breakdown
External environmental factors dramatically accelerate plastic degradation and chemical leaching. High temperatures are the most potent factor, causing a significant increase in the migration of compounds like antimony. For example, storing a bottle at 70°C can cause antimony concentrations to rise rapidly in days, a process that takes months at standard room temperature.
Leaving a bottle in a hot car or direct sunlight quickly subjects it to these high temperatures, compromising the container’s integrity. Ultraviolet (UV) light exposure also contributes by initiating photo-hydrolysis, which breaks down the chemical structure of the PET. Additionally, reusing disposable plastic bottles is not recommended due to physical wear, as repeated use can create scratches that harbor bacteria and accelerate microplastic release.