The question of how long bottled water remains good is a common source of confusion for many consumers. Water itself, being a simple inorganic compound, does not chemically expire or spoil in the traditional sense. The true concern lies with the polyethylene terephthalate (PET) plastic container that holds the water, which changes the quality and safety profile over extended periods. This interaction between the liquid and the packaging material is the primary factor that determines the actual shelf life of sealed bottled water. The integrity of the plastic slowly degrades over time, especially when exposed to environmental stresses.
Regulatory and Practical Shelf Life
The presence of a “best by” date on a bottle of water often leads people to believe the water will spoil, yet the U.S. Food and Drug Administration (FDA) does not mandate an expiration date for bottled water. Instead, the date is typically a commercial or regulatory requirement related to the packaging, not the water quality itself. Bottling companies often use the same machinery for water as they do for perishable beverages, making it more efficient to apply a “best by” date to all products.
Most manufacturers voluntarily print a recommended shelf life of approximately one to two years from the bottling date. While sealed bottles can technically last indefinitely without microbial contamination, the practical shelf life reflects the manufacturer’s confidence in the container’s integrity. Once a bottle is opened, the water is exposed to the environment and should be consumed within a few days, similar to any other opened beverage.
Chemical Interaction Between Water and Plastic
The true limitation on bottled water’s longevity is the gradual breakdown of the plastic container, a process known as chemical leaching. Polyethylene terephthalate, or PET, is the plastic most commonly used for single-serving water bottles, and it is not entirely inert. Over time, and particularly when exposed to heat and sunlight, components of the plastic can migrate into the water.
One such substance is antimony, a metalloid used as a catalyst in the production of PET plastic. Other compounds of concern include phthalates, which are plasticizers that can act as endocrine disruptors and leach into the water, particularly when the bottle is exposed to high temperatures.
Heat is the most significant accelerator of this chemical migration; leaving a bottle in a hot car or direct sun drastically increases the rate at which these compounds and microplastics shed into the water. A recent analysis found that a liter of water in a plastic bottle can contain hundreds of thousands of microscopic plastic fragments. This shedding affects the water’s taste and introduces foreign substances whose long-term health implications are still under study. The taste change, often described as stale or chemical-like, is the most immediate sign that the plastic is degrading and affecting the water quality.
Maximizing Longevity Through Proper Storage
Proper storage conditions significantly extend the practical shelf life. The most effective way to slow the chemical leaching process is to keep bottled water in a cool, dark environment. Ideal temperatures are between 50 and 70 degrees Fahrenheit (10–21°C), away from sources of fluctuating heat like ovens, water heaters, or direct sunlight.
Storing bottled water in areas with high temperatures, such as a garage in summer or a hot car, should be avoided completely. Heat accelerates the breakdown of the PET polymer, leading to a faster release of chemicals like antimony into the water. Furthermore, plastic is slightly permeable, meaning that vapors from strong chemicals can seep through the container and taint the water’s taste and odor. Therefore, bottled water should never be stored near household cleaners, gasoline, paint, or other volatile substances. Even if the bottle is sealed and within the “best by” date, water that has been stored improperly—for instance, left in a hot environment for weeks—should be discarded to prioritize safety and taste quality.