The date printed on a bottle of water often confuses consumers, suggesting the liquid has spoiled. However, pure water (H₂O) does not biologically expire. The paradox lies in the packaging, not the liquid itself. Understanding why this date exists and the actual changes that occur in the container is key to assessing the safety and quality of old bottled water.
Why Bottled Water Has an Expiration Date
The expiration date on bottled water is not a measure of freshness but a function of regulatory compliance and packaging logistics. In many jurisdictions, including the United States, bottled water is classified as a packaged food product. This classification subjects it to regulations that require all consumable goods to carry a “best by” or expiration date for consumer protection and traceability.
Manufacturers use the same machinery for water as they do for perishable beverages like soft drinks. Applying a standardized date code to every product line is more efficient, regardless of the contents’ spoilage risk. This date also serves as a quality assurance tool, helping with inventory rotation and providing a traceable batch number for recalls.
The date ultimately relates to the integrity of the container, typically made of polyethylene terephthalate (PET) plastic. This expiration date is the manufacturer’s guarantee that the seal will remain intact and the plastic will not significantly degrade before that time.
The Actual Health Risks of Old Water
The most significant health risk associated with old bottled water is the chemical breakdown of the plastic container itself. Polyethylene terephthalate (PET) plastic contains a compound called antimony, which acts as a catalyst in the manufacturing process. Over time, especially when subjected to heat, this antimony can leach from the plastic and into the water.
Studies have shown that antimony concentrations can increase significantly during storage. Although initial levels are often well below regulatory limits, exposure to high temperatures, such as those found in a hot car or garage, dramatically accelerates this leaching process. Storing a PET bottle at 50°C can cause antimony levels to rise rapidly, potentially exceeding safe drinking water standards within days.
Antimony is classified as a toxic metal. While acute poisoning from bottled water is rare, the concern lies with chronic, low-level exposure. Long-term exposure is linked to severe issues, including organ damage and an increased risk of certain cancers. The plastic may also leach plasticizers, which can disrupt the body’s endocrine system.
The other risk involves bacterial contamination, which is distinct for sealed versus opened bottles. In a perfectly sealed, commercially bottled container, the water is sterile, and the risk of harmful bacterial growth is extremely low. Once the seal is broken, however, the water is immediately exposed to microorganisms from the air and the drinker’s mouth.
A single sip introduces bacteria that multiply rapidly in the nutrient-poor, neutral-pH environment of the water. Bacterial levels in an opened bottle left at room temperature can rise dramatically, with growth accelerating quickly after the first 24 to 48 hours. This bacterial buildup, especially in frequently reused bottles, is the primary source of short-term health risk.
Changes in Taste and Quality Over Time
Even when sealed, bottled water can undergo noticeable sensory changes that affect its palatability without making it unsafe to drink. Plastic bottles are slightly permeable, allowing for the slow absorption of ambient gases and odors from the surrounding environment. If water is stored next to household chemicals or strong-smelling cleaners, the water can take on those unpleasant aromas or tastes.
The water can also develop a distinct “plastic” or “metallic” taste over extended storage due to the minor leaching of compounds like antimony. The loss of dissolved gases contributes to the water tasting stale. Water naturally contains dissolved air, including carbon dioxide, which slowly permeates through the plastic walls, leaving the water tasting flat.
Aesthetic degradation can also occur due to simple exposure to air in an opened container. As water sits, it absorbs carbon dioxide from the atmosphere, which forms a small amount of carbonic acid. This subtle change in pH can alter the taste, often described as a flat or muted flavor.
Safe Storage and Disposal Guidelines
The longevity and safety of bottled water depend almost entirely on how it is stored. The most important measure is to keep the bottles away from any source of heat, including direct sunlight, hot garages, and vehicles. High temperatures are the main catalyst for chemical leaching from the PET plastic into the water.
Water should be stored in a cool, dark environment, ideally at or below room temperature. Because plastic is permeable, bottles must be kept separate from solvents, paints, cleaning supplies, and petroleum-based products. Storing water away from these items prevents the absorption of chemical vapors that can significantly impact the water’s taste and odor.
If a sealed bottle has been stored correctly—cool, dark, and away from chemicals—it is generally safe to consume even past the printed expiration date. The water itself does not spoil under these conditions. However, if the water has an off-taste, a strange smell, or if the bottle appears compromised, it should be discarded.