Drinking water is a stable compound that does not spoil, but its safety and quality change significantly once placed in a container. How long water can be kept depends on the source, the bottle material, and the storage environment. The main concerns for consumers are potential microbial growth and chemical leaching from the container over time. Understanding these variables provides guidance for safe water storage.
Storage Timelines for Tap Water Versus Commercial Bottled Water
The longevity of stored water is influenced by whether it is tap water or commercially sealed bottled water. Tap water, treated with disinfectants like chlorine, can be safely stored in a clean, sealed container for approximately six months. This timeframe accounts for the gradual dissipation of chlorine, the primary agent preventing microbial growth. Once chlorine evaporates, the water becomes susceptible to bacterial contamination. Unsealed tap water, such as a glass left out, should be consumed within 24 hours, as air exposure quickly depletes chlorine and allows for the absorption of ambient contaminants.
Commercial bottled water often has an expiration date, typically ranging from one to two years after bottling. This date relates primarily to the integrity of the plastic packaging, not the water itself, which is microbiologically safe when sealed. The FDA requires the date for the container because plastic can degrade over time. If stored under optimal conditions (cool and dark), unopened bottled water may remain safe indefinitely, though manufacturers suggest replacement every two years to maintain quality. Once opened, commercial water should be treated like tap water and consumed within a few days to minimize contamination risk.
How Container Material Affects Water Quality Over Time
The container material plays a direct role in preserving water quality and safety during storage. Glass containers are the most inert option because they do not leach chemicals into the water or allow permeability to gases or odors. This makes glass an excellent choice for long-term storage, provided the seal remains intact.
Plastic bottles, particularly those made of polyethylene terephthalate (PET) used for single-use water, are a concern due to their permeability and potential for chemical migration. Over time, manufacturing compounds like bisphenol A (BPA), antimony, or phthalates may leach into the water, especially when exposed to temperature changes. Stainless steel, commonly used for reusable bottles, is durable and non-corrosive, performing well for maintaining water quality. While generally safe, low-quality or damaged stainless steel may introduce a metallic taste, but it avoids the leaching risks associated with many plastics.
The Impact of Heat and Light on Stored Water
Environmental factors like heat and light significantly accelerate the degradation of stored water quality, especially in plastic containers. Heat is a major catalyst for chemical leaching from plastics into the water. When plastic bottles are exposed to high temperatures, such as inside a hot car, the rate at which compounds like antimony or BPA migrate increases substantially. Temperatures above 70 degrees Fahrenheit cause the bottle’s chemicals to degrade more rapidly, affecting both the taste and safety profile of the water.
Direct sunlight introduces a dual threat: heat and ultraviolet (UV) radiation. UV light breaks down the molecular structure of plastic, further promoting the release of chemical components into the water. Sunlight also creates a warmer environment highly conducive to microbial growth. This combination of accelerated chemical leaching and increased bacterial proliferation means all stored water, regardless of the container, should be kept in a cool, dark location.
Maintaining Safety in Reusable Water Bottles
Reusable water bottles pose unique challenges because repeated use and handling introduce external contamination. The most common source of microbes is backwash from the mouth, which deposits oral bacteria, and contact with hands, which transfers environmental germs. Because a reusable bottle is a warm, moist, and enclosed environment, it becomes an ideal breeding ground for bacteria and mold if not cleaned regularly. Studies have shown that unwashed reusable bottles can harbor significant levels of bacteria, including potentially harmful strains.
Proper sanitation requires a two-step approach: daily cleaning and periodic deep sanitization. Daily cleaning should involve washing the bottle with warm water and dish soap, paying close attention to the mouth, lid, and any straws or spouts. Using a bottle brush to reach all interior surfaces is important, as this prevents the formation of biofilm—a slimy layer where bacteria thrive.
For deep cleaning, weekly sanitization is recommended using a solution of one teaspoon of bleach per quart of water, or a mixture of white vinegar and water, which kills microorganisms resistant to regular soap. After washing or sanitization, the bottle must be allowed to air-dry completely with the lid off to ensure no residual moisture encourages further microbial growth.