Water is a simple molecule, H₂O, which does not contain the sugars or proteins that cause perishable foods to spoil. The question of whether water “goes bad” is therefore not about the water itself, but about the container it is stored in and the environment surrounding it. Safety concerns arise from two main sources: the potential for the storage material to degrade and release chemicals, and the introduction and growth of microorganisms once the seal is broken. The timeline for when water is no longer safe or pleasant to drink depends entirely on whether the container is sealed or open.
The Stability of Sealed Bottled Water
The expiration date printed on commercial bottled water is generally not a safety indicator. Water has an indefinite shelf life when kept sealed in ideal conditions. Expiration dates often reflect state regulations, or are used by manufacturers for inventory management and quality control. They indicate the point at which the company can no longer guarantee the water’s peak taste and odor.
The true shelf life limitation for sealed bottled water is the plastic packaging itself, typically polyethylene terephthalate (PET). Over extended periods, even at room temperature, minute amounts of chemical compounds can migrate from the plastic into the water. This process is known as leaching, and one substance of concern is antimony, a metalloid used as a catalyst in PET manufacturing.
Storage temperature is the biggest factor determining the rate of chemical migration. Studies show that while antimony concentrations remain low at typical room temperatures, they increase significantly under heat. For example, exposure to 60°C (140°F) can accelerate antimony leaching, potentially exceeding safe contaminant levels within months. Therefore, sealed bottles should never be stored in hot environments like a car trunk or a non-air-conditioned garage.
Why Water in Open or Reusable Containers Degrades Rapidly
Once the seal is broken, the risk shifts dramatically from chemical degradation to microbial contamination. The water is immediately exposed to airborne particles, dust, and bacteria on the rim or cap. Drinking directly from the container introduces a significant load of microorganisms from the mouth, including bacteria from saliva.
The interior of an open bottle, especially a reusable one, becomes a humid environment where bacteria multiply rapidly. Within 24 hours at room temperature, the bacterial count can increase to over a million colony-forming units per milliliter. This proliferation can lead to the formation of a slimy layer known as a biofilm on the interior surfaces, particularly around the mouthpiece.
This microbial risk is significantly higher in reusable containers that are not cleaned daily. Unwashed reusable bottles can harbor millions of bacteria, far exceeding recommended limits for drinking water. The combination of warmth, residual moisture, and repeated mouth contact creates an ideal breeding ground for pathogens. Heat accelerates this process, causing increased bacterial growth and faster chemical leaching from the plastic.
Practical Rules for Safe Water Storage
To ensure the safety and quality of your water, simple storage rules apply based on the container type. Unopened commercial bottled water should be kept in a cool, dark location away from direct sunlight and strong-smelling chemicals (e.g., cleaning supplies or gasoline). Since PET plastic is slightly permeable, it can absorb odors, affecting the water’s taste. Following these practices allows the water to remain safe to drink well past any printed “best by” date.
The timeline for opened water is much shorter due to microbial growth risk. If you have drunk directly from a single-use plastic bottle, discard any remaining water within 24 to 72 hours. If the water was poured into a clean cup and the bottle neck was not touched, the water may remain safe for a few more days, especially if refrigerated.
Reusable water bottles require diligent hygiene, regardless of the material (stainless steel or hard plastic). The water inside should be consumed or discarded daily, and the bottle itself must be cleaned frequently. Washing the container with hot water and soap, or using a mild vinegar solution, helps break down accumulating biofilm and remove thriving bacteria.