Water itself doesn’t expire. The molecule H₂O doesn’t break down or go bad over time. What changes is everything around the water: the plastic container slowly degrades, chemicals migrate into the liquid, dissolved gases escape, and if the seal is broken, microorganisms move in. So when you see an expiration date on a bottle of water, it’s really a expiration date for the packaging, not the water inside.
Why Bottled Water Has Expiration Dates
The FDA considers bottled water to have an unlimited shelf life as long as it’s produced properly and stays sealed. There’s no federal requirement to stamp an expiration date on it. The dates you see are voluntary, placed there by manufacturers for practical reasons.
The story traces back to a 1987 New Jersey law that required all bottled water sold in the state to carry an expiration date of two years from bottling. Because major bottlers distributed nationally, many companies just started printing dates on every bottle rather than making separate labels for one state. New Jersey eventually repealed the requirement after federal FDA regulations caught up, but the industry habit stuck. Today, manufacturers keep the dates mostly for inventory rotation and because consumers expect them. The dates reflect concerns about taste and odor, not safety.
What Actually Happens Inside the Bottle
The real expiration story is about the plastic. Most water bottles are made from PET (polyethylene terephthalate), a lightweight, durable plastic that has one significant drawback: it slowly releases chemicals into the water it holds. These include a group of compounds called phthalates, which are used to make plastics flexible. Over time, phthalate molecules migrate from the bottle walls into the water.
Three factors speed this up dramatically. Heat is the biggest one. Water stored at around 104°F (40°C), roughly the temperature inside a car on a warm day, releases significantly more chemicals than water kept in a refrigerator at 40°F (4°C). At cooler temperatures, storage time barely matters. At warmer temperatures, the longer the water sits, the more chemicals accumulate. Sunlight is the second factor. Research published in 2025 found that PET bottles exposed to sunlight had significantly higher phthalate migration compared to bottles stored in the shade. The third factor is reuse: refilling and reusing PET bottles accelerates chemical release because the plastic surface degrades with each cycle.
The amounts involved are typically small, well below regulatory safety limits for short-term consumption. But the principle matters if you’re storing water for months or keeping bottles in hot environments.
Why Old Water Tastes Off
Even if the water is perfectly safe, it can start tasting stale or plasticky. PET is not a perfect barrier. It’s slightly permeable to gases, which means two things happen simultaneously: dissolved carbon dioxide and oxygen slowly escape through the plastic walls, and odors from the surrounding environment can seep in.
This permeability has caused real problems. In two separate incidents in Japan, consumers reported strange smells in imported mineral water that had been shipped in PET bottles. Investigations found that the bottles had absorbed odorous chemicals from their surroundings during shipping or storage. The chemicals weren’t harmful, but the smell made the water undrinkable to anyone who opened it. Unlike glass, plastic absorbs and retains smells, so water stored near cleaning products, gasoline, or other strong-smelling substances can pick up those odors right through a sealed cap.
Water that’s been sitting for weeks or months also loses its “fresh” quality simply because dissolved oxygen levels drop. That flat, stale taste you notice in a forgotten water bottle isn’t contamination. It’s the absence of the dissolved gases your palate expects.
Bacterial Growth in Opened Water
Sealed, commercially bottled water stays microbiologically stable almost indefinitely. The moment you open a bottle, that changes. Your mouth introduces bacteria with every sip, and the warm, still environment inside a half-finished bottle gives those organisms a place to multiply.
Bacteria thrive in stagnant water, especially at room temperature. They form biofilms, thin colonies that cling to interior surfaces. Once a biofilm establishes itself, it continuously sheds bacteria back into the water. Research on water systems shows that even one hour of stagnation leads to measurable increases in bacterial counts, driven primarily by cells detaching from biofilm on surfaces. The narrower the container opening (like a bottle neck), the higher the surface-to-volume ratio, and the faster bacteria accumulate in that zone.
This is why a water bottle you’ve been sipping from all day can develop a funky taste or smell by evening. It’s not dangerous for most healthy people in the short term, but it’s the closest thing to water actually “going bad.”
How to Store Water Long Term
If you’re building an emergency supply or just want to keep bottled water in your garage, the CDC recommends three simple rules: keep it cool (between 50°F and 70°F), keep it out of direct sunlight, and replace it every six months. That six-month rotation isn’t because the water becomes unsafe on day 181. It’s a practical buffer that accounts for imperfect storage conditions and ensures you’re never drinking water that’s been slowly absorbing chemicals or losing freshness for years.
For the longest possible shelf life with the least concern about chemical leaching, store water in glass containers or food-grade stainless steel. These materials don’t degrade or release compounds into the water. If you’re using plastic, keep the bottles in a dark, climate-controlled space and away from anything with a strong odor. A cool basement or interior closet is ideal. A hot garage or the trunk of your car is the worst possible choice.
If you find a forgotten bottle that’s been sitting in a cupboard for a year or two, it’s almost certainly safe to drink if the seal is intact. It might taste flat or slightly plastic, but that’s a quality issue, not a health emergency. Trust your senses: if it smells odd, looks cloudy, or has visible particles floating in it, pour it out. Otherwise, it’s fine.