Unopened distilled water stored in a sealed container lasts indefinitely. Most manufacturers print a best-by date of one to two years on the label, but that reflects peak quality rather than a safety cutoff. The water itself doesn’t spoil. What changes over time is the container it sits in and the conditions around it.
Unopened Distilled Water
Because distillation removes minerals, microbes, and dissolved solids, there’s nothing in the water to degrade. A factory-sealed bottle or jug kept in a cool, dark spot will remain chemically stable for years beyond its printed date. The best-by label exists largely because plastic containers slowly interact with their contents, and manufacturers want to guarantee optimal quality within a defined window.
The main concern with long-term storage is the container, not the water. PET plastic, the material used in most store-bought bottles, contains trace amounts of antimony from the manufacturing process. At room temperature (around 22°C or 72°F), antimony leaches into the water at extremely low levels, well below the EPA’s safety limit of 6 parts per billion. But heat accelerates the process dramatically. At 60°C (140°F), concentrations can exceed that safety limit within 176 days. At 80°C (176°F), it takes just over two days. So a case of distilled water left in a hot garage or car trunk for months is a different story than the same case stored in a pantry.
For the longest shelf life, store sealed distilled water in a cool, dark location away from direct sunlight and temperature swings. Glass containers or BPA-free, food-grade plastic are the best options if you’re storing water you’ve distilled at home.
After Opening the Container
Once you break the seal, the clock starts ticking. Exposure to air introduces two problems: microbes and carbon dioxide.
Even though distilled water has almost no nutrients, certain bacteria and algae can still grow in it over time, especially in warm or sunlit conditions. Resealing the container tightly after each use slows this down considerably but doesn’t stop it entirely. Opened distilled water is generally best used within a few days to a few weeks, depending on how carefully you reseal it and where you store it.
The other change is chemical. Distilled water exposed to air absorbs carbon dioxide and forms carbonic acid. Within 24 hours of sitting open, the pH can drop from a neutral 7.0 to somewhere between 5.5 and 6.0. This slight acidity won’t hurt you if you’re drinking it, but it matters if you’re using the water for sensitive applications like laboratory work, cosmetics mixing, or art conservation.
Storage Rules for Common Uses
How strict your storage needs to be depends on what you’re using the water for.
Drinking: An unopened bottle stored at room temperature away from sunlight is fine for years. After opening, refrigerate it and use within one to two weeks, just as you would with any other bottled water. If it develops any cloudiness, unusual taste, or off smell, discard it.
CPAP machines: Sleep equipment manufacturers recommend filling the humidifier chamber with fresh distilled water each night and emptying any leftover water each morning. Water that sits in the warm, enclosed reservoir for more than a day can harbor bacteria, and inhaling contaminated mist directly into your airways is a genuine concern.
Irons and steamers: Distilled water stored in a sealed container for months or even a couple of years works fine for appliances. The purpose here is to prevent mineral buildup, and long-stored distilled water still has no minerals. Slight pH changes from CO2 absorption won’t affect your iron.
Car batteries and cooling systems: Similar to appliances, the goal is mineral-free water. A sealed jug stored in the garage for a year or more is perfectly usable, though you’ll want to avoid containers that have been repeatedly exposed to high heat due to the plastic leaching issue described above.
Signs Your Distilled Water Has Gone Bad
Distilled water that has been contaminated will usually show visible or sensory clues. Cloudiness or floating particles are the most obvious sign. Green or brown discoloration suggests algae growth, which happens when water is stored in a sunlit area. Any unusual odor, whether musty, chemical, or otherwise off, means the water should be discarded. A slimy film on the inside of the container also indicates bacterial colonization.
If you’re using distilled water for drinking and it tastes flat or slightly acidic, that’s normal. Distilled water lacks the dissolved minerals that give tap and spring water their familiar taste. But a metallic, salty, or chemical taste is not normal and signals contamination from the container or environment.
Best Containers for Long-Term Storage
Glass is the gold standard. It doesn’t leach chemicals, doesn’t interact with the water, and blocks nothing from your view so you can spot contamination easily. The downside is weight and breakability.
For plastic, look for HDPE (recycling code 2) or food-grade containers labeled BPA-free. These are more chemically stable than PET bottles and leach fewer compounds over time. Avoid reusing containers that originally held other liquids, since residues can be difficult to fully remove.
Whatever container you choose, ensure the seal is airtight. This is the single most important factor in preserving distilled water’s purity once you’ve filled or opened it. A loose cap invites dust, airborne bacteria, and CO2 absorption, all of which shorten the water’s useful life.