You find a jug of distilled water in the garage or under the sink, maybe left over from topping off a car battery or filling a steam iron. Does distilled water go bad?
The short answer: Essentially no, and the reason is genuinely different from almost everything else in this household series. Distillation removes minerals, dissolved solids, and microorganisms, leaving water with very little dissolved material to support microbial growth, which is a large part of why properly produced, unopened bottled water doesn’t have a meaningful expiration point for safety. Bottled water manufacturers aren’t required under federal FDA rules to print an expiration date, although many do so voluntarily as a quality marker. What actually limits how long a jug stays useful isn’t the sealed water breaking down, it’s contamination once the seal is broken, since distilled water isn’t sterile forever just because it started out mineral-free, plus a specific, measurable chemistry quirk: distilled water absorbs carbon dioxide from the air once exposed, gradually lowering its pH.
For more household shelf-life guides, check out our full Household Shelf Life Guide.
This article is for general informational purposes and isn’t medical advice. For CPAP machines, humidifiers, infant formula, and nasal irrigation devices in particular, always follow the specific instructions provided by the device or formula manufacturer, and consult a doctor or pediatrician with questions about your own or your child’s situation.
Key Takeaways
Unopened distilled water: Doesn’t have a meaningful expiration point for safety when it’s properly produced, sealed, and stored. Many jugs still carry a printed “best by,” “sell by,” or similar date, but it’s a voluntary quality marker, not a federally required safety cutoff.
The reason it’s so stable sealed is what distillation removes. Distillation removes minerals, dissolved solids, and microorganisms from the source water, leaving very little dissolved material that can support microbial growth. That doesn’t mean it stays sterile forever once opened, since contamination can still be introduced from the air or the container.
Once opened, distilled water absorbs carbon dioxide from the air, which measurably lowers its pH from close to neutral toward the acidic side over time. This is real, gradual chemistry, not spoilage in the traditional sense.
How long an opened container stays “good enough” depends heavily on what it’s for. Drinking water and baby formula preparation call for the shortest windows, while water for a steam iron, humidifier, or car battery can reasonably be kept longer as long as the container stays clean.
CPAP machines and humidifiers deserve special caution. Contaminated water used in a CPAP specifically carries a real risk of respiratory illness, which is a higher-stakes situation than most other household uses of distilled water.
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