Distilled water is chemically pure water (H₂O), produced by boiling water into steam and condensing it back into a liquid. This process removes nearly all minerals, salts, and contaminants, resulting in a highly purified product. Although the water molecule itself does not degrade, environmental factors and packaging integrity can compromise its pristine state over time. The expiration date on the label is a guide for maintaining the water’s initial high quality, not a sign of chemical spoilage.
Understanding Distilled Water Purity
Distillation is a purification technique that mimics the natural water cycle. The process involves heating water until it vaporizes into steam, leaving behind substances like dissolved minerals and salts. The collected steam is then cooled, changing it back into pure liquid water. This method effectively removes over 99.9% of dissolved solids, making the resulting water virtually free of minerals and most microorganisms.
This purity allows distilled water to be used in sensitive applications, preventing mineral scale buildup in machinery and avoiding contaminants in laboratory or medical settings. The electrical conductivity of distilled water is very low, reflecting the absence of conductive mineral ions. This baseline purity is the standard against which its “expiration” is measured, as any change indicates contamination.
The Real Reason for the Expiration Date
The expiration date on distilled water refers less to the water itself and more to the container it is packaged in. Unopened distilled water, if stored correctly, can remain pure for years, often exceeding the date printed on the bottle. The date primarily serves as a guideline reflecting potential changes in the packaging over time.
A primary concern is the leaching of chemicals from the plastic container into the water. If exposed to heat or sunlight, the plastic material can slowly degrade and release trace organic compounds. Low-grade plastics are susceptible to this process, potentially introducing substances like phthalates or BPA and compromising purity.
Atmospheric contamination compromises purity once the seal is broken. Distilled water lacks dissolved solids and readily absorbs gases from the air, quickly drawing in carbon dioxide. This carbon dioxide reacts with the water to form a weak carbonic acid, slightly lowering the water’s pH. Airborne mold spores, dust, and bacteria can also enter an opened container, leading to microbial contamination over time.
Practical Safety Guide: When to Use and When to Discard
The decision to use expired distilled water depends on the intended application and the level of risk involved. Uses involving inhalation or direct ingestion of contaminants require a higher degree of caution. Health-related applications require the highest standard of purity.
High-Risk Uses
If distilled water is intended for a Continuous Positive Airway Pressure (CPAP) machine, a nebulizer, or mixing baby formula, it is safest to discard expired or long-opened water. Inhaling water vapor from a CPAP machine introduces airborne bacteria or mold spores directly into the lungs, posing a health risk. Water used for infant consumption must also be free of contaminants or leached chemicals. For these high-risk uses, replace any bottle past its date or one that has been open for more than a few weeks.
Low-Risk Uses
For applications where the water is not aerosolized or ingested, the risk posed by minor contamination is much lower. Expired distilled water can often be used safely in steam irons, car batteries, and for cleaning purposes. Using it in a humidifier is also lower risk, provided the water is not directly inhaled. The slight presence of leached plastic compounds or absorbed carbon dioxide will not negatively affect the function of these devices.
Before using any old distilled water, a simple visual inspection is necessary to assess its quality. Look closely for signs of cloudiness, floating particles, or visible biological growth, such as mold or a slimy film. The water should also be checked for any unusual odors, which can indicate microbial activity or chemical leaching. If the water appears perfectly clear and odorless, it is likely acceptable for low-risk, non-medical applications.