Distilled water is often used in applications where purity is a priority, such as in medical devices or laboratory work. The question of how long it remains suitable for use once the seal is broken is a common concern. While water itself does not expire, its quality can be quickly compromised once exposed to the environment. Understanding the factors that degrade this purified water is key to preserving its integrity after opening.
The Unique Purity of Distilled Water
Distilled water is created by boiling water and collecting the steam as it condenses back into a liquid state. This method leaves behind nearly all dissolved solids, including minerals, salts, and microbes, resulting in a product that is over 99.9% pure water. This lack of dissolved matter leaves the water in a chemically “hungry” or aggressive state. The water will readily seek to absorb whatever it contacts to reach chemical equilibrium. This purified state, while ideal for sensitive uses, also makes it vulnerable to contamination after opening.
How Environmental Factors Impact Shelf Life
The high purity level of distilled water means its shelf life once opened is greatly influenced by its storage environment. For general household use, such as for steam irons or humidifiers, opened distilled water may remain suitable for approximately two to four weeks. However, for sensitive uses, like in a CPAP machine or medical applications, the water should ideally be used within one week. This variation is tied to specific environmental factors that encourage the introduction and growth of contaminants.
Temperature is a major accelerator of microbial growth, as warmer conditions promote the faster multiplication of bacteria or mold spores. Exposure to air is another significant factor, allowing the water to absorb airborne microorganisms, dust, and carbon dioxide. Absorbing carbon dioxide gas slightly lowers the water’s pH level, which alters its chemical properties. Furthermore, the container’s material and cleanliness play a role, as the aggressive water can leach trace elements from certain plastics or metals it contacts.
Identifying Signs of Contamination
Because the purity of distilled water can degrade rapidly after opening, consumers should always perform sensory checks before use. A noticeable change in the water’s appearance is a primary indicator that it is no longer suitable. This includes any sign of cloudiness, the presence of visible particulate matter, or the development of a thin film on the water’s surface, which may indicate the growth of mold or algae.
The water should also be checked for any change in smell, as pure distilled water is odorless. The presence of a musty, sour, or metallic odor suggests that microbial growth has begun or that the water has absorbed chemicals from the container or the air. If there is any doubt about the water’s quality, especially if it is intended for a medical device, it should be discarded to avoid potential health or equipment issues. The risk of introducing contaminants outweighs the cost of replacing the water.
Maximizing Longevity Through Proper Storage
Implementing preventative steps can significantly extend the useful life of opened distilled water. The container must be clean, and an airtight cap is necessary to limit the introduction of airborne contaminants and reduce the absorption of atmospheric gases. Glass or high-grade, food-safe plastic containers are the best choices for storage, as they are less likely to leach substances into the water compared to certain other materials.
The storage location should be cool and dark, ideally maintaining a stable temperature between 59 and 77 degrees Fahrenheit (15–25 degrees Celsius). This keeps the water away from direct sunlight or heat sources that accelerate degradation. Careful handling is also important to prevent the introduction of microorganisms. Users should avoid touching the container’s rim and should not pour the water directly from the mouth of the container into a device, which minimizes the transfer of microbes.