Distilled water is water that has been purified through a process of vaporization and subsequent condensation. The question of whether you can leave it exposed is not a matter of immediate danger, but rather a question of maintaining its defining characteristic: extreme purity. While short-term exposure is generally harmless, the water’s highly pure state is quickly compromised when it comes into contact with the ambient environment.
What Makes Distilled Water Different
The unique quality of distilled water is quantified by its extremely low level of Total Dissolved Solids (TDS). Standard tap water often contains TDS levels ranging from 50 to over 500 parts per million (ppm), while properly distilled water should register between 0 and 5 ppm. This near-zero measurement means the water is almost entirely stripped of ions, minerals, and salts.
This lack of dissolved content creates what is often referred to as “hungry water.” The water is thermodynamically unstable and possesses a strong drive to return to a natural equilibrium. Because it contains so few dissolved substances, it readily absorbs anything it encounters to satisfy this chemical need.
Environmental Factors That Cause Contamination
When distilled water is left uncovered, it immediately begins to absorb substances from the surrounding air, leading to three primary contamination pathways. The most rapid and noticeable change is the absorption of atmospheric carbon dioxide (\(CO_2\)). As \(CO_2\) dissolves into the water, it reacts to form carbonic acid, which lowers the water’s pH and increases its electrical conductivity.
This gaseous absorption process causes the water to become slightly acidic, typically dropping its pH from a neutral 7.0 to between 6.0 and 6.5. The increase in conductivity due to the formation of ions means the water is no longer suitable for applications requiring ultra-low conductivity. Furthermore, airborne particulate matter, including dust, spores, and microorganisms, settles onto the exposed surface of the water.
A third pathway involves leaching, which occurs if the water is stored in an inappropriate container. The water’s powerful tendency to dissolve substances causes it to pull chemicals, stabilizers, and monomers directly from certain plastics or metal surfaces. For this reason, the container material itself must be non-reactive to maintain the water’s integrity.
Purity Standards for Different Applications
The acceptable level of contamination in distilled water depends entirely on its intended use, meaning the functional shelf life of exposed water varies widely. For applications requiring the highest level of purity, such as in medical equipment, the window for safe use is extremely short.
Continuous Positive Airway Pressure (CPAP) machines and nebulizers require distilled water because mineral deposits can cause scale buildup and equipment damage. More significantly, using contaminated water in these devices aerosolizes any microbes present, introducing a risk of respiratory infection from organisms like Legionella or Pseudomonas. For such uses, the water should be sealed until the moment of use.
A medium level of purity is necessary for household appliances like steam irons or humidifiers. In these devices, the primary concern is the mineral scale that can cause clogs and damage over time. While the small amount of mineral content gained from \(CO_2\) absorption is usually not an immediate issue, it contributes to eventual scale buildup, making fresh or sealed water preferable.
How to Store Distilled Water Correctly
Maintaining the purity of distilled water after the original seal has been broken requires proper storage practices. The most straightforward step is to use an airtight container to prevent the exchange of gases with the atmosphere. Sealing the water immediately after use minimizes the absorption of \(CO_2\), which keeps the water’s pH stable and its conductivity low.
The choice of container material is another factor in preventing contamination through leaching. Non-reactive materials such as glass or food-grade, high-density polyethylene (HDPE) plastic are generally recommended. Avoid using containers made from low-grade plastics or reactive metals that the water’s pure nature could degrade over time.
Finally, the storage location should be cool and dark to inhibit microbial growth. Storing the water in a pantry or cupboard away from direct sunlight prevents the potential for UV light and warmth to encourage the growth of bacteria and algae.