CPAP therapy is a common treatment for obstructive sleep apnea. Modern CPAP machines often include a heated humidifier to add moisture to the pressurized air, preventing dryness and irritation in the user’s airways. Humidifiers require distilled water, which is pure and free of minerals and contaminants, to protect both the machine and the user’s health. Understanding how long this water remains safe, both in the bottle and in the device, is important for maintaining therapy standards.
Shelf Life of Bottled Distilled Water
Commercial distilled water, sealed in its original container, can remain viable for years past any printed date. This long shelf life is due to the distillation process removing all organic materials and minerals, leaving nothing for microbes to feed on. To preserve purity, the bottle should be kept in a cool, dark location away from direct sunlight and heat.
Once the factory seal is broken, the water becomes susceptible to contamination from airborne particles and microorganisms. Exposure to air also allows the water to absorb carbon dioxide, slightly altering its purity. For CPAP use, it is best practice to use opened distilled water relatively quickly, ideally within a few days to two weeks, depending on the storage environment. If the water appears cloudy, discolored, or has any visible foreign matter, discard it immediately.
Water Replacement Frequency in the CPAP Humidifier
The water inside the CPAP humidifier reservoir must be replaced daily, even if the chamber is not completely empty. The CPAP environment—warm water in a dark, enclosed space—is an ideal breeding ground for bacteria, mold, and other microorganisms. These microbes can multiply significantly within 24 hours.
Discarding the remaining water each morning prevents the formation of biofilms and the rapid growth of pathogens. Stagnant water sitting at room temperature or warmer significantly increases the microbial load. After emptying, the reservoir should be rinsed and allowed to air dry completely during the day before being refilled for the next night’s use. This daily maintenance prevents a buildup of contaminants that could be aerosolized and inhaled during therapy.
Risks of Using Stale or Contaminated Water
Failing to use distilled water or neglecting the daily replacement schedule introduces health and equipment risks. Health risks stem from inhaling aerosolized microorganisms that thrive in stale or non-distilled water. Tap water, classified as “unsterile” by the Centers for Disease Control and Prevention (CDC), contains pathogens like Legionella spp. and Pseudomonas aeruginosa. When heated and aerosolized by the humidifier, these microbes bypass the body’s natural defenses and are delivered directly into the sensitive respiratory system, potentially causing severe infections.
Using tap water also presents a direct threat to the CPAP machine’s functionality due to its mineral content, such as calcium and magnesium. As water evaporates from the heated humidifier chamber, these minerals are left behind and accumulate as a hard, white residue known as scale. This mineral buildup can damage the heating element, reduce the machine’s efficiency, and shorten the lifespan of the humidifier chamber. Even if the water is boiled, the mineral content remains, making daily distilled water use the only reliable way to protect both the equipment and the user’s health.