A Continuous Positive Airway Pressure (CPAP) machine treats sleep apnea by delivering pressurized air to keep the user’s airway open. Many users rely on an integrated humidifier to add moisture to the airflow, preventing dryness and irritation in the nasal passages and throat. Manufacturers consistently recommend using only distilled water in this heated humidifier chamber to maintain the equipment and ensure patient safety. However, the convenience of filtered water often leads users to wonder if it is an acceptable substitute.
Why Distilled Water is the Only Standard
Distilled water is the standard for CPAP use due to its near-zero content of Total Dissolved Solids (TDS). Distillation is a purification process that involves boiling water and collecting the resulting steam as it condenses back into a liquid state. This phase change leaves virtually all inorganic compounds and microorganisms behind.
Minerals like calcium, magnesium, and iron, common in tap water, cannot convert into steam and are removed during this process. The resulting product is water that is 99.9% pure, with less than 1 part per million (PPM) of TDS. This purity prevents the formation of mineral deposits, often called scale, inside the machine’s humidifier.
The Limitations of Filtered Water
Filtered water is not chemically equivalent to distilled water and retains a measurable amount of TDS. Standard filtration methods, such as pitcher or refrigerator filters, primarily use activated carbon to remove biological contaminants, chlorine, and chemicals that affect taste and odor. These filters improve water quality for drinking but are not engineered to eliminate the hard minerals that cause scaling.
Advanced purification processes, like reverse osmosis (RO), force water through a semipermeable membrane to remove sediment and chemicals. While RO water is purer than standard filtered water, it still contains trace dissolved solids, making it less suitable than distilled water for long-term CPAP use. The term “filtered” means particles and some impurities have been strained out, but it does not guarantee the absence of minerals.
Practical Impact on CPAP Equipment and Health
Using filtered water introduces mineral content into the heated humidifier chamber. When the CPAP humidifier heats the water, the water evaporates, but the non-volatile minerals remain behind, concentrating in the chamber. Over time, these minerals form a hard, crusty residue, or scale, on the heating plate and the water tub interior.
The buildup of mineral scale reduces the efficiency of the humidification system by impeding the heating element’s ability to warm the water. The residue requires increased cleaning, and in severe cases, scale can cause the equipment to malfunction or fail. Manufacturers often stipulate the use of distilled water, and evidence of mineral damage from non-approved water sources may void the machine’s warranty.
While the primary concern is machine damage, health considerations also exist. Inhaling aerosolized mineral particles is not ideal, and the presence of dissolved solids creates a surface where bacteria or fungi can colonize and grow. If distilled water is unavailable, using filtered or bottled purified water for a night or two is a safer temporary alternative than tap water, provided the humidifier tank is emptied and cleaned immediately the next morning. A better emergency option is to boil tap water for five minutes and allow it to cool; boiling kills microorganisms, though it does not remove the minerals.