Continuous Positive Airway Pressure (CPAP) therapy is a standard treatment for obstructive sleep apnea, delivering pressurized air through a mask to keep the user’s airway open during sleep. Many CPAP devices include a heated humidifier, which adds moisture to the air to prevent common side effects like nasal congestion, dry throat, and sinus irritation. Manufacturers strongly recommend that only distilled water be used in the humidifier reservoir. Distilled water is essentially pure H₂O, devoid of the minerals and contaminants found in other water sources.
The Immediate Answer: Using Tap Water for One Night
In a situation where distilled water is unavailable, using tap water for a single night is considered an acceptable exception rather than a recommended practice. The two primary concerns associated with non-distilled water—equipment damage and health risk—are low for one instance. This short-term use will likely not cause immediate, irreversible harm to the machine or the user.
However, this allowance is conditional and must be followed by immediate and thorough action the next morning. It is important to empty the humidifier reservoir as soon as you wake up, preventing the water from sitting and allowing any present microorganisms to multiply. You should then clean the chamber with mild soap and water to remove any film or residue left behind. Long-term use of tap water is never advised, and distilled water should be obtained as quickly as possible.
Understanding the Primary Risk: Mineral Scale Buildup
The main reason distilled water is the standard recommendation is to protect the CPAP machine’s components from mineral scale buildup. Tap water, even if filtered, contains dissolved inorganic minerals such as calcium, magnesium, and lime. When the CPAP humidifier heats this water, the pure water molecules evaporate into the air, but the mineral solids are left behind in the reservoir.
Over time, these leftover minerals form a hard, white, or grayish crust known as scale on the heating plate and the reservoir walls. This buildup reduces the efficiency of the humidifier, forcing the heating element to work harder to warm the water. The scale can eventually damage the heating element, shorten the lifespan of the water chamber, and may even void the manufacturer’s warranty on the equipment. While one night of use will not create a noticeable buildup, repeated use will cause this scaling to become a maintenance issue.
Potential Health Concerns from Contaminants
The risk for one night is minimal, but tap water, categorized by the Centers for Disease Control and Prevention (CDC) as “unsterile water,” may contain various microorganisms like bacteria, fungi, and protozoa. These are typically harmless when swallowed but can pose a risk when aerosolized. Unlike the digestive tract, which has strong acids to neutralize many pathogens, the respiratory system is more vulnerable to inhaled contaminants.
When the CPAP humidifier heats and aerosolizes the water, any microbes or chemical irritants present become airborne and are delivered into the user’s sinuses and lungs under continuous pressure. Contaminants such as Legionella or Pseudomonas bacteria can be inhaled, potentially leading to respiratory irritation or, in rare cases, serious infections. The risk of these health issues is compounded with repeated use, as the humidifier chamber can become a breeding ground for these organisms.
Safe Temporary Alternatives to Distilled Water
When distilled water is unavailable, temporary alternatives are safer than using straight tap water. The most effective short-term substitute is tap water that has been boiled for at least five minutes and then allowed to cool completely before being added to the reservoir. Boiling kills microorganisms that may be present in the water supply, addressing the immediate health risk.
Boiling does not remove the dissolved minerals, so this method will still lead to scale buildup with regular use, but it does significantly reduce the biological risk. Bottled water, such as purified or spring water, still contains varying levels of minerals. If only standard tap water is available, you can minimize the amount of water needed by turning down or switching off the CPAP humidifier entirely, if tolerated, to reduce the evaporation and aerosolization of non-distilled water.