Purified water and distilled water are not the same thing, and the difference matters for your CPAP machine. Distilled water has been boiled into steam and condensed back into liquid, leaving behind virtually all minerals, bacteria, and other contaminants. Purified water can be produced through several different methods, and some of those methods leave trace minerals behind or add them back in for taste. CPAP manufacturers specifically recommend distilled water for humidifier chambers.
How the Two Types of Water Differ
Distilled water goes through a single, straightforward process: water is heated until it becomes steam, and that steam is collected and cooled back into liquid form. Because minerals, bacteria, and dissolved solids are too heavy to evaporate with the water, they stay behind in the boiling container. The result is water that contains essentially no organic or inorganic material.
Purified water is a broader category. Under FDA labeling rules, “purified water” is an umbrella term that includes distilled, demineralized, deionized, and reverse osmosis water. So all distilled water qualifies as purified water, but not all purified water is distilled. Commercial purification often involves filtration through sand, charcoal, and gravel, followed by chemical disinfection with chlorine or similar agents. These steps remove bacteria and many contaminants effectively, but they don’t necessarily strip out dissolved minerals the way distillation does.
Some bottled water brands labeled “purified” also add minerals or electrolytes back in after processing to improve taste. If the label lists ingredients like sodium, potassium, or magnesium, those minerals are in the water and will end up in your CPAP humidifier chamber.
Why CPAP Manufacturers Recommend Distilled
Philips Respironics, one of the largest CPAP manufacturers, explicitly recommends using room temperature distilled water “to prevent mineral build up on the humidifier tank base.” This isn’t just about keeping your equipment clean. Your CPAP humidifier heats water and turns it into fine vapor that you breathe directly into your airways all night. Whatever is in that water gets aerosolized along with it.
The mineral issue is twofold. First, minerals leave a white, chalky residue inside the humidifier chamber over time. This buildup can damage the heating plate, shorten the life of the chamber, and make cleaning harder. Second, studies have shown measurable concentrations of minerals in the aerosol output of humidifiers. Breathing in aerosolized mineral particles night after night is an unnecessary exposure that distilled water eliminates entirely.
The Infection Risk From Non-Distilled Water
Mineral buildup is the visible problem, but the invisible one is more serious. Tap water, and even some forms of purified water, can harbor microorganisms that form biofilms inside warm, moist environments like a CPAP humidifier chamber. Pathogens commonly found in water systems include Pseudomonas, nontuberculous mycobacteria (NTM), and Legionella. In the United States, these biofilm-associated waterborne pathogens account for roughly 120,000 hospitalizations and 7,000 deaths each year.
What makes CPAP use particularly risky is the route of exposure. Most NTM and Legionella infections are acquired through inhalation, which is exactly what a CPAP machine facilitates. The device pushes humidified air directly into your nose and throat for hours at a time. A CDC survey noted that “water quality affects the quality of aerosolized air emitted from humidifiers, CPAP machines, and vaporizers, underlying the need for sterilized or distilled water to be used in those devices.”
Distilled water removes both the organic contaminants (bacteria, amoebas) and the inorganic ones (minerals, dissolved solids). Sterile water kills microbes but can still contain minerals. Purified water may or may not address either concern fully, depending on how it was processed.
What to Do When Distilled Water Isn’t Available
If you’re traveling or can’t find distilled water, you have a few options. Philips advises that tap or bottled water can be used temporarily, but you’ll need to clean the humidifier chamber monthly or more often to manage mineral deposits and bacterial growth. Running your CPAP without the humidifier altogether is also an option for short periods; it won’t affect the air pressure therapy, though your mouth and nasal passages may feel drier.
Boiling tap water kills most bacteria, but it does not remove minerals. In fact, boiling concentrates minerals because some of the water evaporates while the dissolved solids remain. If you boil water and then collect only the steam (essentially building a home distillation setup), that condensed steam is distilled water. Simply boiling a pot of water and pouring it into your CPAP chamber after it cools is not the same thing.
Reverse osmosis water is the closest common alternative to distilled. It removes the vast majority of dissolved minerals and contaminants through a membrane filtration process. If a bottle is labeled “purified by reverse osmosis” and lists no added minerals in the ingredients, it’s a reasonable substitute when distilled water is unavailable.
Reading the Label Correctly
When you’re standing in a store aisle looking at bottled water, the label tells you what you need to know. A bottle labeled “distilled water” with no additional ingredients is exactly what your CPAP needs. A bottle labeled “purified water” requires a closer look. Check for a description of the purification method (distilled, reverse osmosis, deionized) and scan the ingredients for added minerals or electrolytes.
Brands that market “purified drinking water” often add minerals for flavor. These products are fine for drinking but not ideal for a device that will aerosolize that water into your lungs. Spring water and mineral water are even worse choices, as they naturally contain higher levels of dissolved minerals. Stick with water that has been stripped of everything, not water that had things removed and then added back in.