Distilled water is the recommended water for CPAP humidifiers. Every major manufacturer and sleep medicine organization agrees on this point, and the reason is straightforward: distilled water contains no minerals, bacteria, or chemical additives that could damage your machine or irritate your airways.
Why Distilled Water Is the Standard
Distilled water is made by boiling water into steam, then condensing that steam back into liquid. This process strips out essentially everything that isn’t water: minerals like calcium and magnesium, microbes, chlorine, and other dissolved substances. What you’re left with is as close to pure water as you can get in a bottle.
That purity matters because your CPAP humidifier heats water and turns it into moisture you breathe all night. Anything dissolved in that water either gets deposited inside your humidifier chamber or potentially carried into your airways. Minerals leave a white, chalky residue (scale) that builds up on the chamber walls, degrades the plastic over time, and forces you to replace parts sooner. Bacteria or other microbes in the water create an infection risk, since the warm, moist environment inside a humidifier is ideal for microbial growth.
Distilled water eliminates both problems at once. You can find it at virtually any grocery store or pharmacy, typically for about a dollar per gallon.
How Other Water Types Compare
If you’ve ever stood in the water aisle wondering whether “purified” or “spring” water would work just as well, the short answer is no, not quite.
Purified water has been filtered to remove chemicals, but it often still contains trace minerals. Some brands use reverse osmosis, which does a good job of stripping impurities, but then add minerals back in for taste. That remineralization is fine for drinking but defeats the purpose for your CPAP. Purified water may also still contain microbes depending on the filtration method.
Spring water is naturally rich in dissolved minerals picked up as it filters through underground rock. It’s refreshing to drink, but those minerals will leave deposits in your humidifier and can cause components to break down faster than expected.
Mineral water is, by definition, high in dissolved minerals. It’s the worst option for a CPAP humidifier.
Tap water contains varying levels of minerals, chlorine, and potentially bacteria depending on your local water supply. Hard water areas are especially problematic because the high calcium and magnesium content causes rapid scale buildup. Even in areas with soft water, the chlorine and trace contaminants make tap water a poor choice for nightly use.
What About Reverse Osmosis or Home Filters?
If you have a reverse osmosis (RO) system at home, the water it produces is close to distilled in purity. RO works by forcing water through a membrane that blocks most dissolved solids and microbes. The catch is that many RO systems include a remineralization stage as their final step, adding minerals back for better taste. If yours does, that water will still leave deposits in your humidifier over time.
Standard pitcher filters and fridge filters reduce chlorine taste and some contaminants, but they don’t remove dissolved minerals. Filtered tap water will still cause scale buildup in your CPAP.
Can You Boil Tap Water in a Pinch?
Boiling tap water for at least one minute (three minutes at higher elevations) kills bacteria and other microbes, making it safe from an infection standpoint. Ohio State University’s Wexner Medical Center lists boiled and cooled tap water as an acceptable alternative to distilled or sterile water for medical devices.
The limitation is that boiling doesn’t remove minerals. In fact, it slightly concentrates them since some water evaporates as steam while the minerals stay behind. Boiled water is a reasonable short-term solution when you can’t get distilled water, but using it every night will still cause mineral buildup in your humidifier chamber over time.
What to Use When Traveling
Finding distilled water on the road, especially internationally, can be tricky. In many countries it’s sold in pharmacies rather than grocery stores, sometimes labeled as “agua destilada” or “eau distillée.” If you genuinely can’t find it, plain bottled water that isn’t carbonated or mineral water is an acceptable temporary substitute. Look for the simplest bottled water available, ideally one labeled “purified” rather than “spring” or “mineral.”
Some travelers skip the humidifier entirely for short trips. Most CPAP machines work fine without water in the chamber. You’ll lose the moisture that prevents dry mouth and nasal irritation, but the therapy itself still works. If your destination has dry air or you’re prone to congestion, this tradeoff may not be worth it, but for a few nights it’s a practical option.
Cleaning Away Mineral Deposits
If you’ve been using non-distilled water and notice white residue in your humidifier chamber, a vinegar soak will remove it. Mix one part white vinegar with three parts warm water and let the chamber soak for 15 to 30 minutes, then rinse thoroughly. This dissolves the calcium and magnesium scale without damaging the plastic.
Even with distilled water, you should wash the humidifier chamber daily with warm water and mild soap, and do a vinegar soak weekly as a preventive step. Let all parts air dry completely before reassembling. Keeping the chamber clean isn’t just about mineral deposits. Standing water of any kind can harbor bacteria if left sitting for days, so empty and rinse the chamber each morning rather than topping off old water at night.