Distilled water is the best water for a CPAP humidifier. It’s free of minerals and bacteria, which prevents scale buildup inside the chamber and reduces your risk of inhaling waterborne pathogens. Both the CDC and major CPAP manufacturers recommend distilled or sterile water for everyday use.
That said, distilled water isn’t always easy to find, especially when you’re traveling or your local store is sold out. Here’s what you need to know about every common water type and when each one is safe to use.
Why Distilled Water Is the Standard
Distilled water has been boiled into steam and then condensed back into liquid, leaving behind virtually all minerals, chemicals, and microorganisms. This matters for two reasons. First, minerals like calcium and magnesium create a white, crusty scale on the humidifier’s heating plate over time. That scale shortens the life of your equipment and is difficult to fully remove. Second, and more importantly, a CPAP humidifier aerosolizes water into tiny droplets you breathe directly into your lungs. Any bacteria or other organisms in that water get a direct route past your body’s normal defenses.
Hospitals, labs, and factories rely on distilled water for the same reason: it doesn’t leave residue on sensitive equipment and it doesn’t introduce contaminants.
What Makes Tap Water Risky
Tap water is safe to drink because your stomach acid and digestive system handle most common microorganisms without trouble. Breathing aerosolized tap water is a different story. The CDC notes that water systems can harbor organisms including Legionella, nontuberculous mycobacteria (NTM), and Pseudomonas, all of which cause lung infections when inhaled. In the United States, these waterborne pathogens are responsible for roughly 120,000 hospitalizations and 7,000 deaths each year.
There’s a documented case of a woman with sleep apnea who developed severe Legionella pneumonia linked to her contaminated CPAP equipment. The suspected cause: rinsing her mask and tubing with tap water instead of distilled water, and using tap water in the humidifier. Legionella thrives in warm, standing water, which is exactly the environment inside a heated humidifier chamber.
Beyond infection risk, tap water leaves mineral deposits. Depending on how hard your local water is, you may see visible white scaling within just a few weeks.
Other Water Types Compared
Sterile Water
Sterile water has been treated to kill all microorganisms and is perfectly safe for CPAP use. The CDC lists both sterile and distilled water as acceptable options. The downside is cost: sterile water sold for medical purposes is typically more expensive than a gallon of distilled water from the grocery store. If you’re immunocompromised or recovering from a respiratory illness, sterile water offers an extra margin of safety.
Purified and Filtered Water
Purified water (including reverse osmosis) removes most contaminants but may still contain trace minerals or organisms depending on the filtration method. It’s a reasonable short-term substitute if distilled water isn’t available, but it can still leave some mineral residue over time. Standard pitcher filters like Brita reduce chlorine and some heavy metals but don’t remove the dissolved minerals that cause scaling or the bacteria that pose inhalation risks.
Bottled Spring Water
Spring water contains natural minerals, sometimes at higher concentrations than tap water. It will leave deposits in your humidifier chamber and is not a good long-term choice. In a pinch for one night, it won’t damage your machine, but don’t make it a habit.
Boiled Water
Boiling tap water for at least one minute (three minutes at higher elevations) kills bacteria, viruses, and parasites. This makes it microbiologically safe, which is the more important concern. However, boiling doesn’t remove dissolved minerals. It actually concentrates them slightly as some water evaporates. Boiled and cooled tap water is a solid emergency option when you can’t get distilled water, especially while traveling.
What to Do When Traveling
Finding distilled water in an unfamiliar city or country isn’t always straightforward. A few practical approaches:
- Pack a small supply. A gallon of distilled water lasts most people several nights. For short trips, just bring one along.
- Boil tap water. If you have access to a kettle or microwave, bring water to a rolling boil, let it cool to room temperature, and fill your chamber. This eliminates the pathogen risk even if minerals remain.
- Skip the humidifier. Your CPAP works fine without water in the chamber. You’ll lose the humidification feature and may wake up with a dry mouth or nose, but the therapy itself is unaffected. For a night or two, this is the simplest solution.
- Use bottled water as a last resort. Bottled purified water is better than untreated tap water. Check the label for “purified” or “distilled” rather than “spring” or “mineral.”
Daily Water Habits That Protect Your Equipment
Using the right water only solves half the problem. How you handle the water matters just as much. Empty your humidifier chamber every morning and let it air dry. Standing water, even distilled water, becomes a breeding ground for bacteria and biofilm within a day. Refill with fresh water each night rather than topping off yesterday’s water.
Philips Respironics specifically recommends using room temperature distilled water, never hot. Hot water can warp the chamber or release compounds from the plastic.
Cleaning the Humidifier Chamber
Even with distilled water, a thin biofilm can develop over time. Wash the chamber daily with warm water and a mild liquid detergent, then rinse thoroughly. Once a month, soak the chamber for 30 minutes in a solution of 2 parts white vinegar to 3 parts water (about 1 cup of vinegar to 1½ cups of water). This dissolves any mineral deposits that have accumulated. Rinse well with warm water afterward so you’re not breathing vinegar fumes. The same vinegar solution and ratio works for your tubing, though tubing benefits from a weekly soak rather than monthly.
Never put the humidifier chamber in a dishwasher or use bleach, rubbing alcohol, or scented soaps. These can degrade the plastic or leave residues that get aerosolized into your airway.