Why does CPAP need distilled water?

CPAP humidifiers heat water. As the water evaporates, dissolved minerals in tap water stay behind and build up on the chamber walls and in the tubing. This mineral scale is hard to remove and creates surfaces where bacteria and mold can attach. Distilled water has essentially zero dissolved solids, so no mineral residue accumulates. That's the practical reason manufacturers specify it.

Can I use filtered water in my CPAP?

Standard carbon-filtered water (pitcher filter, faucet filter) still contains dissolved minerals. It's better than unfiltered tap for taste reasons, but it doesn't prevent mineral buildup in your CPAP chamber. Reverse osmosis water (TDS under 10 mg/L) is much closer to distilled and works well in practice for many CPAP users, though it's not what the manufacturer specifies. Distilled water from a grocery store is the manufacturer-recommended option.

What happens if you use tap water in a CPAP?

Two things happen. First, mineral deposits accumulate in the humidifier chamber and tubing. These deposits are hard to clean fully and shorten the life of your equipment. Second, the mineral scale creates surfaces where bacteria and mold can establish. Over time, tap water use leads to more frequent chamber fouling, potential respiratory irritation, and the need to replace components earlier than with distilled water.

Is RO water okay for CPAP?

Reverse osmosis water (TDS under 10 mg/L) is much closer to distilled than standard tap water and works well for many CPAP users in practice. It's not what the manufacturer specifies, so technically your warranty may not cover mineral damage. But for users who produce their own RO water at home, it's a reasonable daily option. Standard filtered water from a pitcher or faucet filter is not equivalent.

Where do I get distilled water for CPAP?

Most grocery stores carry distilled water in 1-gallon jugs for $1-2. Home Depot, Walmart, and many pharmacies also carry it. If you use your CPAP nightly with a humidifier at about 1 liter per night, that's roughly one 1-gallon jug every 3-4 nights, or $10-15 per month. A countertop water distiller ($80-200) produces distilled water at home for about $0.05-0.10 per gallon if you want a home production option.

Why CPAP Machines Require Distilled Water (and What to Use Instead)

Using tap water in your CPAP humidifier leaves a white mineral crust inside the chamber and along the tubing. It also creates conditions for bacterial and mold growth that filtered water doesn’t fully prevent.

This isn’t a vague manufacturer caution. It’s a practical engineering problem.

Why Distilled Specifically

CPAP humidifiers work by heating water until it evaporates into the airflow. When water evaporates, the water molecules leave, but the dissolved minerals don’t. They stay behind in the chamber.

Tap water, even in cities with relatively soft water, contains calcium, magnesium, and other dissolved minerals. Heat them repeatedly and they precipitate onto the chamber surface as a white crystalline scale. The same thing happens inside your tubing.

Carbon-filtered water (Brita, faucet filter, most pitcher filters) still contains dissolved minerals. It tastes better and has less chlorine, but the mineral content isn’t meaningfully lower than unfiltered tap. For a CPAP humidifier, filtered tap water behaves like tap water.

Only distilled water, which has essentially zero dissolved solids (typically under 1 mg/L TDS), prevents mineral accumulation. That’s why every CPAP manufacturer specifies it.

The Bacteria and Mold Question

Mineral deposits do more than foul your equipment.

Warm, moist surfaces with mineral texture give biofilm a physical foothold. Bacteria and mold attach more easily to rough mineral surfaces than to clean, smooth chamber walls. This is why tap water use doesn’t just create a cleaning problem, it creates a biological growth problem.

Distilled water’s near-zero TDS produces a less hospitable environment. Without mineral deposits to anchor to, biofilm has a harder time establishing.

This matters because the air from your CPAP goes directly into your airway during sleep. What’s in your chamber air matters.

RO vs. Distilled: A Practical Distinction

Reverse osmosis water (TDS typically under 10 mg/L) is considerably closer to distilled than tap water is. Many CPAP users run RO water daily without visible mineral buildup.

It’s not what the manufacturer specifies. Technically, using RO water means you’re outside the scope of their warranty guidance. In practice, the low TDS of RO water produces substantially less scale than tap water, and many users find it works well.

If you have a home RO system and generate your own filtered water anyway, RO water is a reasonable daily choice. Standard pitcher-filtered or faucet-filtered water is not.

What Distilled Water Costs

Grocery stores carry 1-gallon distilled water jugs for $1-2. Most pharmacies, Home Depot, and Walmart also carry it.

At roughly 1 liter per night, a gallon covers 3-4 nights. Monthly cost: $10-15.

If that feels like a recurring inconvenience, a countertop water distiller runs $80-200 and produces water at about $0.05-0.10 per gallon including electricity cost. At 1 liter per night, home distillation pays for itself in 1-2 years compared to buying jugs.

Traveling with a CPAP

Travel makes distilled water harder to find. Here are the practical options:

Some CPAP users boil tap water and let it cool before using it. Boiling kills bacteria and viruses, but it doesn’t remove minerals. This is better than using cold tap water directly, but it won’t prevent mineral buildup over a multi-week trip.

Bottled mineral water is a poor choice. It has higher mineral content than tap water in many cases. Avoid it.

Bottled purified water or reverse osmosis water (labeled “purified” or listing the treatment as RO) is a better travel option than mineral or spring water.

If you’re traveling internationally or somewhere grocery stores aren’t accessible, some CPAP users run the humidifier without water (cold therapy mode, if your device supports it) for a night or two rather than use high-mineral local tap water.

The Bottom Line

Buy a jug of distilled water from the grocery store. It’s $1-2, it’s available almost everywhere, and it directly prevents the two problems that non-distilled water creates: mineral fouling and biofilm conditions.

This is the simplest, cheapest compliance with your manufacturer’s specification. The CPAP chamber is an expensive component and a direct source of the air you breathe while sleeping. It’s not the place to optimize for convenience over specification.

Ask your CPAP equipment provider or care team if you have questions about your specific device’s humidifier requirements.