What water is best for CPAP humidifier tanks?

What water is best for CPAP humidifier tanks?

Read time: 12 minutes

What’s inside?

  • What is distilled water all about?
  • Deionised? Never heard of it!
  • Bottled water is clean… isn’t it?
  • Is tap best?
  • But my tap water has fluoride added… what then?...
  • Where does that leave us then?


When it comes to CPAP users we’re all human, so we all have different opinions on what is right or wrong… and sometimes what is safe or unsafe.

One of those questions that regularly get people engaged… and sometimes enraged… is: What water is safe to use in a CPAP humidifier?

A lot of people swear by distilled. Others say soft tap water is fine. And a newish one on the circuit is deionised.

I didn’t understand the difference between some of these so decided to dig deeper and find out if there are any facts behind the opinions.


What is distilled water all about?

First up was distilled, which can be easily found in some countries such those in North America and mainland Europe but less so elsewhere. The more difficult to find… the more expensive it becomes.

Distilled water is simply water that has been boiled into a vapour and condensed back into a liquid in a separate container. This process removes a lot of impurities such as minerals, bacteria, and chemicals. For CPAP machines, this high level of purity helps prevent mineral buildup in the humidifier tank, which can extend the life of the equipment.

However, the process of distilling water is energy intensive. When considering the environmental impact, using distilled water regularly for a CPAP machine adds to energy consumption, which might be a concern for you if you are an environmentally conscious user.

Not all impurities are removed either.

Volatile Organic Compounds (VOCs) are organic chemicals that have a high vapour pressure at room temperature. These can include substances like pesticides and solvents. Since some VOCs boil at, or below, the boiling point of water, they can vaporise along with the steam and potentially condense back into the distilled water.

Some gases dissolved in water, such as chlorine, can also vaporise and recondense into the distilled water. Whether they make it through the distillation process can depend on things like the equipment & process used along with the particular boiling point of those gases. If they boil at, or below, 100C then there is good chance they’ll find their way into the distilled water you are about to pour into your tank.

And if that wasn’t enough, all the good work can be thrown away by incorrect handling of the successfully distilled water. Unsterilised distillation equipment or containers can give bacteria a home in your fresh liquidy breeding ground.  Storing it in unsuitable plastic containers could also allow micro-plastics to leach into your fresh stuff.

Carbon filtering has been mentioned as an option for attempting to remove some impurities from distilled water but it does not always work.

If the nasty bits like heavy metals are gases or attach themselves to finer particles during the boiling process, some could escape removal.

However, the odds are reduced significantly during distillation (which generally removes the vast majority) and carbon filtration adds an additional layer of security by potentially capturing any that vaporise.

‘Potentially’ is an important word here though. Carbon filtering relies on the teeny-weenie nasty molecules sticking to the carbon so they can be removed. As with distillation, it works most of the time but there is always the chance something will slip through.

Overall, the decision to use distilled water in a CPAP humidifier comes down to a balance between health benefits and practicality. While distilled offers the safest option for maintaining both the device and your own health, the associated costs and availability issues might become a blocker… especially if you live in place where supermarkets don’t ordinarily have it on their shelves.


Deionised? Never heard of it!

Deionised water isn’t something we used to hear about in the CPAP world but that has changed recently. To become deionised the water has to go through an ion exchange where contaminants that have an electrical charge (a teeny-weenie charge… nothing harmful here) are switched-out for hydrogen & hydroxide… which combine to make more water.

If you can imagine you had a block of cheese with a little bit of something you didn’t like stuck on it. Scooping that bit out and replacing it with exactly the same sized piece of cheese magically made at exactly the same time you needed it… would get you a clean and edible piece of cheese again. It’s kinda like that.

The process can remove mineral salts and other charged ions but may not effectively remove organic materials or microorganisms unless combined with other purification methods like distilling and carbon filtering mentioned above.

The main differences between distilled and deionised are:

Purity Level: Distilled water is generally more purified because it removes a broader range of bad stuff including both dissolved solids and most organisms. Deionised water mainly removes only mineral ions.

Processes: Distillation is a thermal process, which can be done almost anywhere you can power he equipment, whereas deionisation is a more complex chemical process.

Residual Contaminants: While distilled water is free of almost all forms of impurities, deionised water may still contain non-ionic contaminants and organisms unless treated again by some other means… such as distillation.

Usage: Distilled water is good for medical devices, such as CPAP machines, where mineral deposits could be harmful. Deionised water is favoured in scenarios like pharmaceuticals and cosmetics, where ion concentration needs controlled but complete sterility isn't necessary.


Bottled water is clean… isn’t it?

Bottled mineral water is often chosen for its taste and perceived purity, but it contains various dissolved minerals and other components that can pose risks when used in CPAP humidifiers. Especially over a long period of time.

Depending on the source, there can be a bundle of impurities in bottled mineral water – such as:

Minerals: Okay… a bit of a giveaway here, but mineral water contains minerals. Some of these can be healthy for you when drank but not so healthy when they include bacteria or other teensie-weensie organisms that you inhale when the water is evaporated in your humidifier tank.

The top dissolved minerals in bottled water include calcium, magnesium, sodium, and potassium. Naturally occurring, how much is in your water can vary widely depending on the source.

Chemical Contaminants: Bottled water can also contain trace amounts of chemicals from the bottling process or the plastic containers in which it is stored. These can include bisphenol A (BPA). Although most main brand mineral waters will use BPA free plastic, some cheaper or local versions may not. If these do get into the water then the long-term health risks could be real. Much would depend on what the chemical was though.


When we look at CPAP humidifiers in particular, the risks can become pretty dodgy over time, including:

Mineral Buildup: The minerals in bottled mineral water can lead to deposits in your humidifier tank and on the heating element. Over time, this buildup can impair the function of the CPAP machine, requiring more frequent maintenance and potentially shortening the life of the device.

Bacterial Growth: All of this can lead to bacterial growth in the humidifier tank. This often shows up as a dark-orangey coloured film in the corners of the tank. When water vapor containing these bacteria is inhaled, it can cause respiratory infections or make existing respiratory conditions worse.

I reckon I’ve fallen foul of this myself, but it isn’t only bottled water than can cause this issue…


Is tap best?

Before we get to deciding which is best for us though, there’s a heavy set of people – including sleep practitioners - telling us some tap water is perfectly good enough.

Although a small minority of people are happy to use straight tap water in their humidifier, almost every sleep practitioner agrees this is a bad thing and those who do say tap is good enough will also tell us to boil it first. This at least sterilises the water and kills off the main bugs that can cause serious health issues.

There are 2 main players in the tap water world… Soft & Hard.

Soft water has a low concentration of calcium and magnesium. It is either naturally occurring or produced using a water softener to remove these minerals. The benefits and characteristics of soft water include:

Gentler on your CPAP: Soft water prevents scale buildup, which protects appliances and devices, extending their operational life and efficiency.

Better for Cleaning: It also enhances the effectiveness of soaps and other cleaning agents, reducing the amount required for cleaning tasks. It also leaves fewer residues on your tank.


Hard water, on the other hand, contains a higher concentration of minerals, primarily calcium and magnesium. These minerals are picked up by water as it flows through limestone and chalk deposits.

Hard water is common in many areas and has some specific characteristics and side-effects:

How it effects your CPAP: Due to its high mineral content, hard water leads to scale build-up – including your humidifier tank, potentially reducing its efficiency and lifespan.

Impact on Cleaning: It can also leave heavy scale deposits and film inside your tank. This can take a lot more cleaning than if less mineralised water was used, although a white vinegar solution can do wonders on even heavy scaling.


The mineral content in hard water can have health benefits when you drink it, such as contributing to dietary needs of calcium and magnesium. Soft water, lacking these minerals, fails to provide this free mineral kick but is considered more beneficial for those with certain skin conditions like eczema, as it is less irritating to the skin.

What does this mean for CPAP?

Well, the minerals in hard water can vaporise and be inhaled when used in a CPAP machine. Again, what was good when drank can be harmful over time when inhaled.


But my tap water has fluoride added… what then?...

Fluoride is commonly added to public water supplies in many countries to improve oral health by reducing tooth decay.

The downside for CPAP users is that using boiled water, which has previously had fluoride added, in the humidifier tank of a CPAP machine carries extra potential health implications that users should consider.

What’s the problem with sticking fluoride in my humidifier?

Well, fluoride does not evaporate easily at boiling temperatures and can remain in the water even after it has been boiled. This means that using boiled tap water in a CPAP machine might still expose the user to fluoride, which isn’t meant to be inhaled. On the plus side, this also means if that boiled water is distilled into a different container then the chances of the fluoride making it through are slim.

And, speaking of fluoride not being inhaled, there is some debate about the toxicity of fluoride when it enters the body like this. As you may expect, inhaling fluoride isn’t very common and therefore hasn’t really been studied much. However, it may be worth erring on the side of caution here as not knowing how bad it can be for you isn’t the same as it not being bad for you.

So, while boiling water may remove some contaminants, it does not eliminate fluoride, and its use in a CPAP machine could carry potential risks.


Where does that leave us then?

Well, as someone who mainly lives in a soft water area where fluoride is not added to the water, I was one of those told by my sleep clinic that boiled tap water was okay. I’ve also been known to use bottled water when travelling in areas where my confidence in the tap water is low. Using distilled has been a rare occurrence for me as boiling soft tap water at home is easier and lugging around 5 litre tubs of distilled when travelling is a pain.

I rinse out my humidifier tank daily and give it a deep clean in a warm water & white vinegar solution once a month.

This has not prevented the orangey coloured film growing in the tank before the deep cleaning time comes around.

Maybe once a month isn’t enough?

I’ve certainly been experiencing more long-lasting coughs recently.

When digging through the available evidence I came to a few conclusions I’m going to try…

  1. Do I really need the humidifier?

It seems if I could do away with the tank altogether then the risk of inhaling water-borne nasties would go away. It’s a balance though as drying out the mouth and throat can cause other problems too. It may not even be an option if you live in a warm climate.


  1. How easily and cheaply can I get distilled water?

All evidence seems to point to one thing… if you DO need to use a humidifier, distilled water seems to be the safest bet. It certainly isn’t perfect but appears to be less risky than the other water options available to us. There are home water distillation systems available for around the same price as a few CPAP masks. Just try Googling ‘home water distillation system’ and see for yourself.

I started this journey by not understanding what all the fuss was about. I don’t think I have reached the end yet, but at this point I reckon I’ve been missing something so far.

It appears there could be something to this distilled water malarky. My first attempt will be to see if I can do without my humidifier. If so… problem solved.

If not, I’ll certainly be testing one of those distilling kits it in the near future.

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