# CPAP: how does ResMed's "HumidX" system work?

**URL:** <https://boards.straightdope.com/t/cpap-how-does-resmeds-humidx-system-work/962558>\
**Category:** Factual Questions\
**Created:** [April 9, 2022, 2:08am UTC](https://boards.straightdope.com/t/cpap-how-does-resmeds-humidx-system-work/962558 "2022-04-09T02:08:55Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\
**Post date:** [April 9, 2022, 2:08am UTC](https://boards.straightdope.com/t/cpap-how-does-resmeds-humidx-system-work/962558/1 "2022-04-09T02:08:55Z")

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I’m interested in a compact CPAP unit for travel. **[ResMed’s Air Mini](https://shop.resmed.com/en-us/sleep-store/airmini-autoset)** seems like a pretty good choice, but I’m trying to understand the humidification system. My current home CPAP has a tank that you fill with distilled water for humidification, but the Air Mini uses **[a cartridge](https://shop.resmed.com/en-us/sleep-store/cpap-accessories/humidx)** placed in the hose just upstream of the mask that somehow captures the moisture of your exhaled air and puts it back into the fresh air you then inhale. The cartridges are a consumable item: once you open the sealed wrapper, you’re supposed to discard the cartridge within 30 days.

I thought maybe the 30-day time limit was a sanitary thing, because from what I’ve read, they’re not washable. But now I’m reading some user reviews that claim the cartridges actually stop working effectively after some period of time (anywhere from 1-3 months). This implies some kind of chemical activity taking place, with reactants getting consumed over time.

Anyone have an idea of how these little humidifier cartridges work?

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**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\
**Post date:** [April 9, 2022, 8:14am UTC](https://boards.straightdope.com/t/cpap-how-does-resmeds-humidx-system-work/962558/2 "2022-04-09T08:14:46Z")

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The Resmed site calls it a small “heat and moisture exchanger”, which appears to be a term of art. Wikipedia has a short article on them:

> **[Heat and moisture exchanger](https://en.wikipedia.org/wiki/Heat_and_moisture_exchanger)**
>
> Pages for logged out editors learn more
> 
> Heat and moisture exchangers (HME) are devices used in mechanically ventilated patients intended to help prevent complications due to "drying of the respiratory mucosa, such as mucus plugging and endotracheal tube (ETT) occlusion." HMEs are one type of commercial humidification system, which also include non-heated-wire humidifiers and heated-wire humidifiers.
> HMEs have been in clinical use for over 30 years.
> An HME cassette plays a central par...

There are also various NIH papers on the subject:

> **[Heat and moisture exchangers versus heated humidifiers for mechanically...](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6483749/)**
>
> Invasive ventilation is used to assist or replace breathing when a person is unable to breathe adequately on their own. Because the upper airway is bypassed during mechanical ventilation, the respiratory system is no longer able to warm and moisten...

I’m guessing the HumidX device is probably the hydroscopic type, with calcium chloride salts embedded in paper. I looked for a patent on it, but couldn’t find one, possibly indicating that this is very standard and unpatentable technology.

The salts themselves shouldn’t chemically degrade. But the paper matrix would only last so long in that environment. It’s also possible the salts wash away over time, or migrate in a way that makes them less effective.