Editing Gas mask (section)

==Reaction and exchange==
{{Main|Chemical cartridge#Detection of end of service life}}
{{Unreferenced section|date=October 2017}}

This principle relies on substances harmful to humans being usually more reactive than air. This method of separation will use some form of generally reactive substance (for example an [[acid]]) coating or supported by some solid material. An example is [[synthetic resin]]s. These can be created with different groups of [[atom]]s (usually called [[functional group]]s) that have different properties. Thus a resin can be tailored to a particular toxic group. When the reactive substance comes in contact with the resin, it will bond to it, removing it from the air stream. It may also exchange with a less harmful substance at this site.

Though it was crude, the [[hypo helmet]] was a stopgap measure for British troops in the trenches that offered at least some protection during a gas attack. As the months passed and poison gas was used more often, more sophisticated gas masks were developed and introduced. There are two main difficulties with gas mask design:
* The user may be exposed to many types of toxic material. Military personnel are especially prone to being exposed to a diverse range of toxic gases. However, if the mask is for a particular use (such as the protection from a specific toxic material in a factory), then the design can be much simpler and the cost lower.
* The protection will wear off over time. Filters will clog up, substrates for absorption will fill up, and reactive filters will run out of reactive substances. Thus the user only has protection for a limited time, and then they must either [[Chemical cartridge#Detection of end of service life|replace the filter device in the mask]], or use a new mask.

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File:Humboldt gasmask 1799.jpg|A primitive respirator was designed by Alexander von Humboldt in 1799 for underground mining
File:Various gas masks WWI.jpg|Various gas masks employed on the [[Western Front (World War I)|Western Front]] and [[Eastern Front (World War I)|Eastern Front]] during World War I
File:1930s gas mask.jpg|Finnish civilian gas mask from 1939. These masks were distributed during World War II
File:A mother and baby both in gas-masks during 1941. D3918.jpg|Mother and baby with gas masks, 1941
</gallery>