Editing Gas mask (section)

=== Modern filter classification ===
{{See also|Cartridge (respirator)#Classification and marking of cartridges}}
The filter is selected according to the toxic compound.<ref name=draeger>{{cite web|url=http://www.draeger.com/media/10/01/09/10010967/guide_for_selection_and_use_of_filtering_devices_9045782_en.pdf|title=Guide for selection and use of filtering devices|publisher=draeger.com|access-date=February 22, 2013|archive-url=https://web.archive.org/web/20120526013417/http://www.draeger.com/media/10/01/09/10010967/guide_for_selection_and_use_of_filtering_devices_9045782_en.pdf|archive-date=May 26, 2012|url-status=dead}}</ref> Each filter type protects against a particular hazard and is color-coded:

{| class=wikitable
|+Filter types
|-
! EU Class, color !! US color<ref name=osha_2011>{{cite web |title=OSHA Bulletin: General Respiratory Protection Guidance for Employers and Workers |url=https://www.osha.gov/dts/shib/respiratory_protection_bulletin_2011.html |website=Occupational Safety and Health Administration}}</ref>!! Hazard
|-
| AX, brown || rowspan=2| black || Low-boiling (≤65&nbsp;°C) organic compounds
|-
| A, brown || High-boiling (>65&nbsp;°C) organic compounds
|-
| B, grey || (many) || Inorganic gases ([[hydrogen sulfide]], [[chlorine]], [[hydrogen cyanide]])
|-
| E, yellow || white || Acidic gases ([[Sulfur dioxide]] and [[hydrogen chloride]])
|-
| K, green || green || [[Ammonia]] and [[amine]]s
|-
| CO, black || blue || [[Carbon monoxide]]
|-
| Hg, red || {{N/A}} || [[mercury (element)|Mercury]] vapor
|-
| R(eactor), orange || magenta || Radioactive particles ([[iodine]] and [[methyl iodide]])
|-
| P, white || purple, orange, or teal || Particles
|}

Particle filters are often included, because in many cases the hazardous materials are in the form of mist, which can be captured by the particle filter before entering the chemical adsorber. In Europe and jurisdictions with similar rules such as Russia and Australia, filter types are given suffix numbers to indicate their capacity. For non-particle hazards, the level "1" is assumed and a number "2" is used to indicate a better level. For particles (P), three levels are always given with the number.<ref name=draeger/> In the US, only the particle part is further classified by [[NIOSH air filtration rating]]s.<ref name=osha_2011/>

A filter type that can protect against multiple hazards is notated with the European symbols concatenated with each other. Examples include ABEK, ABEK-P3, and ABEK-HgP3.<ref name=draeger/> A2B2E2K2-P3 is the highest rating of filter available.{{when|date=April 2020}} An entirely different "multi/CBRN" filter class with an olive color is used in the US.<ref name=osha_2011/>

Filtration may be aided with an air pump to improve wearer comfort. Filtration of air is only possible if there is sufficient oxygen in the first place. Thus, when handling [[asphyxiant]]s, or when ventilation is poor or the hazards are unknown, filtration is not possible and air must be supplied (with a SCBA system) from a pressurized bottle as in scuba diving.