Gas masks have a useful lifespan limited by the absorbent capacity of the filter. Filters cease to provide protection when saturated with hazardous chemicals, and degrade over time even if sealed. Most gas masks have sealing caps over the air intake and are stored in vacuum-sealed bags to prevent the filter from degrading due to exposure to humidity and pollutants in normal air. Unused gas mask filters from World War II may not protect the wearer at all, and could be harmful if worn due to long-term changes in the chemical composition of the filter.[citation needed]

An asbestos-containing Russian GP-5 filter and a safe modern one in comparison.
Some World War II and Soviet Cold War gas masks contained chrysotile asbestos or crocidolite asbestos in their filters,[2][3][4] not known to be harmful at the time. It is not reliably known for how long the materials were used in filters.

Typically, masks using 40 mm connections are a more recent design. Rubber degrades with time, so boxed unused “modern type” masks can be cracked and leak. The US C2 canister (black) contains hexavalent chromium; studies by the U.S. Army Chemical Corps found that the level in the filter was acceptable, but suggest caution when using, as it is a carcinogen.[5]

EU Class, color US color[7] Hazard
AX, brown black Low-boiling (≤65 °C) organic compounds
A, brown High-boiling (>65 °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 amines
CO, black blue Carbon monoxide
Hg, red — Mercury vapor
Reactor, orange magenta Radioactive (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.[6] In the US, only the particle part is further classified by NIOSH air filtration ratings.[7]

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.[6] A2B2E2K2-P3 is the highest rating of filter available.[when?] An entirely different “multi/CBRN” filter class with an olive color is used in the US.[7]

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 asphyxiants, 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.

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