A gas filter can protect against different groups of gases, in which case it is said to be "mixed". The juxtaposition of the corresponding letters and the associated colorimetric markings are then indicated on the filter. An AB filter for example protects against organic and inorganic gases and vapors.
Source ED6106 INRS
Gas filter efficiency
Gas protection filters are divided into 3 classes based on their efficiency :
o Class 1 : for the lowest efficiency (round filter)
o Class 2 : for medium efficiency (cartridge)
o Class 3 : for the highest efficiency (canister)
AX and SX specific filters do not have an efficiency class. AX filters are for single use.
Negative pressure respirators must not be used in the following situations:
o ambient oxygen rate < 19%
o confined spaces
o areas with little or no ventilation
o areas in which the concentration of the pollutant is unknown or immediately lethal
o areas in which the concentration of the pollutant exceeds the maximum permissible concentration (OEL) or exceeds the capacity of the filter.
In such cases, the use of self-contained breathing apparatus is highly recommended.
