Filter or supply, first
Filtering devices clean the air that is there; they do not make air. That rules them out in three situations:
- where the oxygen concentration is insufficient,
- where the contaminant is unknown, or its warning properties (smell, taste) are not reliable,
- where the concentration is above the protection limit of the filter and facepiece.
Confined space work is where all three often hold at once; the default there is supplied air, not a filtering mask.
Filter type follows the contaminant
Particle filters capture dust, fume and mist; gas and vapour filters are divided by chemical class and each class has its own limits of use. Combined filters do both. The wrong filter type gives no protection — but it gives the wearer the feeling of protection, which is more dangerous than wearing nothing.
Face fit and seal
Full-face and half-face masks deliver their stated protection only when they seal against the face. Beards, spectacle arms, plasters and facial shape all decide whether they do. Selection is therefore personal: the same model gives different protection on two people. Practical training and a pre-use seal check complete the selection.
Service time and change-out
A filter's life is not the date on the box; it depends on use time, concentration, humidity and temperature. The change-out rule must be written in advance and tied to a record — “change it when you smell something” is not a rule, because some chemicals have no smell.