filters for half face masks

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There are three types of filters for half face masks.
1) Thin particulate filters, often now of purple color, to be discarded.
2) Cartridge type filters, each model or brand has its own system.
3) A few respirators are fitted with NATO wcrew, which allows to use the NBC type cartridges, a worlwide norm for attachment. P3 or P100 will be fine. These cartridges have a tniny opening which protects from direct contamination, and are the easiest to be decontaminated.