In a gas mask made to safeguard against a biological hazard , an extremely fine particulate filter is beneficial. An anthrax bacteria or spore might have a minimum size of one micron. Most liquid particulate filters eliminate particle dimensions as small as 0.3 microns. Any particulate filter finally clogs, and that means you need to replace it because breathing becomes hard.

A chemical hazard needs a different strategy, because compounds come as mists or vapors which are largely resistant to particulate filtration. The most frequent approach with almost any natural chemical (whether it’s paint fumes or a neural toxin such as Sarin) is activated charcoal. You can buy gas masks with protective hood and filtered air supply system.

The use of special manufacturing techniques results in highly porous charcoals that have surface areas of 300-2,000 square metres per gram. These so-called active, or activated, charcoals are widely used to adsorb odorous or coloured substances from gases or liquids. The word adsorb is important here. When a material adsorbs something, it attaches to it by chemical attraction.  The huge surface area of activated charcoal gives it countless bonding sites. When certain chemicals pass next to the carbon surface, they attach to the surface and are trapped.

Activated charcoal is good at ridding carbon-based impurities (“natural” compounds ), in addition to things such as chlorine. Many other chemicals are not attracted to carbon at all — sodium and nitrates, to name a few– so they pass right through. This means that an activated-charcoal filter will remove certain impurities while ignoring others. It also means that, once all of the bonding sites are filled, an activated charcoal filter stops working. At that point you must replace the filter.