This term popularly refers to a device that a person wears on their face to protect them from airborne pollutants and toxins. The mask may cover the eyes and other vulnerable soft tissues of the face, and will always form a sealed cover over the nose and mouth. However toxins may be gaseous (for example the chlorine used in WWI), or particulate (such as many biological agents developed for weapons).
For this reason many gas masks include protection from both types of toxin. The advantage of a gas mask over other breathing devices is that it does not require the user to carry their own air supply (such as in the use of scuba gear). However, this means that the user is dependant on the air in the atmosphere, the very medium in which toxins may be present. Thus, the mask must remove the toxins, and relay cleaned air to the user.
There are three main ways of achieving this:
Filtration: This obviously lends itself to particulate toxins. A filter relies upon the fact that what is to be removed is bigger than what is to be passed through. As many pollutant molecules and particles are much bigger than air molecules (almost all O2 and N2) this works for many types of application. However the smaller the gap through which the air has to pass, the greater the pressure that must be exerted to draw the air through. As the users lungs provide this pressure there is a limit as to how small these passages may be. Thus to extract many gaseous toxins, other methods must be used.
Absorption & adsorption: Absorption is the process of being drawn into a (usually larger) body, or substrate, and adsorption is the process of deposition upon a surface. This can be used to remove both particulate and gaseous toxins. Although some form of reaction may take place, it is not necessary, the principle may work by attractive charges (for example if the target toxin is positively charged, use a negatively charged substrate). Examples of substrates include activated carbon, and zeolites. this effect can be very simple and highly effective, for example using a damp cloth to cover the mouth and nose whilst escaping a fire. Most of the harmful vapours and smoke will be dissolved in the water on the cloth, giving you vital extra seconds to escape.
Reaction and exchange: This principle relies upon the fact that substances that can do harm to humans are usually more reactive than air. This method of separation will use some form of generally reactive substance (for example an acid) coating or supported by some solid material. An excellent example is resins. These can be created with different groups of atoms (usually called functional groups) that exhibit different properties. Thus a resin can be tailored to a particular group of toxins. When the toxin comes into contact with the reactive substance, it will bond to it, removing it from the air stream. It may also exchange with a more harmless substance at this site.
There are two main difficulties with gas-mask design:
The user may be exposed to many different types of toxins. This is especially true of the masks that the military use, they may literally have anything thrown at them. However if the mask is for a particular use (such as the removal of a specific type of toxin in a factory), then the design can be much simpler and the cost lower.
The protection will wear off over time. Filters will clog-up, substrates for absorption will fill-up, and reactive filters will run-out of reactive substance. This means that the user only has protection for so long, and then they must either replace the filter device in the mask, or use a new mask.