| A filter mask provides protection to the wearer | | | | gaseous hazards. Although some form of reaction |
| from harmful airborne substances and usually | | | | may take place, it is not necessary; the method |
| covers only the mouth and nose. It limits the | | | | may work by attractive charges (for example, if |
| course of air so that it must flow through a filter | | | | the target particles are positively charged, use a |
| which removes harmful dusts or toxic gases. | | | | negatively charged substrate). Examples of |
| Such masks range from cheaper, single-use, | | | | substrates include activated carbon, and zeolites. |
| disposable types to reusable models with | | | | This effect can be very simple and highly |
| replaceable cartridges. | | | | effective, for example using a damp cloth to |
| Because of their simple, passive design they are | | | | cover the mouth and nose whilst escaping a fire. |
| most commonly used for filtration of tiny | | | | Gas masks used in World War One were made |
| suspended solid or liquid particles and often | | | | as a result of poison gas attacks that took the |
| referred to as particulate masks. For this purpose, | | | | Allies in the trenches on the Western Front by |
| they usually employ a dense, fine natural or | | | | surprise. Early gas masks were crude as would be |
| synthetic fiber mesh. To aid particulate filtration, | | | | expected as no-one had thought that poison gas |
| the mesh is sometimes coated with substances | | | | would ever be used in warfare as the mere |
| that enhance the tendency of particulates to | | | | thought seemed too shocking. |
| adhere to the fibers. | | | | Most of the harmful vapours and smoke will be |
| For gas filtration, mask cartridges are filled with | | | | dissolved in the water on the cloth, giving you |
| activated carbon or certain resins that will absorb | | | | vital extra seconds to escape. |
| substances such as volatile organic compounds | | | | This principle relies upon the fact that substances |
| (VOCs), eliminating them from the air breathed. | | | | that can do harm to humans are usually more |
| Though less effective for this purpose, and only | | | | reactive than air. This method of separation will |
| suitable to the least demanding applications, | | | | use some form of generally reactive substance |
| single-use masks also sometimes employ | | | | (for example an acid) coating or supported by |
| embedded carbon granules in the fiber mesh. | | | | some solid material. An excellent example is resins. |
| There are different cartridges for different | | | | These can be created with different groups of |
| compounds. When filter cartridges become | | | | atoms (usually called functional groups) that exhibit |
| saturated or particulate accumulation within them | | | | different properties. Thus a resin can be tailored |
| begins to restrict air flow, they must be changed. | | | | to a particular toxic group. When the reactive |
| A gas mask is a mask worn on the face to | | | | substance comes in contact with the resin, it will |
| protect the body from airborne pollutants and | | | | bond to it, removing it from the air stream. It |
| toxic materials. The mask forms a sealed cover | | | | may also exchange with a more harmless |
| over the nose and mouth, but may also cover | | | | substance at this site. |
| the eyes and other vulnerable soft tissues of the | | | | Though it was crude, the hypo helmet was a sign |
| face. Some gas masks are also respirators, | | | | to British troops in the trenches that something |
| though the word gas mask is often used to refer | | | | was being done to help them during a gas attack |
| to military equipment (e.g. Field Protective Mask, | | | | and that they were not being left out for |
| etc.) | | | | slaughter. As the months passed and the use of |
| Airborne toxic materials may be gaseous (for | | | | poison gas occurred more frequently, more |
| example the chlorine gas used in World War I) or | | | | sophisticated masks were developed and |
| particulate (such as many biological agents | | | | introduced. |
| developed for weapons such as bacteria, viruses | | | | There are two main difficulties with gas-mask |
| and toxins). Many gas masks include protection | | | | design: |
| from both types. Unlike other breathing devices, | | | | The user may be exposed to many different |
| gas masks do not require the user to carry an air | | | | types of toxic material. Military personnel are |
| supply as in the use of scuba gear. However, this | | | | especially prone to being exposed to a diverse |
| means that the wearer depends on the air in the | | | | range of toxic gases. However if the mask is for |
| atmosphere, the same medium of the toxic | | | | a particular use (such as the protection from a |
| materials. Thus, the mask must remove them | | | | specific toxic material in a factory), then the |
| and relay clean air to the wearer. | | | | design can be much simpler and the cost lower. |
| There are three main ways of achieving this: | | | | The protection will wear off over time. Filters will |
| filtration, absorption and adsorption, and reaction | | | | clog up, substrates for absorption will fill up, and |
| and exchange. | | | | reactive filters will run out of reactive substance. |
| Absorption is the process of being drawn into a | | | | This means that the user only has protection for |
| (usually larger) body, or substrate, and adsorption | | | | so long, and then they must either replace the |
| is the process of deposition upon a surface. This | | | | filter device in the mask, or use a new mask. |
| can be used to remove both particulate and | | | | |