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