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