Protective Masks

Source: A FOA Briefing Book on Chemical Weapons.

General information regarding protective masks.

In this section:

Development of Protective Masks

The historical development of protective masks or respirators may be roughly characterized as four different generations:

  1. The First World War. The first primitive masks were quickly developed after the initial use of CW agents during the First World War. The basic frame of the mask is made of leather.

  2. The Second World War. The protective capability was greatly improved during the period between the wars when natural rubber was used to make the basic frame of the mask. The elastic rubber material allowed the mask to better adapt itself to different shaped faces.

  3. After the Second World War and up to about 1980. In about 1950, there was a more general trend to equip the mask with an inner mask. This must be regarded as a technical break-through. The inner mask solved the problem with misting of the visors also in low winter temperatures.

  4. The current generation of protective masks. During the 1980's and 1990's, protective masks were improved in many ways, as regards for example comfort, fit and the intake of liquids. It might therefore be justified to regard them as the fourth generation of protective masks.
Example of modern mask.

Characteristics of modern masks

  • A demand frequently placed today on a protective mask is that it can be worn for at least 24 hours. The mask must then also permit the intake of liquids.
  • New technical solutions to the problem of combining protective mask and glasses are available, which permit correction of visual defects without degrading the protection.
  • A protective mask must be capable of adaptation to different face shapes and is therefore manufactured in an elastic material. Modern masks are almost always made of some kind of rubber material. If high demands are placed on a good protective ability to permeation of CW agents, it often results in the choice of halogenated butyl rubber.
  • A device for speech communication is included in all of the new masks. The earlier solution, a speech membrane, is now being replaced by a speech horn, which is easier to manufacture. A speech horn also gives largely the same effect as a speech membrane. New material for the filter canisters, e.g., fibre-reinforced plastic, gives them better resistance to external influence.

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Protective mask's filter

The filter in a protective mask consists of two parts; an aerosol filter and a gas filter. The aerosol filter is built up of a layer of glass fibres where the spacing between the fibres is large in relation to the size of the particles to be filtered. Consequently, an aerosol filter of this kind does not work by screening or filtering off the particles. The particles are removed mainly when they collide with the fibres, to which they adhere. If it is a volatile substance that adheres, it may subsequently evaporate from the aerosol filter. Consequently, it is important to design a filter whereby the gas filter component is located after the aerosol filter.

The gas filter component of the protective filter consists of active carbon. Recently other adsorbants, e.g., different synthetic polymers and zeolites have been tested but none has proved as widely applicable as active carbon. Neither have any other absorbents been found to have higher uptake ability for CW agents than active carbon.

Active carbon is produced by heat-treating different organic materials. A number of commonly-used materials are peat, coconut shell and coal. The material is activated byheating it together with carbon dioxide or steam to 800-1000 oC. The carbon so obtained contains numerous pores and cavities and under magnification looks rather like a face sponge. Active carbon of the type used in protective masks has a total area of 1 000 -1 500 m2 per gramme.

By selecting different starting materials and conducting the activation in different ways, the active carbon obtained has different degrees of pore distribution. Carbon with large pores is the most suitable for cleaning water, whereas carbon with small pores is better for removing gas. Pore distribution and pore size are also important for the carbon's ability to absorb water. The particle size distribution is also important and particularly for properties such as air resistance and the protective ability against different gases.

Certain low-molecular CW agents such as hydrogen cyanide and cyanogen chloride are poorly absorbed by active carbon. In order to improve protection against these substances, the carbon is impregnated with metallic salts of copper, chromium and sometimes silver. Further impregnation with organic substances also occurs, the most common additive being triethylenediamine (TEDA).

Certain types of carbon-fibre based material have higher sorption capacity than normal granulated active carbon. Use of carbon fibres of this kind in a gas filter offers advantages such as lower pressure drop, smaller volume and lower weight.

The degree of leakage in modern filters is maximally 0.001 per cent and, in extreme cases, the filter provides protection against at least 10 but probably up to 100 attacks before CW agents start to leak through. If the protective mask is used in a non-contaminated atmosphere the filter will gradually become loaded since it absorbs moisture and pollution from the air. Long-term use or unsuitable storage may lead to the protective ability against certain CW agents becoming deteriorated.

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Control of air flow.

Control of air flow

The inhalation air is first purified during passage through the filter. The air that has passed through the filter is relatively dry. It is guided up over the visors in order to prevent them from misting. The flow of air then passes through the inner mask and then into the lungs. The exhaled air flows directly from the inner mask through the exhalation valve.

The control of the air flow is achieved by valves. A well-functioning inhalation valve is essential for the function of the mask. It must allow air to pass in through the filter but must also prevent the exhaled air from passing out through the filter. During the inhalation phase, the exhalation valve must be fully closed and thereby prevent contaminated ambient air from entering the mask by that route.

The task of the inner mask is to prevent the exhaled air from filling up the entire volume inside the mask. In this way, the moist exhaled air is prevented from passing over the visors which might cause troublesome condensation. The inner mask is usually fitted with valves to control the air flow. However, it is fully possible to design a functional inner mask without any valves.

Fitting protective masks

A measure of the protective ability of a mask is given by the total inward leakage. This consists of the sum of the leakage in the filter, the leakage through the fit, leakage through the exhalation valve and any other leakage. Instead, the protection factor of the mask is frequently given:

Protection factor = (Concentration of a contaminant in the ambient air) / (Concentration of the same contaminant inside the mask)

The protection factor states how much lower the concentration of a pollutant in the air is inside the mask in comparison with the concentration in the ambient air. Laboratory tests with a non-hazardous test substance can easily determine protection factors up to 100 000. In the field, tear gas can be used as a test substance to obtain a rough measure of the protective ability of a mask. The sensitivity in tests with tear gas (CS) is, however, not larger than that it corresponds to a protection factor somewhere in the range between 1 000 and 10 000.

Even in a modern mask with a good sealing edge, the fit will start to deteriorate already when the soldier has a 24 h-old beard stubble. The protection will deteriorate with the length of the stubble. Whole beards usually result in very poor protection.

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Children Protection

For younger children, a protective jacket can be used instead of a protective mask. Such a jacket protects the respiratory organs and also gives some protection against CW agents in liquid form. A battery powered fan forces air through a filter and the purified air flows in front of the child's face.