Understanding the different types of filters for respirators
Here we will give an overview of the different types of filters for respirators and what contaminants they protect against.
Please note there are many factors that need to be considered in the selection process of respiratory protection which are detailed in AS/NZS1715 The information provided below is not a recommendation in the selection process it is for informational purposes only and an assessment of exposure to airborne contaminants should be done prior to selection.
As per AS/NZS 1715 there are 3 different classes of particulate filters, P1, P2 and P3.
The negative pressure particulate categories are based facepiece coverage. All particulate filtering facepieces that cover the nose and mouth area only can achieve only a P1 or P2 classification. A P3 classification can ONLY be achieved when worn with a full facepiece.
Class P1 particulate filters are used against mechanically generated particulates e.g. silica and wood dust.
Class P2 particulate filters are used for protection against mechanically and thermally generated particulates or both e.g. metal fumes.
Class P3 particulate filters are used for protection against highly toxic or highly irritant particulates e.g. beryllium (when worn with a full facepiece).
NOTE: certain contaminants may have specific respiratory selection criteria outside this guide
Gas and vapour cartridges categories are distinguished by their filter type and class. Refer to AS/NZS 1715 for the complete list of filter types and what they are used for. Some commonly used filter types are:
Filter type A = Certain organic vapours (boiling point above 65 ºC) from solvents such as those in paints and thinners (cartridge label colour = brown)
Filter type B = Acid gases such as chlorine and hydrogen sulfide (sulphide) (cartridge label colour = grey)
Filter type E = Sulfur dioxide (cartridge colour = yellow)
Filter type ABE = are suitable for both certain organic vapours/acid gases and sulfur dioxide e.g. solvents, chlorine and sulfur dioxide (cartridge label colour = brown, grey and yellow)
Filter type K = ammonia gas (cartridge label colour = green)
Filter type ABEK = are suitable for both certain organic vapours/acid gases, sulfur dioxide and ammonia (cartridge label colour = brown, grey, yellow and green)
The gas filters are filled with one or several layers of activated and/or impregnated carbons depending on what type of gases the filter is intended to protect the user from. The activated carbon and impregnated carbon is made from peat, wood, coal or coconut shells. The most common process to “activate” the carbon is a high temperature steam activation (1000 ºC) but it can also be manufactured with chemicals. The activation creates a variety of larger transport pores and smaller adsorption pores giving both a high capacity and good kinetics. This results in excellent adsorption
A mechanical particle filter gets more and more efficient the larger number of particles it has collected. As the breathing resistance increase and gets uncomfortable to use then it should be replaced. It is much more difficult to assess when a gas filter needs replacement. It is very important to change the filter before it becomes saturated because then the filtering function will cease and the user will be breathing unfiltered air from the surroundings
A number of factors affect the usetime of a gas filter
Type of filter: Carbon filters can be classified into two main groups, i.e. those consisting of pure activated carbon, and those consisting of impregnated activated carbon. The latter may have different types of impregnation, depending on the pollutants for which they are intended. A type A filter consists of pure activated carbon, while all other types consist of carbon with various types of impregnation. A filter may obviously be used to provide protection against substances for which it has been granted type approval (A,B,E,K, etc.). On the other hand, filters have varying abilities to absorb chemi- cals outside these chemical groups. A type BE filter, for instance, can adsorb substantial quantities of organic solvents, but the actual value varies from one filter model to the next. This may be very important to bear in mind if a mixture of different pollutant types occurs in the work environment (which it virtually invariably does).
Size of filter: Gas filters can be assigned to three different filter classes, depending on their intended application. Only two classes occur in practice, i.e. class 1 and class 2. A class 2 filter has roughly 2 – 5 times higher adsorption capacity than a class 1 filter, but at the expense of higher weight and greater breathing resistance.
Ambient humidity and temperature: If a type A filter is used for organic solvents, the humidity in the air will be competing with the solvent for space in the filter, i.e. the higher the ambient humidity, the shorter the useful life of the filter. Impregnated carbon filters (B,E,K filters) do not have such sensitivity to ambient humidity. The temperature also affects the useful life of the filter. It is then also largely a moisture problem
Concentration of a pollutant: The influence of this variable is easiest to assess. Considering the concentrations in which filter protection is used, the relationship between useful life and concentration is basically linear, i.e. if the concentration is halved, the useful life of the filter will be doubled. The problem linked to this aspect of the filter is that of determining the concentration of the pollutant in the air.
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