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In many work situations and leisure activities, weuse machines or chemicals that make it desirablefor us to protect our respiratory tracts from harm-ful pollutants. By far the most common protectionmethod is some form of filter respirator. This maybe a full mask or half mask with a replaceablefilter, or some form of short-term mask.

How long will a filter last?This is probably the most common question askedof a person who deals with information concerningrespirators. But the unfortunate problem is thatthere is no simple or clear-cut answer. The usefullife of the filter is affected by so many parametersthat each individual case is virtually unique.

Particle filtersNevertheless, fairly clear instructions can be gi-ven to users of particle filters. What makes thecase of the particle filter somewhat easier tohandle is the fact that there is no actual time limitfor how long the filter will be capable of performingits main task of filtering out particles. In actualfact, the longer a particle filter is used, the betterit becomes. Comfort is the main factor that dec-ides how long a filter can be used. The larger thenumber of particles trapped by the filter, thehigher the breathing resistance, and the filter willeventually become uncomfortable to use. Anotherreason that may make it necessary to replace thefilter is physical damage. It should be borne inmind that, even if the damage is of microscopicsize, it may still significantly affect the performanceof the filter.

So a particle filter should be replaced when theresistance to breathing has become so high that

the filter is uncomfortable to use, if you know orsuspect that the filter has sustained damage, orif you sense that you are beginning to breathe thepollutants in the surrounding air. Prefilters areavailable as accessories for some particle filters.Such prefilters are intended to filter our coarseparticles to relieve the particle filter proper ofunnecessary loading. The use of a prefilter and itsregular replacement extends substantially theuseful life of a particle filter.

Gas filtersIt is much more difficult to assess when a gas filterneeds replacement. It is also very important forthe filter to be changed at the right time, sincewhen a gas filter has become saturated, itsfiltering function will cease, and the user will bebreathing untreated air from the surroundings. Anumber of factors that affect the useful life of agas filter are reviewed below.

Type of filterCarbon filters can be classified into two maingroups, i.e. those consisting of pure activatedcarbon, and those consisting of impregnatedactivated carbon. The latter may have differenttypes of impregnation, depending on the pollutantsfor which they are intended. A type A filter consistsof pure activated carbon, while all other typesconsist of carbon with various types ofimpregnation. A filter may obviously be used toprovide protection against substances for which ithas been granted type approval (A,B,E,K, etc.). Onthe other hand, filters have varying abilities toabsorb chemicals outside these chemical groups.A type BE filter, for instance, can adsorb substantialquantities of organic solvents, but the actual

Filter respiratorsHow long will a filter last?

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value varies from one filter model to the next. Thismay be very important to bear in mind if a mixtureof different pollutant types occurs in the workenvironment (which it virtually invariably does).

Size of filterGas filters can be assigned to three different filterclasses, depending on their intended application.Only two classes occur in practice, i.e. class 1(mainly intended for half masks), and class 2(mainly intended for full masks). A class 2 filterhas roughly 2 – 5 times higher adsorption capa-city than a class 1 filter, but at the expense ofhigher weight and greater resistance to breathing.Consequently, under given conditions, a class 2filter will last about 2 – 5 times longer than a class1 filter.

Ambient humidity and temperatureIf a type A filter is used for organic solvents, thehumidity in the air will be competing with thesolvent for space in the filter, i.e. the higher theambient humidity, the shorter the useful life of thefilter. It should also be borne in mind that the filterwill adsorb moisture even when not in use, unlessit is well sealed while in storage. Impregnatedcarbon filters (B,E,K filters) do not have suchsensitivity to ambient humidity. However, ambienthumidity is not totally insignificant, since certaincarbon types (especially those in K filters) have areverse behaviour. These filters perform bestwhen the moisture content in the carbon is fairlyhigh and, if the filter is used in very dry air, it maydry out and its performance may then be impaired.

The temperature also affects the useful life of thefilter. It is then also largely a moisture problem.Consider a case in which a type A filter is used asprotection against some solvent. If the filter has acertain capacity when the ambient air is at 5°C andat a relative humidity of 80%, its capacity willbasically be halved at 20°C and 80% relativehumidity. This is mainly due to the fact that, in thelatter case, the air contains much more water per

unit of volume. Similarly, if we maintain a constanttemperature of 20°C and change the relativehumidity from 70% to 80%, the adsorption capa-city will again be halved.

The above factors definitely occur in reality. Theuseful life of a type A filter can very well beevaluated for a certain work situation indoorsduring the winter. If this is done on a cold, clear daywhen the indoor temperature is around 20°C, therelative humidity is very likely to be below 30%. Sixmonths later, the weather may be sultry andthundery, with showers from time to time. Therelative humidity may then very well be above 70%,and the adsorption capacity of the filter may behalf that recorded on the evaluation occasion.

Fig. 1. The capacity of a type A filter for hexane atvarious ambient air temperatures and relativehumidities. Tests were carried out at a constant air flowrate of 30 l/min and a hexane concentration of 1000ppm.

Work intensityA further factor that affects the useful life of a gasfilter is the user’s work intensity. Increased workintensity increases the pulse and breathing rates.The increased breathing rates affects the usefullife of the filter in two ways.1. A larger volume of air flows through the filterduring a given period of time, and the useful life ofthe filter is directly proportional to the air flow.2. To pass more air through the filter during a givenperiod of time, the air velocity must be increased,

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and this also affects the useful life of the filter. Thehigher the air velocity through the filter, the smal-ler the total air volume the filter will manage toclean, all other parameters remaining unchanged.

Fig. 2. Capacity of a type A filter for hexane at differentflow rates and temperatures. The tests were run at arelative air humidity of 70% and a hexane concentrationof 1000 ppm. Note that if the flow rate is doubled theuseful life of the filter will be more than halved.

Fig. 3. Capacity of a type A filter for hexane at variousflow rates and temperatures. The tests were run at arelative air humidity of 80% and a hexane concentrationof 1000 ppm. Note that if the flow rate is doubled theuseful life of the filter will be more than halved. Incombination with higher relative humidity, the effect ofthe increased flow rate will be even greater than in thepreceding case.

The most common temperature in which respiratorsare used is ordinary room temperature, and it maythen be of interest to consider how the filtercapacity may vary at 20°C.

Fig. 4. Capacity of a type A filter for hexane at variousrelative humidities and flow rates. The tests were runat an air temperature of 20°C and a hexaneconcentration of 1000 ppm. If the air flow rate isdoubled at the same time as the air humidity increased,the filter capacity will be reduced to one tenth!

While on the subject of work intensity, it should beborne in mind that wide individual differencesexist between different users. Two individualsperforming the same task can very well exhibitsubstantial differences in breathing rate and airconsumption.

Concentration of a pollutantThe influence of this variable is easiest to assess.Considering the concentrations in which filterprotection is used, the relationship between usefullife and concentration is basically linear, i.e. if theconcentration is halved, the useful life of the filterwill be doubled. The problem linked to this aspectof the filter is that of determining the concent-ration of the pollutant in the air. However accuratelymeasurements are made, they will only describethe concentration on one particular occasion. Buthow will it be next week? Or next month? And soon. Moreover, in many work situations in whichrespirators are used, no measurements whateverhave been carried out.

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Fig. 5. Adsorption capacity as a function of theconcentration for hexane and methylene chloride. Testswere run at 20°C, 70% RH and a flow rate of 30 l/min.If the concentration is doubled, the time will be roughlyhalved.

Type of pollutantTo make certain that a filter is suitable for a certainpollutant, it should be tested for that particularpollutant. I actual fact, no such tests have beencarried out for the majority of substances, whichis due to the vast number of various chemicalsubstances surrounding us. However, this does notapply to organic solvents, since results of measure-ments have been published for more than a hundreddifferent solvents from various groups.

Fig. 6. Comparison between the capacity of a type Afilter for hexane and methylene chloride at various airhumidifies. The tests were run at 20°C, a flow rate of30 l/min and a concentration of 1000 ppm.

A common result produced by these tests is thatthey demonstrate that the capacity of the carbonfilter for trapping a solvent increases with increasingboiling point of the solvent. However, the curveflattens out at boiling points in the range of 100 -150°C (depending on the type of solvent) and, incertain cases, the capacity decreases if the boilingpoint increases further. However, the problemsposed by solvents are mainly linked to highlyvolatile substances (with low boiling points) andmixtures of various solvents. For substances withlow boiling points (below 65°C), there are specialfilters (AX filters) and special application rules.However, due to the limited knowledge available,mixtures of various substances represent a greaterproblem. Substances can separate in the filter,whereby one substance replaces another in thefilter and displaces the first one ahead of itself,thus giving rise to fast breakthrough, etc.

Fig. 7. Capacity as a function of the boiling point for anumber of alcohols and monochlorides. Tests havebeen run at 22°C, 50% RH, a flow rate of 53.3 l/minand a concentration of 1000 ppm.

SummaryThe above problems related to the use of filterrespirators may seem as though attention isfocused only on how long the filter will last. Thereader may also be misled into believing that arespirator with gas filter offers such poor protectionthat it should not be used. But that is certainly notthe case. A respirator with gas filter providesexcellent protection in many situations, and is

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often the only protection that can conceivably beused in practice. What I wanted to demonstrate inthis article is that there is no simple answer to thequestion of how long a gas filter will last.

So what can be done?The most widely used way of deciding in practicewhen it is time to change the filter is to utilize thewarning properties of most substances. A warningproperty means that the substance smells, tastesor irritates the mucous membranes. Mostsubstances have usable warning properties.

Substances with poor warningproperties or none at allCertain substances have good warning properties,such as smell, although the sense of smell losessome of its keenness after a long period ofexposure or at high concentrations, and the userwill become accustomed to the substance beingcontained in the air. There are also substancesthat have no warning properties at all.

If a filter respirator is used for such substances,their concentrations and other circumstances mustbe known, so that the useful life of the filter can beassessed and the filter can then be changed whena good margin of safety still remains. An evenbetter approach is to set up a filter test rig on site,pump air at a suitable rate through the filter, andcarry out measurements on the outlet side of thefilter to determine when pollutants begin to passthrough. Although this demands access to suitablemeasuring equipment and knowledge of how suchequipment should be handled, the methodproduces the most reliable results. In this casetoo, the useful life determined for the filter mustinclude a reliable margin of safety, and the ambientair conditions should be kept under observation toensure that they do not change.

An alternative solutionUltimately, the best solution is obviously to improvethe working environment so that no respirator atall will be needed. But this is not what I mean byan alternative solution, since a world in which norespirators are needed is still a few years - orperhaps centuries - ahead in time. What I mean byan alternative solution is the respirator solution Imust adopt if conditions are such that a filterrespirator is difficult or impossible to use reliablyin practice. The alternative solution is then arespirator with compressed air supply. This meansthat the respirator user does not breathe theambient air, but is supplied with clean air from acompressed air system. This eliminates allproblems of selecting an appropriate filter andthen changing it at the right intervals.

NOTE that all measurements reported above havebeen carried out on specially manufacturedlaboratory filters. The readings in minutes are thusnot directly applicable to filters available on themarket. On the other hand, the variations causedby variations in temperature, humidity, etc. areentirely relevant to commercially available filters.By Lennart Bäckman

Sources: The measured values reported havebeen obtained from a project initiated and financedby the National Swedish Board of OccupationalSafety and Health, and carried out by the NationalSwedish Defence Research Establishment(Estimating the useful life of A filters under variousservice conditions - in Swedish) and from theAmerican Industrial Hygiene Journal, July 1974(Effects of Solvent Vapor, G.O. Nelson and C.A.Harder).