aerosol sampling || historical milestones in practical aerosol sampling

Part CAerosol Sampling Instrumentation

A erosol Sampling: Science, Standards , Instrumentation and A pplications J am es H. Vincent 2007 John Wiley & Sons, L td. ISBN: 0-470-02725-8

13Historical Milestones in Practical Aerosol

Sampling

13.1 Introduction

This chapter points out the main highlights of the historical development of practical aerosol samplinginstrumentation, from its origins in the 1800s to the sophisticated approaches which are regarded asroutine today. It will refer to criteria and strategies described in greater detail in previous chapters,and will link them up with some of the instruments that have played important roles during the devel-opments that have taken place, and which will be described in upcoming chapters. So what followsprovides a bridge between the past and the future, and between the theory and the practice of aerosolsampling. It focuses on the separate, but generally interconnected, developments that have taken placein aerosol sampling in workplaces and the ambient atmosphere, respectively, both nationally and inter-nationally. To illustrate these historical time lines, a number of specific instruments are mentioned asimportant milestones. Further details on these and many other aerosol samplers will appear later ingreater detail.

The historical perspective is important because the older sampling methods are of more than justpassing interest. This is because exposures from as far back as 50 years ago remain relevant to currentepidemiology and hence to standards setting. Indeed, there are people still alive today who may yet sufferthe consequences of exposures that occurred even that many years ago. Yet, modern occupational andenvironmental epidemiology, primary sources of information for the purpose of the setting of reliableexposure limits, frequently remain encumbered by the lack of knowledge or understanding about whatwas really being measured in the distant past. It therefore remains important to be able to elucidate thenature and intensity of those early exposures and to be able to relate them to modern concepts of whatconstitutes ‘health-related’ exposure. Retrospective exposure assessment represents a challenging areafor occupational and environmental hygiene scientists, requiring full appreciation of the rationales andphysical principles underpinning the sampling instrumentation used in the past, and how these – andthe instruments themselves – have evolved as scientific knowledge has advanced.

A erosol Sampling: Science, Standards , Instrumentation and A pplications J am es H. Vincent 2007 John Wiley & Sons, L td. ISBN: 0-470-02725-8

312 Aerosol Sampling

13.2 Occupational aerosol sampling

13.2.1 Sampling strategies and philosophies

Workplace aerosol exposures take place over limited periods defined by working shifts and patterns. Inmodern developed societies these are typically 8 h a day, 5 days a week, although in other eras, or eventoday in many other countries, these patterns differ, often tending towards greater proportions of lifespent at work. In reality, occupational exposures are superimposed on the exposures which membersof the general population experience in the ambient environment. However, they are highly specific tothe type of organisation and type of work done, changing as old technologies are phased out and newones emerge. Occupational hygiene addresses only those exposures associated with work. For specificagents characteristic of the work done, these tend to be significantly higher there than might be foundelsewhere, and this argument has usually been sufficient to justify the paucity of effort in combiningthe considerations of ambient and occupational exposures.

As described in earlier chapters, there have been essentially two strategies or methods of approachdriving the development and application of aerosol instrumentation in occupational hygiene: area (orstatic) sampling, involving monitoring the environment in specific work areas or associated with specificprocesses, and personal sampling, involving the determination of the health-related exposures of individ-ual workers. Historically, the approach adopted has been dictated as much by the practical and economicconstraints imposed by the availability of suitable instruments (and personnel to operate them), and byrequirements – if any – to measure the chemical or mineralogical composition of the sampled aerosol,as by the overarching philosophy of hazard evaluation. So instruments and strategies of their deploymenthave evolved in parallel with improved understanding of hazards and their associated risks to workers.Such technical progress has gone through a number of important transitions over the years. Duringthe early 1900s to 1930s, relatively short-period gravimetric samples (for nominally ‘total dust’) wereusually taken with area samplers placed at breathing height in the general vicinities of representativeworkers during active operations, primarily to locate sources and to test the effectiveness of dust preven-tion measures. This invariably required the participation of an attendant operator. During the 1920s to1950s, following realisation that the mass of the nonrespirable large particles in the total dust sampledover-estimated the extent of the risk, short-period or ‘snap’ sampling for microscopic counts of particlesconsidered to be small enough to penetrate deep into the lung was widely carried out, using instrumentssuch as the konimeter (described later in Chapter 15). Such sampling was carried out by an attendantoperator who usually ensured that the samples were taken as close as possible to the workers’ breathingzones. In this way, such samples could be regarded as ‘personal’. But then, from the 1950s onwards,increasing emphasis was given to full-shift time-weighted average (TWA) particle-number sampling, byarea sampling either near representative workers as a measure of health-related exposure or at strategicsampling positions to monitor dust control, again (usually) under the supervision of an attendant oper-ator. This changed in the 1960s, with the emergence of long-running, full-shift area or static samplersthat employed the aerodynamic selection of the respirable fraction of airborne particles, enabling themto operate unattended and providing samples of sufficient quantity that could be assessed in terms ofmass and/or chemical (or mineralogical) composition. Eventually, however, from the 1970s onwards,increasing emphasis was given to TWA personal sampling, especially in countries with highly devel-oped occupational hygiene cultures. This ability arrived with the advent of samplers and (particularly)pumps that could be miniaturised to the point where they could conveniently be carried by – or wornon the person of – the worker. As related in the historical account by Sherwood (1997), the possibilityof small sampling pumps first became apparent in the late 1950s when health physicists at the UnitedKingdom Atomic Energy Research Establishment at Harwell came to realise that small, constant-speedDC electric motors like those developed for the early phonographs might be applied to air pumps, much

Historical Milestones 313

smaller than anything that had previously been conceived. The first practical sampling device based onthis new concept was reported in 1960 (Sherwood and Greenhalgh, 1960). The first commercial personalsampling pumps subsequently appeared around 1962, and by the early 1970s were becoming routineoccupational hygiene tools. Now, 30 or so years further on, personal sampling is widely regarded inmost countries as the only satisfactory way to assess the exposures of a workforce.

Until the 1960s considerable attention was directed at the peak levels of dust concentration duringworking shifts, often confined to periods of aerosol-producing activity. The relevance of full-shift sam-pling to health was not clearly understood at the time, and so the focus of much sampling was placed onthe needs to identify sources in order to guide control measures. However, the eventual introduction offull-shift samplers coincided with acceptance that the TWA aerosol concentration over this period wasthe more appropriate index of risk to health (Wright, 1953; Orenstein, 1960). But another, increasinglyimportant, practical consideration was the desirability of reducing the burden on occupational hygienistsassociated with evaluating multiple short-period samples.

13.2.2 Indices of aerosol exposure

Again, an important issue in aerosol exposure assessment is to identify the most appropriate concentra-tion of aerosol particles. Even as far back as the early 1900s, particles of certain specific composition(e.g. containing crystalline silica) were already acknowledged to be dangerous. Indeed, at that time, manyauthorities considered crystalline silica (or quartz) to be the only significantly dangerous component ofsome mineral dusts. The mass proportion of such chemical species in high-volume gravimetric sampleswas initially inferred from the overall dust collected on filter media, or – when the sample mass wassufficient (usually of the order of grams) – quantitated using wet chemical or X-ray diffraction methods.Often, however, the amount of particulate matter collected was insufficient for the use of such methods,and so compositional analysis was often performed on settled dust collected from floors or ledges or takenfrom industrial filter units. In some cases, samples taken for microscope-based particle counting wereheated to burn-off carbonaceous material and acid-washed to remove the soluble component, allowinglimited mineralogical discrimination of the residue when viewed under the microscope. Later, however,more advanced analytical instrumentation became available for analyzing samples of particulate matterin situ on the collection filters. During the 1930s to the 1960s, some authorities considered that, forfree crystalline silica, the surface area of the respirable particles might be a better hazard index thanthe mass (Orenstein, 1960). Here it was suggested that surface area concentrations could be assessed bymeans of light scattering or absorption measurements on liquid suspensions of sampled particles (afterlarge particles had been removed by sedimentation) or on particles sampled onto glass slides.

13.2.3 Early gravimetric samplers for ‘total’ aerosol

Gravimetric sampling involves determination of the overall mass of particulate matter collected, fromwhich, knowing the sampling time and the sampling flow rate, the airborne mass concentration of thesampled aerosol may be determined. South Africa led the way in the early days of gravimetric sampling,followed by the UK and the USA. From about 1900 onwards, cotton-wool filters were used in mine dustsampling, where the mass of sampled aerosol was determined from weighing of the cotton-wool filtermedia before and after sampling. In the method described by Thomas and McQueen (1904), the cottonplug was incinerated and the residue weighed. Around the same time, from about 1902 to 1903, someearly dust determinations were made using the sugar tube, in which a packed bed of sugar granules wasused as the filter medium, and the mass of collected insoluble material was determined after the sugargranules had been dissolved in water (e.g. Boyd, 1930). Here, incidentally, it is of interest to note that


this method first emerged much earlier as a bioaerosol sampler (Frankland, 1886). The sugar tube wasused routinely in South African mines from 1914 until 1936. Indeed, records show that over 35 000such samples were taken and analyzed in South African underground mines during 1919 alone. From1923 onwards, paper thimble-type collectors became popular. In these, porous paper was used as thefilter media, and concentration was again obtained from determination of the change in the filter massafter sampling.

The disadvantages of these earlier devices soon became apparent. Firstly, in industries such as miningand elsewhere, the observed prevalence of lung disease did not fall consistently with reductions in dustlevels as measured by such sampling methods. Secondly, the earlier instrumentation was cumbersome,a special disadvantage in many confined working environments like those found in mines. Thirdly, itlater emerged also that very variable results were obtained because particles of different sizes werenot collected with the same efficiency, depending on the aerodynamics governing how particles wereaspirated into and collected within sampling instruments. This problem was particularly acute for thesampling of coarser aerosols. To overcome this particular problem in the case of the early sugar tube,crude attempts were made to separate the over-large particles from collected samples. But, in due course,the cumulative impact of disadvantages like those mentioned became overwhelming, leading to a switchof emphasis to aerosol measurement based on the counting of individual particles.

13.2.4 Particle count samplers

According to Drinker and Hatch (1934), the earliest mention of particle sampling and counting bymicroscope was by Cunningham (1873). Somewhat later, as mentioned in Chapter 11, McCrea hadnoted in 1913 that particles in the lungs of miners observed post mortem were between 1 µm and 7 µmin diameter, thus identifying the need to selectively measure the finer particles as an appropriate indexof exposure for certain types of dust-related lung disease. This appears to be the first reference to whatwe now refer to as ‘particle size-selective’ exposure assessment for aerosols.

The basic problem for particle-count sampling is to collect particles in a form that can be examinedmicroscopically, classified according to their size and counted. In the early days this raised some technicaldifficulties because filters were not available that would permit collected particles to be examineddirectly. The membrane filters which later became the primary means for the routine collection andcounting of asbestos fibers by optical microscopy did not become available until the 1960s. So, for thesampling instruments that were used pre-1960, particles were deposited onto glass slides by combinationsof impaction, impingement or thermal precipitation. Such slides were very convenient for examinationby conventional optical microscopy.

One such instrument which found very wide usage is the portable konimeter first described by Kotze(1916), which allowed short snap samples to be taken close to a worker followed by microscopic analysisof particles deposited inside the instrument by impaction (more details are given in Chapter 15). Varioustypes of konimeter were widely used from the 1930s until as recently as the 1980s, particularly in theextraction and metals industries in Canada, Australia and South Africa. Before the advent of gravimetricstandards, the konimeter was the basis of dust exposure standards in those and many other countries.Le Roux (1970) reported that, in South Africa, mine officials took about half a million konimetersamples per annum while government inspectors took about 30 000 samples during routine inspections.In Western Australian underground mining operations, of the order of 40 000 konimeter samples wererecorded for the period from 1925 to 1977 (Hewson, 1996).

Other, related aerosol sampling instruments were fashionable during the same period, including theimpinger proposed by Greenberg and Smith (1922), followed by a miniaturised version, the midgetimpinger. These were popular for many years among occupational hygienists in North America in the


period before gravimetric sampling of fine particles was introduced, providing the basis of occupationalexposure limits for many years. Also in the 1920s, the Owens jet appeared (Owens, 1922), and wasused in early investigations of airborne asbestos dusts in the UK, the USA and Germany.

The thermal precipitator that first appeared during the 1930s was attractive in that it provided a‘gentle’ means of depositing the particles such that particles that were subsequently observed under themicroscope would not be damaged by fluid mechanical shear forces or impact fracture during deposition.It was felt by aerosol scientists of the day that particles would be more representative of those inhaled bythe exposed workers, in particular in the sense that the individual airborne particulate entities would nothave undergone any significant changes during the act of sampling. The standard thermal precipitator(STP) developed by Green and Watson (1935) operated at the low continuous flow rate of 8 mLpm,and thermophoretic particle deposition took place onto glass slides placed either side of a heated wire.It was used as the standard instrument in British coal mines during 1949 to 1965. Later a modifieddevice, the long-running thermal precipitator (LRTP) (Hamilton, 1956), was built which excluded thecollection of large particles by virtue of a laminar-flow horizontal elutriator placed ahead of the thermalcollection section. This instrument became the standard instrument in British coal mines during 1965 to1970, and was also used elsewhere and for collecting asbestos fibers. A similar instrument was used inSouth Africa (Kitto and Beadle, 1952). In most versions of the thermal precipitator, collected particleswere usually assessed by optical microscopy.

Particle counting approaches had become popular when it became clear that the earliest ‘total dust’gravimetric methods were fundamentally flawed, based on emerging new knowledge derived fromaerosol science. But it later transpired that particle counting too had its disadvantages. Firstly, theeffort to visually count the collected particles was very labor intensive and there were considerableinter- and intra-observer variabilities. Secondly, for instruments like those described, aspiration andcollection efficiencies were poorly defined, so that biases associated with particle size-dependent entryand deposition effects were unknown. Thirdly, measured concentrations based on particle count werefound not to correlate particularly well with health effects. Rather, it was shown quite early on thatthe mass concentrations of fine particles provided much better correlations, in particular for certaintypes of lung disease such as pneumoconiosis (e.g. Bedford and Warner, 1943). So, from the 1950sonwards, new knowledge of the physics of how particles are transported in the air, are inhaled, andare deposited in the respiratory tract, facilitated a return to gravimetric sampling, now much improved(compared with earlier efforts) by the identification of health-relevant aerosol fractions defined in termsof their aerodynamic properties and the development of instruments that could measure them. As aresult, particle counting-based sampling devices were gradually superseded by gravimetric samplers.

Meanwhile, however, interest in particle counting approaches have continued to the present day incertain key areas. For fine fibrous particles, the interest was stimulated originally by concerns about thevery serious health effects arising from the inhalation of asbestos fibers, but has since been extendedto related concerns about all fine fibrous particulate matter. Although primary asbestos manufacturingor using industries virtually disappeared during the 1990s, at least in developed countries, occupationalexposures still occur during maintenance or demolition work involving older facilities or equipment.Occupational hygienists have therefore needed to remain conversant with the appropriate samplingand analytical methods. From the 1960s onwards, the membrane filter method has been critical in thisapplication, providing means for the efficient collection of particles which are aerodynamically very fineand then – after sampling has been completed – the ability to make the filter media itself transparent(i.e. ‘cleared’) so that the collected particles can be viewed by optical microscopy. Here, phase contrastmicroscopy is needed because of the small difference in refractive index between the fibers and themembrane filter material. In Britain, Holmes (1965) described the application of the membrane filtermethod towards the assessment of the concentration of ‘respirable’ airborne asbestos fibers, defined as


particles which, when viewed under the microscope, had length greater than 5 µm and aspect ratiogreater than 3 to 1, later adapted in some quarters to refer additionally to fibers with diameter less than3 µm. These definitions have continued to be the basis of regulatory standards for airborne asbestosfibers almost world-wide.

From the beginning, bioaerosols have been assessed in terms of the number of organisms collected,expressed in terms of colony forming units (CFUs). Interest in bioaerosols in working environments hasrisen quite sharply in recent years, as better understanding has been gained into the nature and causesof lung disease such as asthma.

13.2.5 Emergence of gravimetric samplers for the respirable fraction

In the early 1950s, there was sufficient information, experimental and theoretical, to enable considerationof the sizes of inhaled particles which were capable of penetrating to the alveolar region of the lung(see Chapter 11). In particular, this highlighted the significance of particle aerodynamic diameter (dae),as distinct from the geometrical measure of particle size that is associated with microscope counting.Driven at the time largely by concerns about the high incidence of pneumoconiosis among coal miners,this led to a succession of conventions for what has been referred to ever since as the ‘respirablefraction’. Most sampling for the respirable aerosol fraction has subsequently been based on the premisethat the index of interest is the mass of overall respirable aerosol or of particular health-related species.However, in view of the continuing interest in some quarters in particle surface area concentration as arelevant metric (e.g. for crystalline silica in relation to silicosis), Talbot (1966) devised an instrument,based on the analysis of optical diffraction patterns, for assessing respirable surface area concentrationsfor samples collected by thermal precipitator. This instrument was used for a while in South Africa.

The first respirable sampling instruments took the form of horizontal elutriators, in which the particlesof interest corresponding to the British Medical Research Council (BMRC) curve were those whichpenetrated in laminar flow through the narrow rectangular channel of an elutriator pre-separator andwere collected on a filter. The very first commercial sampler was the British HEXHLET (Wright, 1954),a large device with a flow rate of 100 Lpm. This was used widely in British foundries in the 1950sand 1960s. Later, a smaller 2.5 Lpm version, the portable MRE Type 113A (Dunmore et al., 1964),was developed specifically for use in British coal mines, where it was used as the basis of regulatorystandards from 1970 until quite recently. This device, although portable, was intended for use as a static(or area) sampler. Other horizontal elutriator-based devices were used in European coal mines and otherworkplaces during the same period.

Initially in the USA, and later elsewhere, respirable aerosol samplers emerged with pre-selectors basedon cyclones. Indeed, the 1968 American Conference of Governmental Industrial Hygienists (ACGIH)respirable aerosol curve was originally derived from consideration of the penetration characteristics ofminiature cyclones. The great advantage of the cyclone over the horizontal elutriator was that it couldbe miniaturised to the point where the sampler could be worn on the lapel of the worker, and so bedeployed as a personal sampler along with a miniaturised sampling pump worn on the worker’s belt.Many commercial versions of cyclone samplers have since appeared, most with sampling flow ratesbetween 1.5 and 2.5 Lpm, some with performance characteristics matching the ACGIH curve, somematching the BMRC curve, and an increasing number matching the contemporary respirable fractiondefinitions that have come along in recent years.

Finally, it should be noted that, during the transition from particle counting to gravimetric samplingthat took place during the 1960s and 1970s, there was the need – at least from the point of view ofoccupational epidemiology, and hence standards setting – to be able to convert particle count exposuredata to equivalent respirable mass concentration data. This set difficult problems for aerosol science


and occupational hygiene researchers, because each index was determined by local factors such asaerosol type and particle size distribution such that any single conversion could not be generalised forall situations. So such conversions were carried out as needed on the basis of field studies in specificindustrial situations (see Chapter 22).

13.2.6 Emergence of gravimetric samplers for ‘total’ and inhalable aerosol

As described in Chapter 10, it has long been recognised that ill-health arising from aerosol exposureis not necessarily confined to the lung. There are substances which can produce health effects afterdeposition anywhere in the body, including not only the lung but also other parts of the respiratorytract, as well as elsewhere in the body if the aerosol material is soluble. During the 1970s, occupationalhygiene scientists began to question ‘total airborne particulate’ as a valid, health-related metric ofaerosol exposure. In addition, by the late 1970s, the science of aerosol sampling had advanced to thepoint where it became clear that simply drawing a known volume of air through a sampling orifice ofarbitrary dimensions and onto a filter did not – in itself – provide a measure of true total aerosol. So, inthe late 1970s, a new approach was first proposed by Ogden and Birkett (1977), in which ‘total’ aerosolwould be represented by what is actually inhaled, based on knowledge of the particle size-dependentefficiency with which particles were aspirated into the nose and/or mouth during breathing. Out of thisthe inhalable fraction emerged. Wind tunnel studies during the 1980s and 1990s, and field studies duringthe 1990s, confirmed that the performances of most of the samplers previously used for collecting ‘total’aerosol did not adequately match this definition (e.g. Mark and Vincent, 1986; Kenny et al., 1997). Soa new generation of sampling instruments began to emerge to fill this instrumentation gap. The firstwas the ORB sampler suggested by Ogden and Birkett (1978). It was originally suggested as a samplerthat could be applied in both the area and personal sampling modes. Although the ORB was developedonly as a prototype, was never applied in field studies and was never made available commercially, itrepresented an important first milestone in the development of instruments that, from the outset, wereintended for collecting specifically the inhalable fraction. This prompted other aerosol scientists towardsthe development inhalable aerosol samplers that could realistically be commercialised and be appliedin the real world of occupational hygiene. Out of this, the Institute of Occupational Medicine (IOM)personal inhalable aerosol sampler first appeared during the late 1980s, and its commercial availabilitywas a significant factor in decisions by some leading standards-setting bodies to begin the establishmentof occupational exposure limits based on this new fraction. Subsequently other inhalable aerosol samplershave been proposed, and some of these too are commercially available.

13.2.7 Other aerosol fractions

As described in Chapter 11, the past two decades has seen a widening of the scope of particle size-selective criteria for aerosol exposure assessment. In the early 1980s, ISO produced its first set ofcriteria, encompassing the coarser inhalable, the intermediate thoracic and the finer respirable fractions.These were followed by a somewhat similar, but quantitatively different, set of criteria from ACGIH,and another from the Comite Europeen de Normalisation (CEN). In the late 1980s, however, thesecombinations were harmonised into a single set of criteria for the three fractions indicated, and this waswidely accepted by many of the world’s standards-setting bodies (Soderholm, 1989).

13.2.8 Sampling to measure aerosol particle size distribution

For investigational purposes, occupational hygienists have frequently sought aerosol information beyondknowledge of the airborne concentration alone, including particle size distribution. In the occupational


hygiene context, such information can be especially valuable in (a) enabling estimation of the doseof particulate matter deposited in – or penetrating to – particular regions of the respiratory tract, and(b) the choice of control strategy or technology. In the early years of aerosol sampling, the most commonapproach to estimating particle size distribution was by the microscopic analysis of samples collected onsuitable substrates, usually glass slides. For many years, for example, samples obtained using the thermalprecipitator were analyzed to provide quite accurate particle size number distribution information overthe particle size range from about 0.2 to 10 µm, enabling other parameters – such as the particle massdistribution – to be estimated based on assumptions about particle shape and density. By the use of thethermal precipitator, it was possible to argue that the particle size distribution was relatively unbiasedwith respect to the collection efficiency characteristic of the sampler. In such an approach, particlescollected on glass slides were counted and sized visually under the microscope by comparison withcircular profiles depicted on an appropriate eyepiece graticule. The Patterson-graticule used by Greenand Watson had 10 such circles with apparent diameters ranging from 0.2 to 5 µm. The procedure wastedious and subject to error for ‘overcrowded’ samples. It was from exposure data obtained in this way,coupled with corresponding health effects data, that Bedford and Warner were able to make their highlyinfluential conclusion that it was the mass of coal dust particles smaller than 5 µm that provided theindex of exposure most relevant to the risk of pneumoconiosis for workers in underground coal mines.

The term ‘aerosol spectrometer’ has been used to refer to sampling instruments that segregate particlesdirectly into a continuous size spectrum or, more correctly, into well-defined size bands, from whichthe continuous particle size distribution can be inferred or calculated. A wide range of such instrumentshas emerged over the years, utilising a variety of aerosol mechanical principles. These include iner-tial, gravimetric and centrifugal spectrometers, some of which will be described later in Chapter 18.Many such devices had sampling flow rates that were too small to enable the determination of particlesize distribution according to mass, and so were of limited value in relation to occupational hygieneapplications. They nevertheless have provided valuable information about the relationships between‘microscopical’ and aerodynamic particle size. One class of aerosol spectrometer has risen above allthe others and found very widespread use by occupational hygienists in the workplace environment.This is the cascade impactor which first emerged in the 1940s (May, 1945). The general principle ofthe cascade impactor is that sampled aerosol is passed through a succession of impactor stages withprogressively smaller particle ‘cut’ sizes. The amount of particulate matter collected masses on thevarious stages of the instrument define the cumulative particle size distribution (by mass) to an accuracydetermined – indeed limited – by the sharpness of the cuts of the impactor stages. The earliest cascadeimpactors were relatively large devices, and so could be used in the workplace only as area samplers.When small personal sampler versions appeared during the late 1980s, interest in their potential asinvestigational exposure assessment tools for occupational hygienists rose sharply. In particular theeight-stage personal cascade impactor first described by Marple and his colleagues (Rubow et al., 1987)has been commercially available for several years and has been widely used by occupational hygienists.

13.2.9 Direct-reading instruments

In parallel with the direct-reading sampling instruments that have found use for the measurement ofambient atmospheric aerosols in the past three decades, similar instrumentation has been of increasinginterest to occupational hygienists. Today it is seen as an approach to aerosol sampling that, after theinitial capital investment, can provide fast results and yet be economic in terms of the time and effort onthe part of occupational hygienists. The earliest practical direct-reading aerosol instrument for workplaceapplications appears to have been the ‘hazard’ device described by Drinker and Hatch as early as 1934,since when a large variety of instruments has appeared, many of which were portable but were used


Table 13.1 Summary of time lines of approaches to aerosol sampling in occupational settings

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Strategy/Index‘Snap’ in workers’ breathing zones

Number concentrationFull shift TWA fixed point, static (or

area) Number concentrationFull shift TWA fixed point, static (or

area) Mass concentration (‘total’)Full shift TWA fixed point, static (or

area) Mass concentration(respirable)

Full shift TWA personal Massconcentration (‘total’)

Full shift TWA personal Massconcentration (respirable)

Full shift TWA personal Massconcentration (inhalable)

Fixed point and portabledirect-reading instruments

as fixed point static samplers. The development of direct-reading instruments in the 1990s that weresufficiently miniaturised that they could be deployed as personal monitors opened up the possibilityof some very interesting and useful control strategies for workplace aerosols. For example, real-timemeasurements of worker exposure using an appropriate personal monitor have been correlated – throughthe use of video techniques – directly with worker activity (e.g. Rosen, 1993). Such an approach isproving especially helpful in identifying what types of activity leads to the greatest exposure and, inturn, has helped educate workers on how to minimise such exposures.

13.2.10 Overview

Table 13.1 provides a concise summary of the history of aerosol sampling in occupational settings, draw-ing together the main threads and time line of what has been reviewed above. It shows the considerableoverlap between philosophies and approaches over the years. It reveals the primary milestones, mostnotably the transitions between particle number and particle mass concentration measurement, betweenstatic and personal sampling, and the emergence of the different phases of particle size-selective sam-pling. It hides the differences between countries or between industry sectors, and so paints only a verybroad picture.

13.3 Ambient atmospheric aerosol sampling

13.3.1 Sampling strategies and philosophies

The exposures of people to aerosols in the ambient atmospheric environment takes place over 24 ha day, 7 days a week, and for every week of the year. The nature of such exposures changes as


individuals move around during their daily lives and spend differing proportions of their time indoors(at home or elsewhere), at work, or outdoors. Specific such exposures differ widely, both qualitativelyand quantitatively. So an aerosol sampling strategy that is truly representative should ideally involve amethod that allows all those various contributions to exposure to be identified, measured and combinedin a way that reflects the overall risk to health, and so is representative of each exposed individual.However, this presents a very difficult technical and strategic challenge. Sampling approaches haveevolved, therefore, that address aerosol levels in the general ambient environment to which most peopleare exposed most of the time. These are specified mainly by geographical location. Most air pollutionmeasurement for ambient aerosols has so far been carried out using such fixed point, static (or area)sampling instrumentation.

As discussed above, personal aerosol sampling has been the preferred mode of aerosol exposureassessment in occupational settings in many countries since the 1970s. It is also desirable in relationto ambient exposures. Recently, acknowledging that static sampling at fixed point outdoor locationsprovides less-than-adequate assessment of the actual exposures of individuals or groups of people,efforts have been made to introduce personal sampling to the ambient atmospheric environment. But,so far, such measurements have been confined to specific research enquiries and there are as yet no airquality standards based on personal exposures.

13.3.2 Indices of health-related aerosol exposure

The primary index of aerosol exposure is the concentration of aerosol particles, since this is what maybe related to the dose received by exposed people. However it has long been an important question asto how concentration should be defined. Particle size fraction? Species? Mass or some other index of‘amount’? In these, as for occupational aerosol exposures, there has been a long history of changingviewpoint.

In Britain, Cohen and Rushton (1912) described air pollution data records, taken mainly in the City ofLeeds in the north of England, that went back as far as the late 1800s. Even then, the primary focus ofattention was atmospheric pollution in the form of smoke arising from combustion, from both industrialand domestic sources. For the latter, much of it was the result of the long culture of home heating bythe burning of coal. Although very variable, the smokes in question were later universally characterisedby the presence of carcinogenic hydrocarbon species (e.g. 3, 4-benzpyrene), thus confirming a clearlink with public health. Eventually, as a result of the 1956 Clean Air Act, a National Survey wasintroduced in Britain in 1961 that required the measurement of particulate air pollution expressly interms of ‘black smoke’. Somewhat later this was formalised still further by the prescription by theBritish Standards Institution (BSI, 1969) of the ‘smoke stain’ method, involving optical assessment ofthe ‘blackness’ of particulate matter deposited on a filter (see Chapter 17). The black smoke approachhas persisted in Britain and elsewhere in Europe to this day, where aerosol sampling and analysis alongthe lines of the BSI method still features in atmospheric environmental regulations (e.g. as in EuropeanCommunity Directive 80/779/EEC). The APHEIS (Air Pollution and Health: a European InformationSystem) program, coordinated by the Institut de Veille Sanitaire (InVS) in Saint-Maurice, France and theInstitut Municipal de Salut Publica de Barcelona (IMSPB) in Spain, recently reported measurements ofparticulate air pollution in 26 cities in 12 European countries taken during 2001, expressed in terms oftwo indices, one of which was black smoke (the other was PM10, see below) (Medina et al., 2002). TheWorld Health Organisation (WHO) also continues to list guidelines based on the black smoke index.But the United States Environmental Protection Agency (EPA) lists no such requirement.

The term ‘smog’ was first used by Des Voeux in 1905 to describe the aerosol that he observed whenfog occurred in the presence of smoke. Smog episodes were most likely to occur during meteorological


conditions associated with calm air and fog, along with high concentrations of smoke. The healthimplications of the effects of such combinations were later dramatically illustrated during the 1952smog incident in London where as many as 4000 people were thought to have died; another 700 or sowere thought to have died in the subsequent 1962 episode. In general, it is a reasonable assumptionthat smog levels would have been quite well reflected in the measurements of black smoke, and this islikely to have spurred the continuing use of the black smoke index in Britain and elsewhere in Europe.More recently, however, use of the term ‘smog’ has been extended to describe the complex form ofaerosol pollution that occurs in the presence of photochemical reactions.

More recently, the term ‘total suspended particulate’ (TSP) has referred in principle to the massconcentration of all particles considered to be airborne, although in reality it has always been defined bywhatever sampling instrument was chosen to collect it. In Britain and elsewhere in Europe, this fractionwas measured using sampling instruments whose design varied over time, depending on the technologycurrently available at the time. In the early 1900s, filter media were quite rudimentary, and cottonwool filters were commonly used. Later, however, improved filter technology increased the options forsampling. Many samplers for TSP were proposed, and some of them are described later in Chapter 17.But, as will be seen there, they are notable for the general lack of consistency in performance fromone to the other. This is partly because TSP has never been defined in terms of any ‘target’ samplerperformance criterion.

TSP, regardless of how poorly defined, formed the basis of primary standards in the USA for manyyears, indeed up until 1987. Then, however, in a flurry of activity, EPA published revised particulatestandards to account for the deeper penetration of particles into the respiratory tract, and hence to a fineraerosol fraction. The aerosol mass concentration for particles with aerodynamic diameter nominally lessthat 10 µm was chosen for defining the new index, providing in turn a new criterion for sampling.This index is what has become widely known as ‘PM10’, and it became the primary atmosphericaerosol standard in the USA. As such it was the first particle size-selective index (or criterion) forambient atmospheric aerosol sampling that was linked directly to the physical nature of human exposureto aerosols, representing the penetration of particles which, after inhalation, may penetrate beyondthe larynx and into the lower respiratory tract. Many new samplers appeared to meet the new need,and the old TSP standard was relegated to the status of a secondary standard. The PM10 criterionsubsequently became widely accepted in other countries, as reflected in the APHEIS report mentionedabove. Then in 1997 EPA proposed a new, additional fine particulate criterion for those particles that,nominally, have aerodynamic diameter less than 2.5 µm, aimed at providing measures of combustion-related aerosol. This new criterion was referred to as ‘PM2.5’ and it became a second standard (on apar with PM10), while the TSP standard was dropped. With the cementing of EPA’s philosophy onparticulate matter (PM10 and PM2.5), and its wide acceptance in other countries, a whole new generationof sampling instruments, with particle size-selective performance curves matching the definitions inquestion, emerged.

Meanwhile, in 1981 the ISO had first proposed a quantitatively different but similar definition for thesame intermediate fraction, with a view to applications both in ambient atmospheric and occupationalaerosol exposure assessment. ISO referred to this as the ‘thoracic’ fraction. In addition to the ‘thoracic’fraction, ISO also recommended criteria for the coarser, inhalable fraction that can be inhaled throughthe nose and/or mouth during breathing and also for a finer, respirable fraction. But neither of the ISOcriteria were adopted for routine ambient aerosol monitoring and harmonisation of criteria and methodsnow appear to have coalesced around the EPA PM10 and PM2.5 criteria.

Interest in airborne fibers grew from the 1960s onwards as long, thin particles of asbestos fibers wereacknowledged to be especially hazardous. Apart from potential exposures in some workplaces, therewas special concern about exposures to such particles in indoor living spaces, particularly in locations


inhabited by vulnerable populations such as children (e.g. schools). For these, mass concentration wasfound to be difficult to measure at low concentrations, especially in the presence of other, nonfibrousparticulate material. So the particle number concentration was determined by the manual counting ofparticles of appropriate shape and dimensions after they had been collected on filters when viewedunder the light or electron microscope. Later, in the 1990s, although they had long been of interest toaerobiologists, concerns about biological organisms in indoor living environments rose sharply. Suchorganisms were sampled and assayed, for example, by collecting them onto, and culturing them in,nutrient media and observing the colonies visually. The concentrations of collected organisms were thenexpressed in terms of the numbers of colony forming units (CFUs), following basic methodology thatwent back even as far as the 19th century.

13.3.3 Indices for coarse ‘nuisance’ aerosols

Interest in sampling atmospheric aerosol has extended in some cases beyond the adverse health effectsexperienced by people after inhalation. A significant economic burden may be placed on individualsor businesses impacted by the unwanted deposition of coarse, ‘nuisance’ particles. Such effects werefelt in the near vicinities of certain industrial operations, including quarrying, electricity generatingstations, landfill sites, etc. and have been manifested, for example, in psychological stress and loweringof property values (Ridker, 1970). Such concerns surfaced in Britain and a number of other countriesduring the 1970s and were serious enough to require action, beginning with monitoring by aerosolsampling.

13.3.4 Direct-reading instruments

Development and applications of direct-reading instrumentation for the measurement of atmosphericaerosols began to appear during the second half of the 20th century. These derived from the rangeof potential physical interactions involving airborne particles, or by particles after they have beendeposited onto surfaces. In all such devices it was a general feature that information about aerosolproperties was converted into electrical signals that could be processed and recorded. Some of theearliest automatic instruments for the direct measurement of airborne particles were reviewed by Greenand Lane (1964), including photometers and particle counters based on light scattering by ensemblesof particles or just single particles, and detectors operating on the basis of the electrical mobility ofcharged particles. In later decades other devices appeared, operating on the basis of particle growth bycondensation in supersaturated vapors (condensation particle counter) or time-of-flight in a changingflow field (aerodynamic particle sizer). Others were based on mechanical properties associated withthe changing mass of particulate material on substrates as particles were collected there, includingpiezoelectric and tapered element mass balances. The commercial development and availability of suchinstruments has increased sharply in subsequent years as more sophisticated and cost effective electronicsystems have become available, accelerating still further with the arrival of enhanced computer-basedcapability. Direct-reading aerosol instrumentation is now seen as an approach to aerosol sampling that,after the initial capital investment, can provide fast results that are also economic in terms of the timeand effort on the part of the environmental hygienist. More details are given later in Chapter 20.

13.3.5 Overview

Some of the main points of what has been described are summarised in Table 13.2, providing a shortoverview of the history of aerosol sampling in the ambient atmospheric environment. It shows the con-siderable overlap between philosophies and approaches over the years. It reveals the primary milestones,


Table 13.2 Summary of time lines of approaches to aerosol sampling in the ambient atmospheric environment

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Strategy/IndexFixed point‘Nuisance’ dustFixed pointMass concentration(TSP)Fixed pointBlack smokeFixed point Mass

concentration (PM10)Fixed point Mass

concentration (PM2.5)Fixed point and portable

direct-readinginstruments

Personal sampling PM10

and PM2.5

most notably the introduction of the black smoke index in some countries, the emergence of PM10 andPM2.5, and the beginnings of personal sampling for members of the general population.

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aerosol sampling || historical milestones in practical aerosol sampling

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