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This paper reviews how aerosol exposure assessment, for people in both working and living environments, has evolved over the years. It charts the main scientific developments that led to progressively improved ways of thinking and methods to assess exposure to airborne particulate matter in a manner more relevant to human health. It has been a long scientific journey as one generation of pioneering contributors has handed off to the next. In the process a consistent rationale has emerged, producing aerosol sampling criteria – and in turn exposure standards – which have been increasingly relevant to actual human exposures. The journey continues as a new generation of scientists steps up to deal with the new challenges that are emerging. An appreciation of the history of what went before is essential to charting the most effective path looking forward.

Knowledge of the particle size distributions for workplace aerosols is invaluable in the assessment of aerosol-related health effects. Cascade impactors have been widely used for obtaining such information, including a small number that have been developed as personal samplers of the type that can be used for the assessment of the exposures of individual workers. Common limitations for most samplers of this type have been that (a) the aspiration efficiency has not been well defined (leading to biases in particle size distribution measurement), and (b) the range of particle size has been constrained by particle bounce in impactors for particle sizes beyond about 20 micro m. This article describes a modification of the Marple personal cascade impactor that addresses these limitations. The sampler has a new entry whose aspiration efficiency is known and a new top stage that employs porous plastic foam filtration media and significantly extends the particle size range of the instrument. The new instrument is referred to as the modified-Marple sampler. A numerical simulation was performed to investigate the ability of the new instrument to accurately retrieve particle size distributions over the range typical of aerosols found in workplaces. The retrieval process was carried out using a simple inversion algorithm of the "zeroth-order" type. The results are presented in terms of the ability of the new sampler to retrieve the masses contained in the inhalable, thoracic, and respirable fractions. They suggest that the more narrowly distributed the particle size distribution, the more restricted the ability to accurately retrieve the particle size distribution. However, for most aerosols of the type encountered in the real world of industrial hygiene, the modified-Marple sampler provides particle size information of sufficient quality for most practical purposes.

The American Conference of Governmental Industrial Hygienists (ACGIH) and some regulatory authorities have revised their exposure limits for nickel and nickel compounds in workplaces based upon new sampling standards for inhalable nickel exposures. Others may be in the process of doing so. Safe standards for workplace exposures should utilize the most up-to-date health data on individual nickel species and should incorporate the principles of new sampling conventions that have been developed over the recent decades. The purpose of this paper is to review the basis for setting inhalable occupational exposure standards for the principal inorganic nickel species. It is hoped that this paper will (1) prompt companies in various nickel industry sectors to begin collecting the necessary inhalable aerosol measurements, speciation data, and particle-size information required to implement health-based sampling programs in the future, and (2) encourage regulators to derive species-specific, inhalable-based workplace standards for nickel and its inorganic compounds.

In 2002 the Comité Européen Normalisation (CEN) published its document Workplace atmospheres-assessment of performance of instruments for measurement of airborne particle concentrations (EN 13205) that describes a standard protocol by which to carry out the testing and validation of personal aerosol samplers of the type widely used for occupational aerosol exposure assessment. It emerged from more than a decade of discussion and a large body of research experience involving several laboratories. The protocol that is described, however, still poses significant technical and economic challenges, not least because it involves laborious-and hence costly-procedures in large, specialized wind tunnel facilities. More recent research has identified a number of areas by which the protocol may be improved and made more accessible to testing laboratories, including a set of validated aerosol sampler scaling laws, a better understanding of the reduced role of the bluff body of the wearer on sampler performance, and the availability of new options for rapid sampler testing methods. Taking these into account, a dummy new protocol is offered for discussion.

New automated instrumentation for the rapid acquisition of aerosol sampler aspiration efficiency data has been applied to an investigation of a range of personal aerosol samplers of the type developed during the 1980s at the Institute of Occupational Medicine (IOM) in Edinburgh, Scotland, U.K. The experimental research was carried out in a small wind tunnel, and the relation of the results for IOM-like samplers to full-scale life-size personal aerosol sampling scenarios—like those encountered in occupational aerosol exposure assessment—was investigated by reference to the scaling laws that have been developed based on familiar aerosol mechanics as they apply to the physics of aerosol sampling. In the small-scale experimental study, the IOM-like sampler was mounted centrally on a rectangular bluff body, simulating the wearing of the sampler on the body (e.g., as by a worker in an industrial setting). Scaling with respect to the corresponding, more-realistic full-scale system for a corresponding full-scale ...

A new experimental system was recently developed for the rapid acquisition of data for aerosol sampler aspiration efficiency, and was applied in this latest work to the experimental study of thin-walled probes oriented at 90° to the freestream. Previous experimental studies from the 1980s, backed up by a physical model, suggested that aspiration efficiency for this orientation ( A 90 ) may be uniquely described as a function of St R 1 / 2 (where St is the Stokes number for particle motion in the region of the sampler entry, and R is the ratio of the freestream air velocity to the average air velocity across the plane of the sampler inlet). But in this new work the experimental system was sufficiently selective as to allow detection of an additional dependency on R not revealed by the previous work. It is proposed that this additional dependency is associated with effects associated with the non-uniformity of the airflow distribution across the plane of the test sampler at such extreme orientations, derived from the inertia of the air motion as it approaches the sampler. One result of such non-uniformity is to reduce the effective cross-sectional area of the inlet as R increases. The experimental results suggest that A 90 is now better described as a unique function of St R 1 / 4 (instead of St R 1 / 2 ). Such new insights into the basic physical behavior of a simple aerosol sampling scenario may be useful in helping to explain some aspects of the performances of more complicated aerosol samplers like those used in practical occupational and environmental air sampling situations.

A large experimental study has been conducted to make definitive measurements of the aspiration efficiencies of idealized cylindrical thin-walled aerosol samplers in perfectly calm air, using a method that involved the direct visual observation of falling streams of particles of well-defined size for sampling under specific conditions of sampling flowrate and orientation (downwards facing and horizontal). These data augment an earlier set of experimental data for upwards-facing sampling, and those results are included again in this article for the sake of completeness. In addition to the experimental study, a numerical study was also carried out, first for the purpose of comparison with the experimental results in ranges where such results are available, and second for providing information in ranges where experiments could not be conducted satisfactorily. From this combined approach, a very comprehensive set of new data was generated. In general, the experimental and numerical results were seen to be in ...

In a recent previous paper (Su & Vincent, J. Aerosol Sci. 33 (2002) 103) we described a new method by which to investigate the relationships between aspiration efficiency, particle inertia, gravitational effect and sampling orientation for aerosol sampling in perfectly calm air. All previous experimental work to elucidate the basic nature of aerosol sampling in calm air described has been carried out for thin-walled tubes, and none has yet been reported in relation to blunt samplers. To begin to fill this important gap, the present paper describes the application of our new method towards acquiring new measurements of aspiration efficiency for simple blunt samplers. Experiments were carried out to determine the aspiration efficiencies of simple, idealized, spherical blunt samplers for a range of sampling scenarios, for two sizes of blunt samplers and different sampling inlet diameters, and for upwards and downwards sampling scenarios (and hence a range of governing dimensionless physical quantities). It was shown that aspiration efficiency decreased both with increasing inertia (as represented by the Stokes’ number) and with increasing gravitational effect (as represented by the ratio of particle settling velocity to the air velocity at the sampler inlet). The results enabled qualitative physical explanation of the difference between what was observed for upwards and downwards-facing sampling, respectively, in terms of (a) the role of the sampler body in deflecting the air flow in the region close to the body of the sampler (in turn influencing the performance of the sampler), and (b) the interception of particles in the downwards-facing scenario falling within the ‘shadow’ projected upwards by the sampler body. The significant contribution of this work has been the acquisition of a definitive set of new experimental data. These data will be valuable in the future development of understanding of the physics underlying aerosol sampler performance. Such knowledge will be of practical value because (a) blunt samplers are generally the most representative of the types of instruments used in practical occupational, and (b) calm or slowly moving air is characteristic of many indoor situations.

In the development of aerosol samplers with desired particle size-selective properties, it is important to understand and be able to predict the physical processes that determine the aspiration of aerosols into their nozzles. An idealized sampling scenario is one involving a disc-shaped blunt nozzle facing the wind. A few earlier studies have investigated this scenario but only for relatively narrow ranges of particle aerodynamic diameter (dae) and velocity ratio (R, the ratio between the air velocity in the approaching freestream and that in the plane of the sampling orifice). The purpose of the current study is to expand this knowledge to larger dae-values pertaining to the inhalable aerosol fraction and to larger R-values. The issue of sampling at large R is seen to be especially important in relation to our desire to develop low flowrate samplers for occupational hygiene applications. New experiments were conducted for disc-shaped blunt nozzles, varying in disc diameter and orifice diameter, for R ranging from 0.5 to 25. The experiments were conducted in a small wind tunnel using narrowly graded fused alumina powders to produce aerosols with mass median particle aerodynamic diameters up to 90 μm . A highly repeatable set of experimental results showed a broad trend of A increasing with R, with the magnitude of its increase proportional to particle size. This was expected from what is known about aerosol sampling theory. However, a comparison of these data with predictions from an earlier mathematical model showed that, for R increasing above about 2, the predictive model significantly overestimated aspiration efficiency in relation to what was measured in these new experiments. The model, therefore, was modified to include this more expansive set of data, including additional terms involving both R and the dimension ratio (r, the ratio of the size of the sampling orifice to that of the sampler body). The new, modified model was found to agree very well indeed with the whole data set, as well as with the limited data sets reported earlier. Another notable observation was the relatively small change in A that resulted from a two-fold increase in r. This suggests that within a certain range, r may play only a minor role in determining A. The new model provides physical insights into the performance of blunt samplers when facing the wind, and it will ultimately aid our ability to design new samplers for specific applications.

The research described in this paper extends the body of work contained in earlier publications in which aerosol sampling aspiration efficiency was studied in a small wind tunnel for simple, idealized sampler geometries. These studies are aimed at providing generic knowledge about the physical and functional behaviors of such systems that can then be generalized to sampling systems of more practical interest. In this paper, attention is focused on the little-researched case of a blunt sampler facing away from the prevailing wind velocity. The results show, that for angles greater than 90 ∘ with respect to the wind, aspiration efficiency and particles losses inside the sampler close to the entry are constant as a function of α . This in itself may have some practical usefulness since it enables the definition of a regime of aerosol performance that is relatively “well-behaved”. These results are in marked contrast to what has been learned from the same body of work for sampler behavior when facing forward with respect to the wind. Here not only does aspiration efficiency vary sharply with orientation, but so too does the particle entry loss. From this it is concluded that no general model of aspiration efficiency can be available so long as experimental results embody unknown entry loss biases. With this in mind, caution is recommended in field sampling or in laboratory experiments where the measurement does not explicitly include such entry losses.

The study of thin-walled aerosol sampling probes facing the wind was originally driven by practical applications (e.g., stack sampling). But more recently it has provided useful scientific insights into the performances of more complicated blunt aerosol samplers. Previous work has been carried out for a relatively narrow range of conditions, especially for R (the ratio of freestream air velocity to probe suction velocity), leading to the widely accepted model of Belyaev and Levin (J. Aerosol Sci. 5 (1974) 325). This model has been shown to predict aspiration efficiencies (for sharp-edged, thin-walled samplers facing the wind) that agree well with experimental data, for R ranging from 0.03 to 11. Recently, there has been renewed interest in studying aspiration for a much wider range of R, driven primarily by the desire for a new generation of particle size-selective blunt samplers, including the need to sample at lower flowrates in order to use lighter sampling pumps for industrial hygiene applications. The study of thin-walled sampling for wider ranges of R, especially in the range above about 10 is the first step towards improved understanding of blunt sampling as it might be applied to practical samplers under such conditions. This is because much of what is currently known about the aspiration of aerosols into blunt samplers derives from what is known about the less complex thin-walled sampler configurations. New experiments have been conducted for thin-walled cylindrical sampling probes for R ranging from 0.5 to 50. The experiments were conducted in a small wind tunnel (cross-section 0.30 m ×0.30 m ) using narrowly graded fused alumina powders to produce aerosols with mass median particle aerodynamic diameters up to 90 μm . The external wind velocity was held constant at 1 m / s , so that Stokes’ number (St) ranged from about 0.05 to 3.7. An extensive, highly repeatable set of experimental results showed that, for R increasing above about 6, the Belyaev and Levin model increasingly overestimated aspiration efficiency compared to what was measured. An improved model for aspiration efficiency of thin walled sampling probes facing the wind is proposed that describes what happens over this much wider range of conditions.

Wind tunnel experiments and a visualization procedure are described with the purpose of investigating the effects of inertia on the dispersion of particles in flows around a bluff body. Four different grits of narrowly graded aluminum oxide optical powders were used to generate the test aerosol, which was released from a point source upstream the obstacle. The resultant aerosols covered the range of median particle aerodynamic diameter from 14 to 47 μm . Measurements were taken for the residence time of the particles in the near-wake region of a long flat plate positioned perpendicularly to the free-stream after the aerosol injection had been suddenly interrupted. The experiments covered air velocity and particle size ranges corresponding to Reynolds’ number ( Re ) ranging from 1400 to 10,200 and Stokes’ number ( St ) from 0.02 to 1.8. Results for the dimensionless residence time H (defined as H = τU / D , where τ is the time constant for the concentration decay for particles in the near wake, and U and D are the free-stream velocity and characteristic dimension of the bluff obstacle, respectively) for the larger particles varied between about 5 and 13, while the H -values for small inertialess particles (obtained in previous experiments; Gomes, Vincent, and Pui (Aerosol Sci. Tech. 26 (1997) 269) had varied between about 7 and 10. This is in accordance with the general finding that the apparent diffusivity of the large particles may, depending on the flow and particle characteristics, be either smaller or larger than the diffusivity of the fluid, as has been proposed by several authors in recent years. An important result from the visualization of the particle trajectories was the identification of organized structures, or “particle vortices”. These appear as a consequence of centrifugal inertial effects acting on the large particles which are responsible for concentrating them in the outer regions of the large coherent eddy structures while keeping the center practically clear. The same phenomenon has also been discussed for several free shear flow configurations by Crowe (Proceedings of the 11th Annual Meeting of the American Association for Aerosol Research (AAAR), Plenary Lecture, San Francisco, 1992), Tang et al. (Phys. Fluids A 4 (1992) 2244) and Yang, Crowe, Chung, and Troutt (Int. J. Multiphase Flow 26 (2000) 1583). Most notably, it was predicted numerically by Chein and Chung (Chemical Engineering Science 43 (1988) 1621) for particles in the near wake of the same two-dimensional flat plate configuration studied here.

Typical air movements in many indoor living and working places are as low as, or less than, 0.1 m / s , and so conditions might best be described in terms of calm, as opposed to moving air—or perhaps some combination of the two. However, the history of aerosol sampling science has been dominated by studies of the aspiration efficiencies of sampling devices in moving air, and there have been very few definitive studies of calm air sampling, even for very simple sampling systems. With this in mind, this paper sets out the basis for the development of a larger body of experimental work, and describes an experimental method by which unambiguous measurements may be made for a range of simple aerosol sampling systems (i.e., shape and orientation). The method, referred to as the “direct method”, owes much to the ‘thread-of-droplets’ approach proposed by Lipatov et al. (J. Aerosol Sci. 17 (1986) 763) for studies of aspiration efficiency in moving air. In the new method, monodisperse large droplets of olive oil with aerodynamic diameter in the range from 40 to 70 μm , were generated using a vibrating orifice aerosol generator and the experimental conditions were controlled very carefully so that a well-defined, vertically-falling droplet stream was obtained. By the application of appropriate tracking apparatus, visual observation of this droplet stream was used to map the pattern of particle trajectories near a given sampling inlet and to identify limiting particle trajectories in order to calculate aspiration efficiency. In an initial application of the new method, measurements of aspiration efficiency were carried out for upwards-facing thin-walled sampling tubes, and the results were found to be in satisfying agreement with the numerical results of Agarwal and Liu (1980, in the Am. Ind. Hyg. Assoc. J. 41 (1980) 191) and the numerical and experimental results of Yoshida et al. (Kagaku Kogaku Ronbunshu 4 (1978) 123). The results of this initial study provide confidence that the method described may now be applied to a wider range of sampling systems.

An understanding of the scaling laws governing aerosol sampler performance leads to new options for testing aerosol samplers at small scale in a small laboratory wind tunnel. Two methods are described in this paper. The first involves an extension of what is referred to as the “conventional” approach, in which scaled aerosol sampler systems are tested in a small wind tunnel while exposed to relatively monodisperse aerosols. Such aerosols are collected by test and reference samplers respectively and assessed gravimetrically. The new studies were carried out for a modified, low flowrate version of the IOM personal inhalable aerosol sampler. It was shown that such experiments can be carried out with a very high level of repeatability, and this supported the general validity of the aerosol sampler scaling laws. The second method involves a novel testing system and protocol for evaluating the performances of aerosol samplers. Here, scaled aerosol samplers of interest are exposed to polydisperse aerosols, again in a small wind tunnel. In this instance, the sampled particles are counted and sized using a direct-reading aerodynamic particle sizer (the APS). A prototype automated aerosol sampler testing system based on this approach was built and evaluated in preliminary experiments to determine the performance of another modified version of the IOM personal inhalable aerosol sampler. The design of the new test system accounts for the complex fluid mechanical coupling that occurs near the sampler inlet involving the transition between the external flow outside the sampler and the internal airflow inside the sampler, leading in turn to uncontrolled particle losses. The problem was overcome by the insertion of porous plastic foam plugs, where the penetration characteristics are well understood, into the entries of both the test and the reference samplers. Preliminary experiments with this new system also supported the general validity of the aerosol sampler scaling laws. In addition, they demonstrated high potential that this approach may be applied in a standardised aerosol testing method and protocol.

The Respicon has been introduced as a sampler for health related measurements of airborne contaminants at workplaces. The instrument is aimed at simultaneous collection of three health related aerosol fractions: (a) the coarser inhalable fraction, defining the aerosol fraction that may enter the nose and mouth during breathing; (b) the intermediate thoracic fraction, defining the fraction that may penetrate beyond the larynx and so reach the lung; and (c) the finer respirable fraction, defining the fraction that may penetrate to gas exchange region of the lung. The instrument has a number of features attractive to occupational hygienists: in addition to providing the three aerosol fractions simultaneously, it is light and compact enough to be used as a personal sampler. yet can be a tripod mounted for area sampling, it can provide samples not only for gravimetric analysis but also microscopic and chemical analyses; and it is also available in a photometric direct-reading version. The instrument has previously been evaluated as an area sampler and, in this mode of operation, has shown reasonable accuracy in collecting respirable, thoracic and inhalable particles, the latter up to particle diameters of ca. 80 microm. Except for some scattered unpublished data there exist no systematic investigations in the Respicon's performance when used as a personal sampler in the industrial environment. In this paper, we will report on a study of side by side comparison of the Respicon with the IOM inhalable sampler, regarded as a reference instrument for the inhalable fraction. The main study was performed at six different workplaces in a nickel refinery. Statistical analysis of the gravimetrically-determined concentration data reveals consistently lower aerosol exposure values for the Respicon as compared to the IOM sampler. The data for the nickel workplaces are compared with findings from other studies. The results are interpreted in the light of the overall results and the possibility of introducing a correction factor is discussed.

A field study was conducted at a range of worksites at a number of plants, for two companies, in the primary nickel production industry. The aim of the field study was to provide direct measurements of the distributions of both particle size and relevant nickel species groups (soluble, sulfidic, metallic and oxidic), and to use the data to characterize health-relevant aerosol exposures of workers at the worksites in question. Since there was no commercially-available sampler that could provide samples of sufficient quantity to enable chemical speciation for the species fractions of interest, as well as particle size distribution information, new instrumentation was developed, based on a modified version of the Andersen cascade impactor, incorporating a porous foam media top stage that produced particle classification over the upper end of the inhalable range (Kerr, Vincent and Ramachandran, Annals of Occupational Hygiene, 2001, in the press). This modified-Andersen sampler was used in extensive field studies, along with other sampling instruments, including, in particular, the IOM personal inhalable aerosol sampler. The results of the field study provided results that could be represented succinctly in terms of the distributions of the four nickel species groups and the three health-related particle-size fractions: inhalable, thoracic and respirable. They showed that, for practical purposes, the distributions of the four nickel species groups were consistently uniform across the full range of particle-size distribution. For the purpose of characterizing the distribution of nickel species for each worksite, this finding enabled incorporation of additional data taken at those same sites using the IOM personal inhalable aerosol sampler. This yielded a data set sufficient to permit the development of exposure ‘fingerprints’ for each worksite. Such ‘fingerprints’ make it possible for the occupational hygienists at the plants in question to estimate workers' exposure to individual nickel species fractions based on measurement only of overall inhalable nickel. In addition, by reference to the differing threshold limit values (TLV) for the various nickel species, the ‘fingerprints’ also allow the individual worksites to be characterized by hazard indices (in the form of ‘equivalent sulfidic fractions’, or ESFs) that reflect the weightings of the risks experienced by workers at each worksite.

A commercially-available, high-volume (28.3 Lpm) Andersen-type cascade impactor was modified in order to extend its operational range further into the range of large inhalable particles for purposes of use in an exposure assessment study in the primary nickel production industry. The modification involved incorporating a 10-ppi (pores per inch) porous plastic foam top stage that has a wide penetration curve with a 50 d ae -value of approximately 27 μm. This enabled the upper end of the range of the instrument to be extended from about 10 to greater than 70 μm. The inlet of the original instrument was also modified to incorporate the new top stage and provide ‘representative’ aspiration of total airborne particulate with an efficiency of close to 100% over the range of aerodynamic particle sizes of interest. A mathematical inversion algorithm developed in earlier research was modified so that it could be applied to this new instrument. This enabled raw data on particulate material recovered from all stages of the instrument (including the new porous foam top stage) to be used for the determination of continuous particle size distributions, as well as chemical speciation, over the inhalable range. The new instrument was deployed in a pilot field study in the nickel primary production industry by which to demonstrate the potential of the new instrument for generating useful information pertaining to health-relevant aerosol size tractions (e.g., inhalable, thoracic, and respirable), and for soluble, sulphidic, metallic, oxidic and total nickel chemical species groups.

This article summarizes the results of an extensive experimental study of sampling losses in thin-walled probes at various values of velocity ratio R and the probe orientation with respect to the freestream. The purpose of this study was to gain insights into the complex interaction of various parameters that influence sampling losses and the consequent effect on the overall sampling efficiency. A 0.635 cm diameter sharp-edged tube was mounted in a small wind tunnel where the freestream velocity could be varied over a wide range of values. Polydispersed spherical glass beads were used as the test aerosol. The number concentration and the particle size distribution were measured using the aerodynamic particle sizer (APS 3310). The sampling efficiency was determined as a function of orientation for a range of particle sizes (or Stokes number). By using an existing model to predict the aspiration efficiency for thin-walled probes, the sampling losses could be isolated from the sampling efficiency. In this ma...

Inhaled ultrafine particles are increasingly being recognized as a potential threat to health. Aerosols in workplace environments may come from a wide variety of sources, depending on the type of activity and processes taking place. Some activities and processes are acknowledged as being ‘dusty’, where aerosol is generated from the mechanical handling and attrition of solid or liquid material, and are not considered to be plausible sources of ultrafine particles. However, hot processes, involving the vaporization of material, and inevitable subsequent cooling, do have the potential to generate significant number concentrations of ultrafine particles. However, consideration of the physical conditions required for the generation of particles in the range below 100 nm suggests that those conditions are not easily met in workplaces. More generally, the conditions are such that particles grow out of this range, either by continuing condensation (as happens at high vapour concentrations) or by agglomeration between smaller particles (as happens at high number concentrations). Not much is known about ultrafine particles in actual workplaces, mainly because our view has been obscured for the past few decades by the fact that most occupational aerosol standards have been based on the mass concentration of airborne particulate matter. Now that a new awareness has set in, it is expected that new research will address the problem. Most current aerosol standards are expressed in terms of the mass concentration of particulate matter conforming to a particle size fraction, where the latter is based on knowledge of how particle size relates to where particles deposit in the human respiratory tract and any subsequent effects. At present no such basis exists for ultrafine particles, but one is needed before progress can be achieved towards meaningful standards for occupational ultrafine aerosols. It is expected that, for ultrafine particles, such a standard may, in the future, be expressed in terms of the number concentration of particles less than a certain size, that size to be determined on the basis of the physical and chemical nature of the particle at that size, human physiology and toxicology.

In this paper a mathematical approach is adopted to model the flow of air and airborne particles into an idealised spherical aerosol sampler whose single circular inlet is oriented at an angle α to the undisturbed freestream. From detailed knowledge of the airflow, the paths of suspended particles in the flow were calculated and the aspiration efficiency of the sampler determined. In tracing the particle paths both inertial and gravitational forces were considered, and values of α up to 60° were investigated. The results were compared with data obtained for the same sampling system in wind tunnel studies, and were generally in good agreement. It was found that the common practise of neglecting gravity when predicting aspiration is only reasonable in certain situations, for example, for samplers facing the airflow the effects of gravity upon aspiration were found to be negligible even at low wind speeds of O(10 −1 ) m s −1 . However, as the sampler orientation to the airflow increases above zero this assumption was found to lead to increasing errors. In the case of an orientation of 60° to the airflow errors in aspiration efficiency greater than 10% were found to occur even in wind speeds as large as 6 m s −1 for the samplers considered.

This paper describes the effects of intermediate and large freestream levels of turbulence (intensity and integral length scale) on the dispersion of fine particles in the near wake regions of long flat plates placed normal to the freestream. The work is particularly relevant to the transport of airborne contaminants in complex flows, especially like those encountered in many ambient and occupational atmospheric environments. An experiment was designed based on the release of fine aerosol particles upstream of the obstacle and the measurement of the concentration decay for the entrained aerosol in the near wake region after the source generation has been suddenly interrupted. Five grids were employed for generating specific levels of freestream turbulence characterized by the parameter, Λ f (defined as l f k 1/2 f /UD , where l f and k f are the length scale and the kinetic energy of the free stream turbulence, and U and D are the freestream velocity and the plate width, respectively). The investigation covered the ranges 1500⩽ Re ⩽11,000 and 0.0015⩽ Λ f ⩽0.071. The dimensionless residence time, H (defined as τU/D , where τ is the time constant for the observed decay of the aerosol concentration in the near wake cavity), appeared to be independent of the freestream turbulence for values of Λ f between 0.0015 and 0.013, for which the average H -value was about 7.2±1.5. For larger freestream turbulence, corresponding to Λ f greater than 0.018, H increased up to about 10. A visualization procedure was used to capture some of the qualitative features of the dispersion process for aerosols in the near wake regions of the plates and provide some support for the physical mechanisms suggested by this work. In particular, by comparing the results with those of other studies, we note the marked contrast in the effects of the freestream turbulence for aerosol flows about two- and three-dimensional bodies, respectively.

A variety of health effects are caused by chronic, cumulative exposure over time to pollutants. In these cases, to establish dose-response relationships for epidemiological and risk assessment purposes, it is vital to determine the exposures of individuals or cohorts as functions of time. Most existing occupational exposure databases, however, do not contain continuous records of historical exposures to airborne contaminants. These gaps in the historical record may be filled by using the knowledge base that experts and professionals in the field possess. In this article we present a new framework, based on Bayesian probabilistic reasoning, for obtaining estimates of exposure histories for airborne particulates from limited historical measurements, using subjective expert judgment. The framework has great potential applications in instances where there is sparse information or missing data on past exposures. Expert judgment, in the form of inputs to physical models, provides additional knowledge to retrospectively estimate exposure as a function of time from discrete and incomplete measurements. The expert judgments are informed by knowledge of historical plant conditions and work practices, and models describing process-dependent aerosol generation, ventilation, and worker activity patterns. The result will be probability distributions of the exposure of task-groups of workers as a function of time, in the form of a matrix.

An investigation of the relationship between observed nickel aerosol exposures and urinary nickel excretion was undertaken at a Scandinavian nickel refinery. The goal of the study was to assess the impact of nickel aerosol speciation, the use of particle size-selective sampling instrumentation and adjustment of urinary levels for creatinine excretion on the usefulness of urinary nickel excretion as a marker for exposure. Urinary nickel measurements and paired 'total' and inhalable aerosol exposure measurements were collected each day for one week from refinery workers in four process areas. The mean observed urinary nickel concentration was 12 micrograms L-1 (11 micrograms of Ni per g of creatinine). The strongest relationships between urinary excretion and aerosol exposure were found when urinary nickel levels were adjusted for creatinine excretion and when exposure to only soluble forms of nickel aerosol was considered. No significant difference was observed between measures of 'total' and inhalable aerosol in the ability to predict urinary excretion patterns. In the light of these results, it is recommended that consideration be given to the chemical species distribution of nickel aerosol in the use of urinary nickel measurements as a screening tool for cancer risk in occupationally-exposed populations.

The study of the basic physical performance characteristics of aerosol samplers, like those used in the occupational hygiene setting, will provide insights to enable the improved development of new instruments and cost-effective testing procedures. These will be required as the new particle size-selective sampling criteria become the basis of new occupational exposure standards. A new body of work is being conducted, in which the factors influencing sampler performance are being investigated using idealized samplers of spherical shape in small wind tunnels. By the experimental methods described, a large amount of performance data can be acquired in a very short time. The results for wide ranges of particle size, wind speed, sampling flow rate, and sampler orientation conditions show that there are strong trends as functions of these variables. Those trends are very complicated. But it is encouraging that they are broadly consistent with recent semi-empirical models, suggesting that extensions of that type of modelling approach--supported by the large amount of new experimental data now being generated by such experiments--might provide new models of aerosol sampler performance accessible to researchers and occupational hygienists.

This paper is the result of an investigation carried out in February 1997, which set out to learn about the development and application of occupational health standards, and in turn approaches to occupational environmental monitoring, in Russia, and to see how these were similar to, or different from, what takes place in other countries. The enquiry involved face-to-face meetings with senior Russian occupational health scientists and officials. It was confirmed that Russian occupational exposure standards are very stringent and provide a very high level of protection of workers. However, they are difficult to enforce, and it is suggested that the development of a strong and distinctive occupational hygiene discipline and profession in Russia would provide a "bridge" to enable more effective implementation and interpretation of those standards.

This article is a report of work carried out during visits to Australia, Britain, Norway, and Russia for the purpose of becoming acquainted with the various processes of setting occupational exposure standards in those countries. The article reviews the processes by which occupational exposure standards are developed from country to country, examines the role of the occupational hygiene discipline in their development and implementation, and reflects on the complex philosophical and practical issues that surround them and the prospects for international harmonization.

The research presented in this article assessed geochemical factors relating to dust produced during primary lead smelting. Bulk dust samples and size-selective airborne dust samples were collected...

In preparation for filter efficiency tests and sampler comparison studies, methods of biological aerosol generation, sampling, and filter recovery were modified from previous studies. Methods described include (1) techniques for generating aerosols that reduced nuisance particles to negligible levels and increased the cell culturability of Mycobacterium abscessus by 30%, (2) sampling techniques that lowered the detectable range of biological particle size from 0.65 to 0.45 micron and reduced the sampling flow from the chamber from 28.3 to 1.5 L/min, and (3) development of methods to remove culturable organisms from respirator filter media. These methods were developed for filter challenge tests with M. abscessus and were applied to two other bacteria. They may also have application to a wider variety of organisms and bioaerosol assessments.

Occupational hygiene has always been very influential in aerosol science—and vice versa. This paper gives an historical overview of this interaction, in particular how aerosol measurement instrumentation has evolved for the measurement of workers' exposures to aerosols in the occupational setting. It shows how health-related criteria for aerosol measurement have shifted from ones based on airborne particulate mass to ones based on particle count concentration, and then back again, depending on the aerosol science knowledge that was available at the time. It also draws the distinction between instrumentation based on time weighted-average sampling and direct-reading measurement, and the factors that govern how the choice of type of measuring instrument was made in the past, the way it is made now, and the way it might be made in the future. *Paper based on presentation made by Professor Vincent at the 1996 Meeting of the American Association for Aerosol Research (AAAR) in Orlando, FL, in the speci...

The research described in this paper was stimulated by the need to understand better the nature of air flow around aerosol samplers of the type widely used in environmental and industrial hygiene. It deals with the application of visualisation techniques to determine the location of stagnation points for air flow about simple two- and three-dimensional bluff bodies (cylinder and sphere, respectively) for the case where there is aspiration of air (i.e. suction) from a point on the body surface. The effect of orientation of the sampling orifice (or sink) with respect to the free stream on the location of stagnation points was compared with theoretical predictions using potential flow models. Good agreement was obtained, even for large angles with respect to the wind. For the two-dimensional cylindrical body, we also experimentally investigated the frequency of the vortex shedding in its near wake and how that is influenced by the aspiration. As represented by the dimensionless Strouhal number, this was found to be strongly dependent on the aspiration flow rate and the slot orientation. The results may be explained qualitatively in terms of the effect of aspiration on the development of the boundary layer over the cylinder surface.

Personal cascade impactors are widely used in occupational aerosol exposure assessment. Appropriate algorithms must be used to determine the total particle size distribution from masses collected on the stages of the cascade impactor. Such algorithms should be regarded as integral components of the measurement system. When evaluating algorithms for reconstruction of size distributions from cascade impactor data, the eventual use of the size distribution must be considered. So, from an industrial hygiene perspective, an appropriate basis for comparison of given measurement systems is the accurate retrieval of the inhalable, thoracic, and respirable aerosol fractions as described by the new, internationally accepted, particle size-selective sampling conventions (which are expected to form the basis of future aerosol standards). This article compares two inversion routines in terms of their abilities to retrieve these aerosol mass fractions relative to the masses that would have been obtained using an ideal sampler that perfectly followed the sampling convention. The routines were used to invert measurements made with the Institute of Occupational Medicine personal inhalable dust spectrometer, a miniature cascade impactor that aspirates the inhalable aerosol fraction, and the results are presented graphically as contours of equal mass bias. The simplest algorithm, based on the a priori assumption of lognormality, appears to provide the best results.

The dispersion of inertialess particles in the recirculation zone immediately behind long, two-dimensional flat plates positioned perpendicular to an aerosol-laden freestream was investigated both experimentally and numerically. The physical situation corresponds to the release of particles from a two-dimensional line source positioned upstream of the flat and consequent entrainment of those particles into the near wake region. After the sudden interruption of the aerosol generation process, the particles persist in the near wake, with the number concentration decaying exponentially with time. The experiments were carried out in a low-turbulence wind tunnel. An optical detection system was developed for the fast observation and recording of changes in the particle concentration in the near wake of the plate. The dimensionless residence time for fine inertialess smoke particles in the near wake region H(= T U/D, where T is the time constant for the exponential concentration decay, U is the undisturbed freestream air velocity, and D is the width of the obstruction) was measured in the range of Reynolds numbers (Re) from about 1000 to 10,000. H was found to be approximately constant at about 9.5 for Re above about 4000. These results were in agreement with a simple model and with numerical calculations based on a two-equation, steady-flow, k - e approach with coefficientschosen to reflect to some degree the actual physical situation which also includes vortex shedding.

Several recent studies have compared worker personal aerosol exposures as measured by the current method with those obtained by a new approach based on collecting the inhalable fraction, intended to represent all the particles that are capable of entering through the nose and/or mouth during breathing. The present study investigated this relationship for a metal machining facility where aerosols were generated from severely refined, nonaqueous ("straight") cutting oils used during the lathe working of metal rod stock. Workers (n = 23) wore two personal aerosol samplers simulataneously, one of the 37-mm type (for "total" aerosol exposure, E37) and the other of the Institute of Occupational Medicine (IOM) type (for inhalable aerosol exposure, EIOM). The data were analyzed by weighted least squares linear regression to determine the coefficient S in the relation EIOM = S.E37. It was found that S = 2.96 +/- 0.60. This ratio-in which exposure to inhalable aerosol was greater than to "total" aerosol-is consistent with previous observations in other industries. The relative coarsenss of the oil mist aerosol, as estimated by cascade impactor measurements, probably explains the difference between the sampling methods. The collection of large "splash" droplets, may also contribute. Future occupational aerosol standards for metalworking fluids will be based on the new, health-related criteria, and exposures will be assessed on the basis of the inhalable fraction. Results of studies like that described here will enable assessment of the impact on future workplace aerosol exposure assessments of introducing new standards.

We examined two inversion procedures for solving the Fredholm integral equation of the first kind to obtain aerosol particle size distributions from a set of measured masses collected on the various stages of a personal cascade impactor. The problem is essentially ill-conditioned, in that many solutions satisfy exactly an integral equation slightly perturbed from the original due to measurement error. The two methods, although derived from different families of inversion techniques, fit into the general framework of Tikhonov regularization. Both try to optimize the a posteriori degree of matching of the solution to the measured data and the a priori judgments about the likelihood of a solution in terms of its smoothness. The first method uses a weighted least squares optimization and zeroth-order regularization to fit a priori bi-modal log-normal distribution functions, using an intermediate step to define an appropriate starting point for the optimization routine. The second involved “blind” inversion of the impactor data to express the second derivative of the particle size distribution function as a linear combination of orthogonal basis functions, chosen so that the resulting solution is smooth and positive. The orthogonal functions are constructed from the eigenvectors and eigenvalues of a kernel covariance matrix. The personal inhalable dust spectrometer (PIDS), used to illustrate the application of these methods, is an eight-stage cascade impactor which selects the inhalable fraction of the aerosol by means of a specially designed inlet. Both inversion methods explicitly include consideration of the aerosol that is collected in the sampler entry between the inlet and the first impactor stage, something that applies to all cascade impactors but which has not usually been taken into account in the past. An important parameter in inversions, the expected value of measurement error for each stage, was estimated from a series of wind-tunnel experiments. Both methods work well for simulated PIDS data as well as for experimental wind-tunnel data for a wide range of sets of aerosol size distribution parameters.

This paper describes a study to determine the most appropriate particle size parameter for polydisperse powders dispersed as aerosol and detected by light scattering. It is relevant to many areas of engineering research. By means of calculations of the intensity of light scattered from particles of different sizes, it has been shown that the surface median aerodynamic diameter (SMAD) is an appropriate measure for polydisperse aerosolised powders. The microscope-based procedure for obtaining the SMAD is described, as well as its implementation for narrowly-graded polydisperse powders of fused alumina of the type which have been widely used in studies of particle transport in wind tunnels. It was shown that, for the fused alumina powders examined, SMAD=1.25x(PASMAD), where PASMAD is the projected area surface median aerodynamic diameter.

Experiments were carried out to investigate the effects of probe electrification on the efficiency of the sampling of particles from air moving at relatively high wind speeds ranging from about 10 to 30 m s −1 . The aim was to gain physical insight which might be extended to the case of the sampling of atmospheric aerosol from aircrafts. Polydisperse NaCI aerosol particles with number median diameter of 0.05 μm were sampled using a metallic thin-walled probe aligned with the air stream. Penetration of aspirated particles through tubing of different materials was measured when a potential of −30 kV DC was applied to the probe. Penetration was found to increase from 0.16 to 0.87 as the external air speed increased, and the results were interpreted as indicating that sampling efficiency was influenced by the electrification of the probe itself. Experiments were also performed with nearly monodisperse liquid DOP aerosols with number median diameters from 0.05 to 0.5 μm and the results were used to examine the charge transfer of ions (from the corona discharge which occurred at the sharp-edged probe tip) to the aerosol particles. In this part of the study, negative and positive corona discharges around the sampling inlet were created by applying potentials of up to ± 20 kV to the probe. It was found that, for given potential, the negative corona was much more effective in charging the aerosol, probably due to the presence of unattached free electrons. Such charge transfer process may be attributed to combined field and diffusion charging mechanisms.

The dispersion of particles in the near wake of a flat disc placed perpendicular to a turbulent free stream was studied by optically measuring the dimensionless near-wake residence time of particles, H ( = t r U D , where t r is the time constant of the decay of the near-wake concentration following removal of the source, U the free-stream air velocity, and D the disc diameter). The dependence of H on the Stokes number (St, expressed in terms of U and D ) was investigated for St up to about 8. The experiments were conducted for aerosols which were generated from narrowly graded fused alumina dust and glass beads, whose surface median aerodynamic diameter (SMAD) ranged from 13 to 96 μm, introduced directly into the near-wake region at the base of the disc. The results showed H decreasing steadily and uniquely with increasing St, leveling off for St > 1. This behavior is well explained by simple physical models which take account of the centrifugal forces exerted on a particle during its motion inside the recirculating body of fluid enclosed in the near-wake region, supplementing the turbulent exchange dispersion processes which occur for particles of all sizes.

This article describes a study that was carried out at two electroplating shops to investigate the levels of personal exposure to inhalable nickel-containing aerosols and the impact of introducing new personal sampling instrumentation with performance consistent with the latest criteria proposed by the International Standards Organisation, the Comite Europeen Normalisation, and the American Conference of Governmental Industrial Hygienists (ACGIH). Experiments were carried out to investigate the effect of changing from the current widely used method for total aerosol (using the closed-face, 37-mm filter holder) to a new method for inhalable aerosol (using the IOM inhalable aerosol sampler). The results showed that measured inhalable aerosol exposures—for both overall aerosol and total nickel—were consistently and significantly higher than the corresponding measured total aerosol exposures. Weighted least-squares linear regression yielded biases for one company ranging from about 1.3 to 2.5 and for...

This paper extends what it known about the aspiration efficiencies of blunt aerosol samplers at large angles and under 'orientation-averaged' conditions to the important case of personal samplers worn by workers. Attention is focused on three sampling devices which have particular relevance to industrial hygiene sampling in Britain, the United States and elsewhere. A semi-empirical model for aspiration efficiency is described, based on physical considerations of the nature of air and particle motion near an asymmetric sampling system, like that relevant to personal sampling. The resultant model contains a number of coefficients which are fitted by non-linear regression to experimental data sets which were available from previously published experiments for the three samplers in question. Agreement for all three data sets is good, with an overall Rcorr2 of 68%. Such semi-empirical models can be useful for predicting and interpreting sampler performance until practical versions of more rigorous mathematical and numerical models become available.

OBJECTIVE To assess the ability of a protective isolation room ventilation system to reduce patient exposure to airborne infectious agents, using a small-scale model that permits cost-effective and unobtrusive study of relevant indices of performance. DESIGN A one-half scale model of a protective isolation room at the University of Minnesota Hospital was constructed and equipped for tracer gas experiments to assess ventilation efficiency. MEASUREMENTS Tracer gas (SF6) was injected into the model supply air. Tracer gas concentration was recorded over time and analyzed to determine local and room mean age of air. Age of air is a direct measurement of ventilation efficiency and can be used to predict patient exposure to contamination. RESULTS Although for the room taken as a whole, ventilation efficiency was close to 50% (a value corresponding to perfect mixing), the experimental results for the local mean age of air indicate that some parts of the model were ventilated much better than others. CONCLUSION Room air exchange rate is only one parameter useful in assessing ventilation in patient areas. Effective distribution of ventilation air also is critical to the control of airborne contamination. Areas of the room with poor ventilation would be expected to have higher concentrations of airborne infectious agents and other contaminants. Patient exposure can be minimized by placing the patient in well-ventilated areas of the room. Improved ventilation designs may reduce patient exposure further without increasing actual airflow rate.

This paper applies what is already known about the aspiration efficiency of blunt aerosol samplers at large angles to the interesting case of sampling under «orientation-averaged» conditions. This is particularly relevant to practical aerosol sampling in workplaces and the atmospheric environment. The resultant semiempirical model of aspiration efficiency contains a number of coefficients which are fitted by nonlinear regression to data sets for the human head and for two rotating-head inhalable aerosol samplers (a 3 L/min sampler intended for applications in workplaces and a 70 L/min sampler intended for particle sampling in the ambient atmosphere). Agreement for all three data sets (with 88, 64, and 37 records respectively) is generally good, with an overall R corr 2 of 68%. Such semiempirical models can be useful for predicting and interpreting sampler performance until practical versions of more rigorous mathematical and numerical models become available

Recently, the protection of health care workers from tuberculosis-containing aerosols has been the subject of considerable debate. An experimental apparatus and test protocol were developed to measure the collection efficiency of surgical mask and respirator filter media using a microbial aerosol challenge. Mycobacterium chelonae (M. chelonae) , used as a surrogate for Mycobacterium tuberculosis , was generated from liquid suspension using a Collison nebulizer. Upstream and downstream concentrations of viable aerosol particles were measured using Andersen cascade impactors, while total particle concentrations were measured with an aerodynamic particle sizer (APS). A monodisperse polystyrene latex (PSL) sphere aerosol (0.804 μm) was used in separate experiments to measure filter efficiency; concentrations were determined with the APS. The mycobacterial aerosol ranged in size from 0.65 to 2.2 μm when measured with the cascade impactor. A similar size range was found with the APS, yielding a count median diameter of about 0.8 μm. Samples of the mycobacterial aerosol were collected on glass slides, stained M. chelonae , as determined by environmental scanning electron microscope, were found to be rod shaped with an average length of 2 μm and average width of 0.3 μm. To evaluate the apparatus over a range of filter efficiencies (10–100%), different layers of fiberglass filter paper were tested for penetration using a 0.12 μm dioctyl phthalate (DOP) aerosol measured with a light scattering photometer, in addition to the mycobacterial and PSL aerosols. For the range of efficiencies tested it was shown that filter collection of DOP was linearly related to that of both mycobacterial and PSL sphere aerosols ( r 2 = 0.99), demonstrating that an inert aerosol may be used to predict the collection of biological aerosols by such filter media.

The assessment of personal inhalable aerosol samplers in a controlled laboratory setting has not previously been carried out at the ultra-low wind speed conditions that represent most modern workplaces. There is currently some concern about whether the existing inhalable aerosol convention is appropriate at these low wind speeds and an alternative has been suggested. It was therefore important to assess the performance of the most common personal samplers used to collect the inhalable aerosol fraction, especially those that were designed to match the original curve. The experimental set-up involved use of a hybrid ultra-low speed wind tunnel/calm air chamber and a rotating, heating breathing mannequin to measure the inhalable fraction of aerosol exposure. The samplers that were tested included the Institute of Occupational Medicine (IOM), Button, and GSP inhalable samplers as well as the closed-face cassette sampler that has been (and still is) widely used by occupational hygienists in many countries. The results showed that, down to ∼0.2 m s(-1), the samplers matched the current inhalability criterion relatively well but were significantly greater than this at the lowest wind speed tested. Overall, there was a significant effect of wind speed on sampling efficiency, with lower wind speeds clearly associated with an increase in sampling efficiency.

Objectives: To provide targets for personal samplers designed for estimating particle deposition at distinct locations in the body, accounting if necessary for inter- and intra-person variation. Proposal:Ultrafineandfine aerosolsamplingconventionsareproposedforapproximatingthe depositionefficiencyforfivedistinctlocioftherespiratorytract.The2 35 510conventionsrepresent averages over variation in physical activity level, posture, sex, and breathing mode. Recognizing three approximate relationships among the 10 deposition efficiencies, the number of independent conventions is reduced to only seven, namely three ultrafine and four fine aerosol conventions.Theultrafineandfineconventionsaredefinedasidealsamplingefficienciesinterms of thermodynamic (independent of particle density) and aerodynamic diameter, respectively. Application: Addition of measured mass, surface area, or particle count from aerosol collected by sampler pairs in agreement with convention can be used to estimate dose (prior to clearance) at a particular locus in the mean over breathing conditions ranging from sitting, to light, or to heavy exercise, normal or mouth breathing, and male or female, with aerosol density effects partially corrected automatically by the separate aerodynamic and thermodynamic sampling. Linear combinations of the conventions can be used for yet simpler sampling, though with limited distinctness as to deposition locus. Alternatively to estimating simply a mean, the large inter- and intra-person variation (relative standard deviation of the order of 100%) corresponding to the wide range of breathing conditions can be approximately corrected by using ‘arrays’ of samplers in agreement with convention, given suitable profiling of the individual whose dose is to be assessed. Outcome: The intent behind the proposed conventions is not to eliminate the current aerosol penetration conventions, which have found international application in exposure assessment particularly for determining compliance with standards on acceptable aerosol levels. The aim is to promote personal sampler design leading to more sharply defined health research than can be done at present.

The current convention for sampling inhalable aerosols was based on several mannequin studies performed in wind tunnels at wind speeds between 0.5 and 4 m s(-1). In reality, as we now know, the wind speed in most modern indoor working environments is generally at or below ∼0.2 m s(-1). Inhalability studies performed in calm air aerosol chambers have shown that human aspiration efficiency at essentially zero wind speed is not consistent with the existing inhalable aerosol convention, calling into question the universal applicability of the current standard. More recently, experiments were carried out in a new hybrid wind tunnel-calm air chamber at more representative workplace wind speeds, between ∼0.1 and 0.5 m s(-1), to fill in this knowledge gap. Comparing these new data to both the existing inhalable aerosol convention and a recently proposed alternative for low wind movement suggests that, while the existing inhalable aerosol convention remains appropriate for wind speeds above ∼0.2 m s(-1), the modified version is more appropriate for the range below ∼0.2 m s(-1).