Ann. occup. Hyg., Vol. 47, No. 6, pp. 437-440, 2003
© 2003 British Occupational Hygiene Society
Published by Oxford University Press
Commentary: Scientific Principles and Pragmatic Solutions for the Measurement of Exposure to Inhalable Dust
Health and Safety Laboratory, Broad Lane, Sheffield S7 1LB
Received 13 March 2003; in final form 14 March 2003
 |
ABSTRACT |
|---|
 TOP ABSTRACT REFERENCES |
|---|
In order to understand and control the risks presented by inhalation of dust at work, research over many years has been focused on understanding how dust present in the air enters the human nose and mouth during the act of breathing. For health-related dust exposure monitoring, sampling devices are needed that collect the same ‘inhalable fraction’ of dust as the human head. Mark and Vincent’s 1986 paper presented a study that has contributed more than any other to the practical realization of this inhalable dust concept. The authors developed a simple solution—the IOM personal inhalable dust sampler—to what we now know is an extremely complex problem. Although scientific understanding has grown in the years since this paper was published, very few other dust sampling instruments have emerged as being able to meet both the scientific criteria and the practical need for inhalable dust measurement. Both authors have continued to build on this work and have made further contributions to our theoretical and practical understanding in this field.
Keywords: dust sampling; inhalable dust
The 1975 Inhaled Particles Symposium included several papers contrasting the ‘total dust’ monitoring practices of different countries. The conference highlighted the major inconsistencies in methodology at that time, which made it very difficult to carry out reliable epidemiological studies using dust exposure data. The conference provoked a discussion that has since proved to be an important milestone in occupational hygiene. It concluded that the application of scientific principles (rather than national preferences) to dust at work required a completely new approach to dust monitoring. Research effort was needed to understand how the dust present in the air entered the human nose and mouth during the act of breathing, and to develop dust monitoring devices that collected the same fraction of the airborne dust as the head itself. Hence, the concept of ‘inhalable dust’ was born. The history of this concept, and how it made its way into scientific research, standards, regulations and occupational hygiene practice, has since been reviewed in detail in a number of articles and books (Vincent, 1995; ACGIH, 1998; Kenny and Ogden, 2000).
The classic paper by Mark and Vincent (1986) presents the scientific study that has contributed more than any other to the practical realization of the inhalable dust concept. The paper proposes the deceptively simple idea that ‘for the personal sampling of "total" dust, the inspirability criterion is upheld when a personal sampler mounted on the body gives the same measured dust concentration and aerodynamic size distribution as that inspired by its wearer, regardless of dust source location and wind conditions’. Note that modern terminology has replaced the term ‘inspirable’ with ‘inhalable’. The paper describes a purely empirical approach to solving this problem. A life-sized manikin was fitted with a mouth orifice behind which the ‘inhaled dust’ could be collected on a filter. Human breathing was simulated using a sinusoidal breathing machine, and all the dust entering the mouth orifice was collected. At the same time, dust was collected through a personal sampling orifice mounted on the upper torso of the manikin. The concentrations at mouth and chest were compared directly by scaling the weights of dust collected by the volumes sampled in each case. Concentration ratios were plotted as a function of particle size and external wind speed. Mark and Vincent tested three commercially available dust samplers to highlight the deficiencies in methodology at that time. They designed a new sampler by trying out three different orifice sizes, before settling on the one that forms the basis for the now widely used IOM personal inhalable dust sampler: a 15 mm orifice aspirated at 2 l/min.
The approach described by Mark and Vincent involves a great deal of abstraction from reality, but it is doubtful that much progress could have been made in the practical design of inhalable samplers without taking such liberties. In the simulated environment in which testing took place, the external winds were regulated, unidirectional, steady, and the dust evenly distributed in the air. Directional effects were averaged, by repositioning the manikin in various orientations with respect to the wind. Subsequent work in this field has convincingly demonstrated that these ‘environmental’ simplifications had remarkably little practical impact on the outcome. The wind tunnel results of Mark and Vincent were later confirmed by a field study, conducted in a wide range workplaces using a breathing manikin that wore personal samplers on its upper torso (Vaughan and Armbruster et al., 1990). In the field study the manikin was articulated; it could be positioned close to sources of dust in positions similar to those adopted by workers themselves. The manikin breathed in a programmable sinusoidal pattern that could be varied to simulate different work rates. Comparisons of the dust concentration collected using IOM samplers mounted on the chest, to that collected by the mouth orifice, showed that across a wide range of work environments the mean ratio was close to 1. The UK seven-hole personal dust sampler, which was also tested in the study, performed adequately in low-wind indoor work environments. However, outdoors, or in stronger winds, the IOM sampler consistently gave results that were closer to the mouth-inhaled concentration. Hence this field-based study confirmed that the early wind-tunnel results obtained with the IOM sampler could be reproduced in real workplace monitoring scenarios.
This outcome highlights the utility of sometimes setting theoretical problems to one side and adopting a purely pragmatic approach. The occupational and environmental hygiene literature contains many papers describing how the concentration of pollutants changes in space, particularly in the personal space close to people (Flynn and George, 1991; Kim and Flynn, 1991a; Rodes et al., 1991, 1995). Changes in local concentration can arise even when the far-field concentration is uniform, and particularly when local or general ventilation interacts with the human body (Flynn et al., 1999; Kim and Flynn, 1992; Smith and Bird, 2002) In real workplaces the aerosol concentration is never uniform, as particles are generated by localized work activity (Lidén and Kenny, 1994; Kim and Flynn, 1991b). Reviewing this literature, one might reasonably conclude that personal dust sampling is simply an impossible task; the concentration of dust close to the sampling zone of a chest-mounted personal sampler is always likely to be different to the concentration in the breathing zone. If this were true, presumably one should not be able to measure inhalable dust at all, except by perhaps inserting a filter into the mouth or nose of the unfortunate worker (this has been tried in laboratory tests by Hsu and Swift, 1999). However, in practice the aerosol concentration variations do not seem so important: perhaps this is because in most workplaces winds are low, air is turbulent, dust is well mixed and workers move around. When workers go outdoors where winds are stronger, they are rarely exposed for long periods to wind from one direction only (Baldwin and Maynard, 1998). Strong, unidirectional winds are only found in a very few situations such as mines. Fortunately therefore, the practical results from workplace dust sampling appear much better than one might anticipate from theoretical considerations.
Another major simplification in Mark and Vincent’s work is that the manikin breathed in through the mouth only, with a regular sinusoidal pattern, but did not exhale through the mouth orifice because of the problem of scouring dust deposits from the inside of the mouth. In this respect it differs greatly from a real human, who rarely breathes through the mouth alone and always exhales. The manikin in effect acts as a ‘reference sampler’ for inhalable dust, and this has caused problems in getting the concept of inhalability accepted, particularly as the manikin was very poorly specified. In fact, right from the earliest work on the inhalability concept by Ogden and Birkett (1977) much effort was expended on testing whether the concentration inhaled by the manikin mouth was sensitive to details of the design. Over many years, several papers have discussed every aspect that might have an influence on the dust inhaled by the manikin: head shape, features, surface, hair, warmth, breathing rate, tidal volume, steady versus sinusoidal inhalation, nasal versus mouth breathing, and no exhalation versus exhalation (Aitken et al., 1999; Armbruster and Breuer, 1982; Homma and Yakiyama, 1988). The only factors found to have a large effect are the breathing pattern (i.e. oral versus nasal breathing), and to a lesser extent, the tidal volume (Vincent and Armbruster, 1981; Kennedy and Hinds, 2002). However, it is recognized that there is a need both to standardize and simplify the ‘manikin head reference sampler’, and David Mark is currently leading an EU-funded research project with this objective (Mark et al., 2001). The resulting ‘Caltool’ that is under development will provide a relatively low-cost and standardized means to test new sampler designs in ordinary workplace situations, a logical and practical extension to the approach described in Mark and Vincent’s paper.
One of the very interesting results in Mark and Vincent’s paper concerns the relative performances of the various dust sampler designs tested. This and subsequent work showed that different designs of personal sampler collect very different fractions of the airborne dust, despite the fact that most personal sampler designs operate at the same flowrate and have fairly similar external dimensions. In practice, the only factor distinguishing the sampler designs tested by Mark and Vincent is the orifice size and hence the inlet velocity. Yet somehow, the IOM sampler (which has an orifice of 15 mm, a flow of 2 l/min and an inlet velocity of 0.2 m/s), collects the same dust concentration as the manikin’s mouth. In contrast to the IOM sampler, the manikin mouth has an elliptical orifice measuring ~6 x 30 mm, an average flow rate of 20 l/min and an average inlet velocity of ~4.7 m/s during the inhalation part of the breathing cycle. It seems quite remarkable that two such different sampling inlets could both collect the same aerosol fraction. This has lead to speculation that this result may simply be a fortuitous coincidence, with the combined effects of the varying concentration field around the manikin and the sampler inlet efficiency just happening in this case to produce a satisfactory outcome (Smith and Bird, 2002). If this were so, one might expect to see big discrepancies between the IOM sampler performance in controlled environments and that in real workplaces (which fortunately we do not).
More than 15 years on, the science behind aerosol sampler design is still not fully understood. Around the time of publishing this paper, James Vincent and his co-workers began to develop semi-empirical models to describe the aerodynamic characteristics of blunt aerosol samplers (Vincent and Mark et al., 1982; Tsai et al., 1996a,b). In recent years, these models have suggested that there may be many different ways to construct dust samplers that select the inhalable fraction (Ramachandran et al., 1998). Professor Vincent continues to work in this field and to explore whether the model predictions can be validated experimentally. If so, we may one day be able to ‘design’ an inhalable sampler, rather than arrive at one through the practical process of trial and error that is so elegantly described in this paper.
|
REFERENCES |
|---|
|
TOPABSTRACTREFERENCES |
|---|
ACGIH. (1998) Particle size-selective sampling for particulate air contaminants. Cincinnati, OH: American Conference of Governmental Industrial Hygienists.
Aitken RJ, Baldwin PEJ, Beaumont GC, Kenny LC, Maynard AD. (1999) Aerosol inhalability in very low winds. J Aerosol Sci; 30: 613–26.[CrossRef]
Armbruster L, Breuer H. (1982) Investigations into defining inhalable dust. In Walton WH, editor. Inhaled Particles V. Oxford: Pergamon Press. pp. 21–32.
Baldwin PEJ, Maynard AD. (1998) A survey of windspeeds in indoor workplaces. Ann Occup Hyg; 42: 303–13.
Flynn MR, George D. (1991) Aerodynamics and exposure variability. Appl Occup Environ Hyg; 6: 36–9.
Flynn MR, Gatano B, McKernan J, Dunn K, Balzicko B, Carlton GN. (1999) Modeling worker exposure to airborne contaminants generated during compressed air spray painting. Ann Occup Hyg; 43: 67–76.
Homma H, Yakiyama M. (1988) Examination of free convection around an occupant’s body caused by its metabolic heat. ASHRAE Trans Part 1; 94: paper 3118, 104–24.
Hsu D-J, Swift DL. (1999) The measurements of human inhalability of ultralarge aerosols in calm air using manikins. J Aerosol Sci; 30: 1331–43. [CrossRef]
Kennedy NJ, Hinds WC. (2002) Inhalability of large solid particles. J Aerosol Sci; 33: 237–55. [CrossRef]
Kenny LC, Ogden TL. (2000) Twenty-five years of inhalable dust. Ann Occup Hyg; 44: 561–3.
Kim T, Flynn MR. (1991a) Air flow pattern around a worker in uniform freestream. AIHA J; 52: 287–96.[CrossRef]
Kim T, Flynn MR. (1991b) Modeling a worker’s exposure from a hand-held source in a uniform freestream. Am Ind Hyg Assoc J; 52: 458–63.[Web of Science][Medline]
Kim T, Flynn MR. (1992) The effect of contaminant source momentum on a worker’s breathing zone concentration in a uniform freestream. Am Ind Hyg Assoc J; 53: 757–66.[Web of Science][Medline]
Lidén G, Kenny LC (1994) Errors in inhalable dust sampling for particles exceeding 100 micrometres. Ann Occup Hyg; 38: 373–84.
Mark D, Vincent JH. (1986) A new personal sampler for airborne total dust in workplaces. Ann Occup Hyg; 30: 89–120.
Mark D, Aitken R, Beamont G et al. (2001) Development of a novel calibration tool for workplace aerosol sampling: review of progress of EU project. In Proceedings of the International Symposium on Dust, Fumes and Mists in the Workplace: Risks and Their Prevention. Toulouse, France, 11–13 June 2001. pp 54–7.
Ogden TL, Birkett JL. (1977) The human head as a dust sampler. In Walton WH, editor. Inhaled Particles IV. Oxford: Pergamon Press. pp. 93–105.
Ramachandran G, Sreenath A, Vincent JH. (1998) Towards a new method for experimental determination of aerosol sampler aspiration efficiency in small wind tunnels. J Aerosol Sci; 29: 875–91.[CrossRef]
Rodes CE, Kamens RM, Wiener RW (1991) The significance and characteristics of the personal activity cloud on exposure assessment measurements for indoor contaminants. Indoor Air; 2: 123–45.
Rodes CE, Kamens RM, Wiener RW (1995) Experimental considerations for the study of contaminant dispersion near the body. Am Ind Hyg Assoc J; 56: 535–45.[Web of Science][Medline]
Smith JP, Bird AJ. (2002). Relationship of sampling efficiency for manikin-mounted personal samplers to efficiency measurements made independent of the manikin. J Aerosol Sci; 33: 1235–60.[CrossRef]
Tsai PJ, Vincent JH, Mark D. (1996a) Semi-empirical model for the aspiration efficiencies of personal aerosol samplers used in occupational hygiene. Ann Occup Hyg; 40: 93–114.
Tsai PJ, Vincent JH, Maldonado G, Mark D. (1996b) The development of the aspiration efficiency predictive model of aerosol samplers. J Aerosol Sci; 27: 650.
Vaughan NP, Chalmers CP, Botham RA. (1990) Field comparison of personal samplers for inhalable dust. Ann Occup Hyg; 34: 553–73.
Vincent JH. (1995) Aerosol Science for Industrial Hygienists. Oxford: Pergamon. ISBN 008042029X.
Vincent JH, Armbruster L. (1981). On the quantitative definition of the inhalability of airborne dust. Ann Occup Hyg; 24: 245–8.
Vincent JH, Mark D. (1982) Applications of blunt sampler theory to the definition and measurement of inhalable dust. In Walton WH, editor. Inhaled Particles V. Oxford: Pergamon Press. pp. 3–19.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  T. OGDEN Annals of Occupational Hygiene at Volume 50: Many Achievements, a Few Mistakes, and an Interesting Future Ann. Hyg., November 1, 2006; 50(8): 751 - 764. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. T. SIMPSON Comparison of Sampling Positions when Measuring Personal Exposure to Solder Fume Ann. Hyg., July 1, 2005; 49(5): 439 - 442. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||