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Inhalable and respirable dust - moving from aerosol science to legislation
0021-8502/92 $5.00 + 0.00 Pergamon Prcss Lid
J. Aerosol Sci., Vol. 23, Suppl. 1, pp. $445-$448, 1992 Printed in Great Britain.
INHALABLE AND R E S P I R A B L E DUST M O V I N G F R O M A E R O S O L SCIENCE TO L E G I S L A T I O N
T.L.
OGDEN
H e a l t h and Safety Executive, 1 C h e p s t o w Place, L o n d o n W2 4TF, U.K.
ABSTRACT The development over the last fifty years of s t a n d a r d s for size-selective sampling for w o r k p l a c e h e a l t h - h a z a r d control is reviewed, leading to the r e c e n t a g r e e m e n t s in CEN and ISO.
KEYWORDS D u s t sampling,
inhalable,
respirable,
health legislation,
standards.
RESPIRABLE The c o n c e p t of s i z e - s e l e c t i v e m e a s u r e m e n t of dust for the i n v e s t i g a t i o n and control of health r i s k is at least 50 years old. B e d f o r d and W a r n e r (1943) wrote: " F r o m the r e s u l t s of recent r e s e a r c h . . . t h e r e is a c o n s e n s u s of o p i n i o n that particles larger than 5 m i c r o n s are of little p a t h o l o g i c a l interest, while some w o r k e r s suggest that p a r t i c l e s larger t h a n 3 m i c r o n s are of little account...(Measure) the mass c o n c e n t r a t i o n below a certain size, say 5 microns, or p e r h a p s the surface area p r e s e n t e d by the dust in unit v o l u m e of the dust cloud." Thus the need to m e a s u r e what we w o u l d call the r e s p i r a b l e m a s s (or s u r f a c e area) was r e c o g n i s e d early. At that time, the o n l y e f f e c t i v e w a y of s i z e - s e l e c t i n g was to size the p a r t i c l e s under the m i c r o s c o p e . B e d f o r d and Warner w e n t on to refer to the good c o r r e l a t i o n o b s e r v e d in some coal-mine studies between "the count of p a r t i c l e s above 1 m i c r o n in size and the mass concentration of p a r t i c l e s not exceeding 5 microns in size." This was the J u s t i f i c a t i o n for the n u m b e r - c o u n t i n g m e t h o d w h i c h was u s e d in British coalmines for another 25 years, until there was a p o r t a b l e size-selective g r a v i m e t r i c s a m p l e r safe for use in coal-mines. The first step t o w a r d s this was the o b s e r v a t i o n by D a v i e s (1952a) that the alveolar d e p o s i t i o n curve could be a p p r o x i m a t e d by the p a r a b o l i c selection c u r v e of a h o r i z o n t a l elutriator. This curve, w i t h a 50% a d m i s s i o n p o i n t at 5 micrometres a e r o d y n a m i c diameter, was adopted by the B r i t i s h M e d i c a l R e s e a r c h Council in 1952 (Hamilton and Walton, 1961), and by the J o h a n n e s b u r g i n t e r n a tional c o n f e r e n c e on p n e u m o c o n i o s i s in 1959 (Orenstein, 1960). A barelyp o r t a b l e i n s t r u m e n t w i t h this s e l e c t i o n curve, the Hexhlet, was described by Wright (1954). The ready a v a i l a b i l i t y of w e i g h t - c o n s t a n t filters m a d e it possible to use smaller flowrates, and the MRE g r a v i m e t r i c sampler, o p e r a t i n g with the J o h a n n e s b u r g curve at 2.5 L/min was d e s c r i b e d by D u n m o r e et al. (1964). This r e m a i n s the s t a n d a r d instrument for B r i t i s h coal-mines. The horizontal e l u t r i a t o r s e l e c t i o n device has t h e r e f o r e had a long and useful $445
$446
T.L. OGDEN
life. Its attraction for the aerosol scientist is that the theory is very simple, and Lid6n and Kenny (1991} have recently confirmed by modern techniques that it obeys the theory if the entry problems can be eliminated. The description by Sherwood and Greenhalgh (1960) of a self-contained portable sampling pump, and the repeated demonstrations that personal sampling gave substantially different concentrations in most environments, meant that any sampling instrument for general use thereafter had to be capable of being hung unobtrusively in the breathing zone. In the United States, a different definition of respirable was adopted, which was approximated by the performance of miniature cyclones (Lippmann and Harris, 1962), so that respirable sampling in the US was dominated by personal sampling from the start. A cyclone to match the Johannesburg definition was described by Higgins and Dewell (1966), and became the usual respirable-dust sampler in Britain. No effective theory of these portable cyclones was available, so empirical calibration was important.
INHALABLE By the early 1970s, the need for personal sampling of respirable dust was well understood for dusts which caused disease when retained in the deep lung. Dusts which might cause disease on deposition elsewhere in the respiratory tract were collected with a wide range of sampler designs, despite the fact that it had been known for many years that inertial and gravitational effects made collection efficiency dependent on a number of environmental and sampler variables. The German occupational exposure limit (MAK) list was the first to recogniee this problem by requiring that samplers have an entry velocity of 1.25 m/sec; it later emerged that this pioneering attempt was flawed because this variable does not determine the entry efficiency (Ogden, 1983). W.H. Walton suggested that a sampler should logically imitate the entry efficiency of the human head, and Ogden and Birkett (1977) and Armbruster and Breuer (1982) made proposals on this basis, which were refined and much extended by Vincent and co-workers (Vincent and Mark, 1982; Vincent, 1989).
COORDINATED DEFINITIONS The early results on human head efficiency were used by a working group of the International Standards Organisation (ISO, 1983) to propose a suite of sampling criteria. It was recommended that an "inspirable" (later renamed inhalable) curve should be used to specify sampling for particles which might be a hazard anywhere in the respiratory tract. This was subdivided into thoracic and reepirable fractions. Both the US and Johannesburg curves were proposed as alternative conventions for the respirable fraction, in slightly modified form, it being felt that there was no hope of getting either side to abandon its well-established practice. (These ISO proposals recalled the suite of curves proposed long before by the Task Group on Lung Dynamics (1966), but the Task Group had not had the benefit of the inhalable measurements, and they had also assumed apparently that sampling would be mainly by measuring the size distribution of the airborne cloud and weighting measurements accordingly, rather than by simple size-selective sampling.) The ISO proposals' chief value was in recognising internationally the inhalable convention, and in stimulating improved later agreements. The UK adopted the inhalable concept for use with its exposure limits, but for practical purposes the 7-hole sampler was adopted for this fraction on the basis of practicability and the entry efficiency results of Wood and Birkett (1979). The first major improvement to the ISO agreement was some proposals by a committee of the American Conference of Governmental Industrial Hygienists (ACGIH, 1985). They provided an excellent review, but the most noteworthy
Inhalableand~spirabled~t
$447
innovation was an improved inhalable proposal. An ISO working group was then formed, to revise the 1983 report in the light of this, as a draft International Standard. Because European Community legislation referred to the inhalable concept, a European Standards Organisation (CEN) working group was set up to examine the definitions. The CEN and ISO groups have since worked very closely together. The ISO and CEN groups quickly adopted the ACGIH inhalable definition. There was not thought at first to be any chance of a reconciliation between the US and Johannesburg respirable definitions, but a compromise respirable definition of Soderholm (1989) was adopted by all three groups, laying the foundation for the end of the 30-year division in respirable definitions. At the time of writing, the position is that the proposals form a draft European Standard, an ISO Committee Draft (one stage back from a draft International Standard), and appear in the ACGIH's Notice of Intended Changes for its Threshold Limit Values. Adoption into legislation will be slower. Ogden (1991) has discussed the position in Europe, where adoption as a CEN standard can be expected to be followed by adoption by reference into European Community legislation. In the US, ACGIH's attitude is clearly influential, but the position of the enforcement agencies, OSHA and MSHA, i8 not yet known. For the inhalable fraction, a difficulty is the long tradition of using the 37-mm plastic cassette. Considering its wide use, it is amazing how little information has been published on this sampler (Buchan et al., 1986). The indications are that it gives lower results than inhalable samplers, so that adoption of the inhalable fraction for US exposure limits implies a tightening of exposure limits if no adjustment is made to the numerical values. The immediate problem for 8tandardisation is a protocol for testing the performance of instruments, to decide whether performance is close enough to the specified curves. This is an important problem in Europe because different countries will prefer to use different instruments, and there need to be agreed tests to determine which are satisfactory. Discussions are taking place on the range of variables to be tested, procedures, acceptable performance, and the place of field and laboratory tests. Some of these factors have been discussed by Kenny and Lid6n (1989). The problem has returned to one of aerosol science, and the agreement on standards will increase rather than decrease the need for good experimental work.
REFERENCES American Conference of Governmental Industrial Hygienists (1985). Particle Size-Selective Sampling in the Workplace. ACGIH, Cincinnati. ISBN 0 936 712 57 O. Armbruster, L. and H. Breuer (1982). Investigations into defining inhalable dust. In: Inhaled Particles V (W.H. Walton, Ed.), pp. 21-32. Pergamon, Oxford. ISBN 0 08 026838 2 Bedford, T. and C.G. Warner (1943). Physical studies of the dust hazard and of the thermal environment in certain coalmine8. Inz Chronic Pulmonary Disease in South Wales Coalminers. Medical Research Council, Special Report Series No, 244. HMSO, London. Buchan, R.M., S.C. Soderholm and M.I. Tillery (1986). Aerosol sampling efficiency of 37 nun filter cassettes. Am. Ind. Hyg. Assoc. J., 47, 825-831. Davies, C.N. (1952). Dust sampling and lung disease. Br. J. Ind. Hed., ~, 120-126. Dunmore, J.H., R.J. Hamilton and D.S.G. Smith (1964). An instrument for the sampling of respirable dust for subsequent gravimetric assessment. J. Sci. Instrum., 41, 669-672.
$448
T.L. OGDEN
Hamilton, R.J. and W.H. Walton (1961). The selective sampling of respirable dust. In: Inhaled Particles and Vapours (C.N. Davies, Ed.), pp. 465-475. Pergamon, Oxford Higgins, R.I. and P. Dewell (1967). A gravimetric, size-selecting dust sampler. In: Inhaled Particles and Vapours II (C.N. Davies, Ed.), pp. 575586. Pergamon, Oxford International Standards Organisation (1983). Air quality - Particle size fraction definitions for health-related sampling. Technical Report ISO/TR 7708-1983 (E). International Organisation for Standardisation, Geneva. Kenny, L.C. and G. Lid~n (1989). The application of performance standards to personal airborne dust samplers. Ann. Occup. Hyg., 33, 289-300. Lid~n, G. and L.C. Kenny (1991). Comparison of measured respirable dust sampler penetration curves with sampling conventions. Ann. Occup. Hyg., 35, 485-504. Lippmann, M. and W.B. Harris (1962). Size-selective samplers for estimating "respirable" dust concentrations. Health Phys., 8, 155-163. Ogden, T.L. (1983). Inhalable, inspirable and total dust. In: Aerosols in the Mining and Industrial Work Environments (V.A. Marple and B.Y.H. Liu, Eds.), pp. 185-204. Ann Arbor Science, Ann Arbor. ISBN 0 250 40531 8 Ogden, T.L. (1991). Legal background and standards: particulate matter. In: Clean Air at Work (R.H. Brown, M. Curtis, K.J. Saunders, and S. Vandendriessche, Eds.), pp.27-33. Royal Society of Chemistry, London. ISBN 0 85186 217 9 Ogden, T.L. and J.L. Birkett (1977). The human head as a dust sampler. In: Inhaled Particles IV (W.H. Walton, Ed.), pp. 93-105. Pergamon, Oxford. ISBN 0 08 020560 7 Orenstein, A.J. (Ed.) (1960). Recommendations adopted by the Pneumoconiosis Conference, III (i), In: Proceedings of the Pneumoconiosis Conference, Johannesburg, February 1959, p.619. Churchill, London. Sherwood, R.J. and D.M.S. Greenhalgh (1960). A personal air sampler. Ann. Occup. Hyg., 2, 127-132. Soderholm, S.C. (1989). Proposed international conventions for particle sizeselective sampling. Ann. Occup. Hyg., 33, 301-320. Task Group on Lung Dynamics (1966). Deposition and retention models for internal dosimetry of the human respiratory tract. Health Phys., 12, 173-207 Vincent, J.H. (1989). Aerosol Sampling Theory and Practice. John Wiley, Chichester. ISBN 0 471 92175 0 Vincent, J.H. and D. Mark (1982) Applications of blunt sampler theory to the definition and measurement of inhalable dust. In: Inhaled Particles V (W.H. Walton, Ed.), pp. 3-19. Pergamon, Oxford. ISBN 0 08 026838 2 Wood, J.D. and J.L. Birkett (1979). External airflow effects on personal sampling. Ann. Occup. Hyg., 22, 299-310. Wright, B.M. (1954). A size-selecting sampler for airborne dust. Br. J. Ind. Med., ii, 284-288.
J. Aerosol Sci., Vol. 23, Suppl. 1, pp. $445-$448, 1992 Printed in Great Britain.
INHALABLE AND R E S P I R A B L E DUST M O V I N G F R O M A E R O S O L SCIENCE TO L E G I S L A T I O N
T.L.
OGDEN
H e a l t h and Safety Executive, 1 C h e p s t o w Place, L o n d o n W2 4TF, U.K.
ABSTRACT The development over the last fifty years of s t a n d a r d s for size-selective sampling for w o r k p l a c e h e a l t h - h a z a r d control is reviewed, leading to the r e c e n t a g r e e m e n t s in CEN and ISO.
KEYWORDS D u s t sampling,
inhalable,
respirable,
health legislation,
standards.
RESPIRABLE The c o n c e p t of s i z e - s e l e c t i v e m e a s u r e m e n t of dust for the i n v e s t i g a t i o n and control of health r i s k is at least 50 years old. B e d f o r d and W a r n e r (1943) wrote: " F r o m the r e s u l t s of recent r e s e a r c h . . . t h e r e is a c o n s e n s u s of o p i n i o n that particles larger than 5 m i c r o n s are of little p a t h o l o g i c a l interest, while some w o r k e r s suggest that p a r t i c l e s larger t h a n 3 m i c r o n s are of little account...(Measure) the mass c o n c e n t r a t i o n below a certain size, say 5 microns, or p e r h a p s the surface area p r e s e n t e d by the dust in unit v o l u m e of the dust cloud." Thus the need to m e a s u r e what we w o u l d call the r e s p i r a b l e m a s s (or s u r f a c e area) was r e c o g n i s e d early. At that time, the o n l y e f f e c t i v e w a y of s i z e - s e l e c t i n g was to size the p a r t i c l e s under the m i c r o s c o p e . B e d f o r d and Warner w e n t on to refer to the good c o r r e l a t i o n o b s e r v e d in some coal-mine studies between "the count of p a r t i c l e s above 1 m i c r o n in size and the mass concentration of p a r t i c l e s not exceeding 5 microns in size." This was the J u s t i f i c a t i o n for the n u m b e r - c o u n t i n g m e t h o d w h i c h was u s e d in British coalmines for another 25 years, until there was a p o r t a b l e size-selective g r a v i m e t r i c s a m p l e r safe for use in coal-mines. The first step t o w a r d s this was the o b s e r v a t i o n by D a v i e s (1952a) that the alveolar d e p o s i t i o n curve could be a p p r o x i m a t e d by the p a r a b o l i c selection c u r v e of a h o r i z o n t a l elutriator. This curve, w i t h a 50% a d m i s s i o n p o i n t at 5 micrometres a e r o d y n a m i c diameter, was adopted by the B r i t i s h M e d i c a l R e s e a r c h Council in 1952 (Hamilton and Walton, 1961), and by the J o h a n n e s b u r g i n t e r n a tional c o n f e r e n c e on p n e u m o c o n i o s i s in 1959 (Orenstein, 1960). A barelyp o r t a b l e i n s t r u m e n t w i t h this s e l e c t i o n curve, the Hexhlet, was described by Wright (1954). The ready a v a i l a b i l i t y of w e i g h t - c o n s t a n t filters m a d e it possible to use smaller flowrates, and the MRE g r a v i m e t r i c sampler, o p e r a t i n g with the J o h a n n e s b u r g curve at 2.5 L/min was d e s c r i b e d by D u n m o r e et al. (1964). This r e m a i n s the s t a n d a r d instrument for B r i t i s h coal-mines. The horizontal e l u t r i a t o r s e l e c t i o n device has t h e r e f o r e had a long and useful $445
$446
T.L. OGDEN
life. Its attraction for the aerosol scientist is that the theory is very simple, and Lid6n and Kenny (1991} have recently confirmed by modern techniques that it obeys the theory if the entry problems can be eliminated. The description by Sherwood and Greenhalgh (1960) of a self-contained portable sampling pump, and the repeated demonstrations that personal sampling gave substantially different concentrations in most environments, meant that any sampling instrument for general use thereafter had to be capable of being hung unobtrusively in the breathing zone. In the United States, a different definition of respirable was adopted, which was approximated by the performance of miniature cyclones (Lippmann and Harris, 1962), so that respirable sampling in the US was dominated by personal sampling from the start. A cyclone to match the Johannesburg definition was described by Higgins and Dewell (1966), and became the usual respirable-dust sampler in Britain. No effective theory of these portable cyclones was available, so empirical calibration was important.
INHALABLE By the early 1970s, the need for personal sampling of respirable dust was well understood for dusts which caused disease when retained in the deep lung. Dusts which might cause disease on deposition elsewhere in the respiratory tract were collected with a wide range of sampler designs, despite the fact that it had been known for many years that inertial and gravitational effects made collection efficiency dependent on a number of environmental and sampler variables. The German occupational exposure limit (MAK) list was the first to recogniee this problem by requiring that samplers have an entry velocity of 1.25 m/sec; it later emerged that this pioneering attempt was flawed because this variable does not determine the entry efficiency (Ogden, 1983). W.H. Walton suggested that a sampler should logically imitate the entry efficiency of the human head, and Ogden and Birkett (1977) and Armbruster and Breuer (1982) made proposals on this basis, which were refined and much extended by Vincent and co-workers (Vincent and Mark, 1982; Vincent, 1989).
COORDINATED DEFINITIONS The early results on human head efficiency were used by a working group of the International Standards Organisation (ISO, 1983) to propose a suite of sampling criteria. It was recommended that an "inspirable" (later renamed inhalable) curve should be used to specify sampling for particles which might be a hazard anywhere in the respiratory tract. This was subdivided into thoracic and reepirable fractions. Both the US and Johannesburg curves were proposed as alternative conventions for the respirable fraction, in slightly modified form, it being felt that there was no hope of getting either side to abandon its well-established practice. (These ISO proposals recalled the suite of curves proposed long before by the Task Group on Lung Dynamics (1966), but the Task Group had not had the benefit of the inhalable measurements, and they had also assumed apparently that sampling would be mainly by measuring the size distribution of the airborne cloud and weighting measurements accordingly, rather than by simple size-selective sampling.) The ISO proposals' chief value was in recognising internationally the inhalable convention, and in stimulating improved later agreements. The UK adopted the inhalable concept for use with its exposure limits, but for practical purposes the 7-hole sampler was adopted for this fraction on the basis of practicability and the entry efficiency results of Wood and Birkett (1979). The first major improvement to the ISO agreement was some proposals by a committee of the American Conference of Governmental Industrial Hygienists (ACGIH, 1985). They provided an excellent review, but the most noteworthy
Inhalableand~spirabled~t
$447
innovation was an improved inhalable proposal. An ISO working group was then formed, to revise the 1983 report in the light of this, as a draft International Standard. Because European Community legislation referred to the inhalable concept, a European Standards Organisation (CEN) working group was set up to examine the definitions. The CEN and ISO groups have since worked very closely together. The ISO and CEN groups quickly adopted the ACGIH inhalable definition. There was not thought at first to be any chance of a reconciliation between the US and Johannesburg respirable definitions, but a compromise respirable definition of Soderholm (1989) was adopted by all three groups, laying the foundation for the end of the 30-year division in respirable definitions. At the time of writing, the position is that the proposals form a draft European Standard, an ISO Committee Draft (one stage back from a draft International Standard), and appear in the ACGIH's Notice of Intended Changes for its Threshold Limit Values. Adoption into legislation will be slower. Ogden (1991) has discussed the position in Europe, where adoption as a CEN standard can be expected to be followed by adoption by reference into European Community legislation. In the US, ACGIH's attitude is clearly influential, but the position of the enforcement agencies, OSHA and MSHA, i8 not yet known. For the inhalable fraction, a difficulty is the long tradition of using the 37-mm plastic cassette. Considering its wide use, it is amazing how little information has been published on this sampler (Buchan et al., 1986). The indications are that it gives lower results than inhalable samplers, so that adoption of the inhalable fraction for US exposure limits implies a tightening of exposure limits if no adjustment is made to the numerical values. The immediate problem for 8tandardisation is a protocol for testing the performance of instruments, to decide whether performance is close enough to the specified curves. This is an important problem in Europe because different countries will prefer to use different instruments, and there need to be agreed tests to determine which are satisfactory. Discussions are taking place on the range of variables to be tested, procedures, acceptable performance, and the place of field and laboratory tests. Some of these factors have been discussed by Kenny and Lid6n (1989). The problem has returned to one of aerosol science, and the agreement on standards will increase rather than decrease the need for good experimental work.
REFERENCES American Conference of Governmental Industrial Hygienists (1985). Particle Size-Selective Sampling in the Workplace. ACGIH, Cincinnati. ISBN 0 936 712 57 O. Armbruster, L. and H. Breuer (1982). Investigations into defining inhalable dust. In: Inhaled Particles V (W.H. Walton, Ed.), pp. 21-32. Pergamon, Oxford. ISBN 0 08 026838 2 Bedford, T. and C.G. Warner (1943). Physical studies of the dust hazard and of the thermal environment in certain coalmine8. Inz Chronic Pulmonary Disease in South Wales Coalminers. Medical Research Council, Special Report Series No, 244. HMSO, London. Buchan, R.M., S.C. Soderholm and M.I. Tillery (1986). Aerosol sampling efficiency of 37 nun filter cassettes. Am. Ind. Hyg. Assoc. J., 47, 825-831. Davies, C.N. (1952). Dust sampling and lung disease. Br. J. Ind. Hed., ~, 120-126. Dunmore, J.H., R.J. Hamilton and D.S.G. Smith (1964). An instrument for the sampling of respirable dust for subsequent gravimetric assessment. J. Sci. Instrum., 41, 669-672.
$448
T.L. OGDEN
Hamilton, R.J. and W.H. Walton (1961). The selective sampling of respirable dust. In: Inhaled Particles and Vapours (C.N. Davies, Ed.), pp. 465-475. Pergamon, Oxford Higgins, R.I. and P. Dewell (1967). A gravimetric, size-selecting dust sampler. In: Inhaled Particles and Vapours II (C.N. Davies, Ed.), pp. 575586. Pergamon, Oxford International Standards Organisation (1983). Air quality - Particle size fraction definitions for health-related sampling. Technical Report ISO/TR 7708-1983 (E). International Organisation for Standardisation, Geneva. Kenny, L.C. and G. Lid~n (1989). The application of performance standards to personal airborne dust samplers. Ann. Occup. Hyg., 33, 289-300. Lid~n, G. and L.C. Kenny (1991). Comparison of measured respirable dust sampler penetration curves with sampling conventions. Ann. Occup. Hyg., 35, 485-504. Lippmann, M. and W.B. Harris (1962). Size-selective samplers for estimating "respirable" dust concentrations. Health Phys., 8, 155-163. Ogden, T.L. (1983). Inhalable, inspirable and total dust. In: Aerosols in the Mining and Industrial Work Environments (V.A. Marple and B.Y.H. Liu, Eds.), pp. 185-204. Ann Arbor Science, Ann Arbor. ISBN 0 250 40531 8 Ogden, T.L. (1991). Legal background and standards: particulate matter. In: Clean Air at Work (R.H. Brown, M. Curtis, K.J. Saunders, and S. Vandendriessche, Eds.), pp.27-33. Royal Society of Chemistry, London. ISBN 0 85186 217 9 Ogden, T.L. and J.L. Birkett (1977). The human head as a dust sampler. In: Inhaled Particles IV (W.H. Walton, Ed.), pp. 93-105. Pergamon, Oxford. ISBN 0 08 020560 7 Orenstein, A.J. (Ed.) (1960). Recommendations adopted by the Pneumoconiosis Conference, III (i), In: Proceedings of the Pneumoconiosis Conference, Johannesburg, February 1959, p.619. Churchill, London. Sherwood, R.J. and D.M.S. Greenhalgh (1960). A personal air sampler. Ann. Occup. Hyg., 2, 127-132. Soderholm, S.C. (1989). Proposed international conventions for particle sizeselective sampling. Ann. Occup. Hyg., 33, 301-320. Task Group on Lung Dynamics (1966). Deposition and retention models for internal dosimetry of the human respiratory tract. Health Phys., 12, 173-207 Vincent, J.H. (1989). Aerosol Sampling Theory and Practice. John Wiley, Chichester. ISBN 0 471 92175 0 Vincent, J.H. and D. Mark (1982) Applications of blunt sampler theory to the definition and measurement of inhalable dust. In: Inhaled Particles V (W.H. Walton, Ed.), pp. 3-19. Pergamon, Oxford. ISBN 0 08 026838 2 Wood, J.D. and J.L. Birkett (1979). External airflow effects on personal sampling. Ann. Occup. Hyg., 22, 299-310. Wright, B.M. (1954). A size-selecting sampler for airborne dust. Br. J. Ind. Med., ii, 284-288.