Predictive Models for Deposition of Diesel Exhaust Particulates in Human and Rat Lungs

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Predictive Models for Deposition of Diesel Exhaust Particulates in Human and Rat Lungs C. P. Yu & G. B. Xu To cite this article: C. P. Yu & G. B. Xu (1986) Predictive Models for Deposition of Diesel Exhaust Particulates in Human and Rat Lungs, Aerosol Science and Technology, 5:3, , DOI: / To link to this article: Published online: 06 Jun Submit your article to this journal Article views: 103 View related articles Citing articles: 18 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 01 December 2017, At: 00:38

2 Predictive Models for Deposition of Diesel Exhaust Particulates in Human and Rat Lungs C. P. Yu and G. B. Xu Department of Mechanical and Aerospace Engineering, State University of New York at Bufalo, Amherst, NY Diesel exhaust particulates are potentially hazardous when they are inhaled into the respiratory system. In this paper, theoretical models were developed to predict the deposition of such particles in humans and rats. It was shown that because of their suhmicron particle size, a significant fraction of diesel exhaust particulates can penetrate into the alveolar region of the lung and deposit there. Under normal breathing conditions, the alveolar deposition for humans was found to be between 7% and INTRODUCTION While diesel cars provide about 25% lugher fuel economy and reduce the exhaust ernissions of carbon monoxide and hydrocarbons as compared to an equal-performance car with a conventional gasoline engine, they produce significantly more particulate matter. These particulates consist principally of combustion-generated carbonaceous soot, on which variable amounts of potentially toxic, solvent-extractable organic materials are adsorbed. Measurements have shown that these particles are submicron in size and can therefore penetrate into the alveolar region of the lung and deposit there. However, in order to have a quantitative assessment of the potential health effects of diesel emissions, the fate of inhaled diesel particles must be understood. Investigations on the health effects of diesel emissions have been conducted for many years, but the results are not conclusive. Among others, it was reported (Barth and Blacker, 1978) that diesel exhaust material had a positive mutagenic response in bacterial assays, indicating that such inhaled 13% depending upon particle size distribution. The alveolar deposition for rats was also investigated and it compared favorably with existing experimental data. Despite their considerable differences in airway size and respiratory condition, humans and rats exhibited similar deposition patterns during a breathing cycle; however, the average minute particle dose per unit pulmonary surface was found to be significantly higher for rats. particulates might be carcinogenic. On the other hand, however, epidemiological studies, such as those conducted by the British Medical Research Council on London bus workers between 1955 and 1974 (Waller, 1980), as well as some recent inhalation studies on animals (Gross, 1980; Chan et al., 1981; Lee et al., 1983), claimed that the1 airborne diesel particulates have no serious threat to public health. The reason for such an apparent contradiction is as yet unknown. One possible explanation may be sought from the level of particle exposure in different experiments. Indeed, Chan et al. (1984) have recently found that lung burden of diesel particles in rats accumulated gradually in high and median exposure concentration groups, but not in the low exposure group. This finding suggests that the longterm burden and hence the potential hazardous effect of diesel particles is closely associated with the particle dose level and dose-time history. The present study is intended to determine the lung dose of diesel exhaust particulates from automobile emissions during Aerosol Science and Technology 5: (1986) Elsevier Science Publishing Co., Inc.

3 C. P. Yu and G. B. Xu various exposure conditions. Mathematical and computer models are developed to calculate the amount of inhaled diesel particles deposited in the respiratory tract of humans and laboratory animals such as rats. Particular attention is given to those particles which deposit in the alveolar region of the lung where the retention time of particles is of long duration. PHYSICAL CHARACTERISTICS OF DIESEL EXHAUST PARTICULATES It is well known that the deposition of aerosol particles in the respiratory tract is strongly influenced by the physical characteristics of the particles. To obtain an accurate dose estimate of diesel exhaust particles in the lung, it is therefore essential to have a correct assessment of their characteristics. Previous studies (Kittelson and Dolan, 1980; Soderholm, 1981) have shown that diesel exhaust particles are aggregates formed by primary spherical particles which are in the size range of nm. These aggregates are irregular in shape and have a size distribution which varies with engine design, fuels used, engine operating conditions, and conditions in the exhaust surroundings. Because the sizes of aggregated particles range from a few molecular diameters to tens of microns, sampling of diesel particulate emission sizes is impossible using any single type of instrument. Cheng et al. (1984) have summarized some data whch were collected by various workers from diesel engine emissions using different measuringdevices. The data show a variation of mass median aerodynamic diameter (MMAD) ranging from 0.19 to 0.54 pm for diesel particles sampled in dilution tunnels before entering the exposure chambers. The size distribution was found to be approximately lognormal, with a geometric standard deviation a, above 4. However, Chan et al. (1981; 1984) have reported a smaller particle size ( pm MMAD) and a much smaller value of a, (1.9) for diesel particles in a quick dilution experimental arrangement. Because of short residence times ( < 1 s) and high dilution ratios (> 1000) at the actual roadway environment, diesel exhaust particles measured under such conditions would more likely have smaller values of MMAD and a, than those measured in the laboratory. The mass density of diesel exhaust particles has not been adequately investigated thus far. The best estimate to date is about 1.5 g/cm3 by Soderholm (1981). PARTICLE MODEL As is well known, the deposition of inhaled particles in the lung is due to the combined mechanisms of impaction, settling, diffusion, and interception. The relative importance of each mechanism is, however, particle size dependent. In the case of diesel exhaust emissions, diffusion is probably the predominant deposition mechanism in view of the particle size range. But large particles in the exhaust, although they make up a small percentage in number of all particles, may also contribute significantly to deposition by impaction, settling, and interception mechanisms. Deposition behavior of irregularly shaped particles, such as diesel exhaust particulates, in model airways is much less understood. To our knowledge, neither experimental nor theoretical results on this problem are available. Such information is essential to the formulation of a mathematical lung deposition model. In this paper, we have attempted to derive expressions for deposition efficiency in an airway by the individual mechanisms of deposition for diesel particles using a simple particle model. We consider each diesel particle to be modeled by a cluster aggregate with a spherical envelope of diameter d,, and consisting of solid primary particles with bulk density p. Then the ratio of the space actually occupied by solid particles in the envelope to the overall envelope volume is the packing density 4 which determines the dy-

4 Deposition Models for Diesel Particulates 339 namic shape factor, K, of the particle (Kasper, 1982). Since the air density is negligibly small compared to p, the effective density p, of the spherical envelope is equal to 3 P P ) =P+, where d, is the volume equivalent diameter of the diesel particle. By definition, the aerodynamic diameter of a diesel particle is given by where p, = 1 g/cm3, C, and C, are, respectively, slip coefficients associated with diameters de and d,, and C is given by the expression (Davies, 1945) in which X is the mean free path of air molecules. Combining Eqs. (1) and (2), we get Since d, determines the mechanical mobility of the particle, the dynamic shape factor, K, can be written as (Kasper, 1982) Equations (4) and (5) together with Eq. (3) determine the values of djd, and K for a given p and +. Table 1 shows these results for different + at p = 1.5 g/cm3. As + TABLE 1. Values of d,/d, and K for different + at p = 1.5 g/cm3 increases, both d Jd, and K decrease, and they reach respective values of 0.75 and 1 when + = 1, which is the case for a solid sphere. AIRWAY DEPOSITION EFFICIENCIES In sampling diesel particles, impactors have often been used to determine particle size, and the results are expressed in the form of aerodynamic diameter. For this reason, it is convenient to consider the diesel particles as an equivalent polydisperse aerosol with same MMAD and a, as measured. For each particle size of aerodynamic diameter d, and its corresponding value of d,, the deposition efficiencies in an airway due to the individual mechanisms of impaction, settling and diffusion are the same as that derived for spherical particles, given by the expressions (Yu and Diu, 1983) q, = 0.768(St)O (6) and qd = exp(- l4.63a) where qi, qs, and qd are, respectively, the efficiencies due to impaction, settling and diffusion, having the same expressions for both inspiration and expiration except that qi = 0 for expiration. In Eqs. (6)-(81, St = d:u/(18~~) is the particle Stokes number, 0 = L/(~R), c = 3susL/(32uR), A = DL/(~R~u), in which u is the air velocity in the airway, p is the air viscosity, L and R are, respec- 1.o tively, the length and radius of the airway,

5 C. P. Yu and G. B. Xu us = C,dt/(l8p) is the particle settling velocity, and D = CekT/(3npde) is the diffusion coefficient, with k being the Boltzmann constant and T the absolute temperature. Deposition efficiency by direct interception at an airway bifurcation has been derived by Harris and Fraser (1976) for an ellipsoidal particle. In a laminar flow, the corresponding deposition efficiency for a spherical particle at inspiration is given by: and qp = 0 at expiration. During the pause, only diffusion and sedimentation are present. We assume that the combined deposition efficiency P is equal to P= 1 - (1 - ps)(l - P ~), (10) where PD and Ps are, respectively, the deposition efficiencies by the individual mechanisms of diffusion and 'sedimentation over the pause period. The expressions for PD and Ps are given by (Yu, 1978) where T~ = Dr/R2, in which 7 is the pause time, and a,, a,, and a, are the first three roots of the equation J&) = 0, (12) in which J, is the Bessel function of the zeroth order, and Ps = ~~ ; for 0 < 7,s 1 (13) and Ps = ~;' ~;~ , where rs = us7/2 R. LUNG DEPOSITION MODEL for T,> 1 (14) The deposition efficiencies given by Eqs. (6)-(14) were incorporated into a lung deposition model that we previously developed for polydisperse aerosols (Diu and Yu, 1983). Deposition calculations were performed for both humans and rats. The respiratory cycle was assumed to consist of a constant flow inspiratory process, a pause, followed by a constant flow expiratory process with time periods equal to, respectively 0.435, 0.05, and of the total period of the cycle. The lung geometry for humans was chosen to be Weibel's dichotomous lung model A (Weibel, 1963), whereas for rats the Whole Lung Anatomical Model proposed by Schum and Yeh (1980) was used. Since the lung dimensions of a rat are related to its body weight and vary by species, we have considered two typical species, Fisher 344 rats and Long Evans rats, for deposition studies. These animals have been frequently used in exposure experiments of diesel emissions and their deposition data are available. The average body weight and the respiratory conditions for Fisher 344 and Long Evans rats are given in Table 2. The airway dimensions given by the Whole Lung Anatomical Model of Schum and Yeh were based upon a rat with a body weight of 330 g and a total lung capacity of cm3. To obtain the airway dimensions for Fisher 344 and Long Evans rats, we assumed that the structure of the airways is independent of species but total lung capacity is propor- TABLE 2. Respiratory Conditions of Rats Long Evans rat Fisher 344 rat Body weight Tidal volume Respiratory frequency (9) (cm3 ' )

6 Deposition Models for Diesel Particulates tional to body weight. The airway dimensions were then calculated based upon a lung volume of 40% of the total lung capacity (Stahl, 1967), assuming that all linear dimensions of the airways vary with the cubic root of lung volume. In constructing the lung model for rats, we also assumed that alveoli are distributed along the last seven generations of airways, similar to that in humans. Particle deposition in the human head via nose and mouth breathing was determined from the empirical formulas that we have derived (Yu et al., 1981). These are given by the following expressions. For mouth breathing, Hi, = g(d:Q) for d:q > 3000, (15) Hi, = 0 for d:q < 3000, (16) and Hex = 0; and for nose breathng, Hi, = log(d;q) for d:q < 337, (18) Hi, = log(d:e) for d:q > 337, (19) and Hex = g(d:Q) for d:q < 215, (20) Hex = log(d:e) for d:q > 215, (21) RESULTS AND DISCUSSIONS Because of the large variation of particle characteristics measured in the laboratory, deposition calculations for diesel particles were made over one respiratory cycle for a wide range of particle sizes and packing and mass densities. Figures 1 and 2 show total and regional deposition results for both human nose and mouth breathing versus particle mass median aerodynamic diameter (MMAD) for p = 1.5 g/cm3, + = 0.3, and a, = 4.5, the latter representing the upper bound of the geometric standard deviation of measured particle size distribution. In these figures, the deposition fraction of die- FIGURE 1. Total (T) and regional (head H, tracheobronchial TB, and alveolar A) depositions of diesel exhaust particles for humans at nose breathing versus particle mass median diameter (MMAD) at + = 0.3, o, = 4.5, and p = 1.5 g/cm3. The initial lung volume is assumed to be 3200 cm3 and the respiratory conditions are tidal volume (TV) = 500 cm3 and respiratory frequency (RF) = 13.7 min-'. where H is the deposition efficiency in the head, the subscripts "in" and "ex" denote inspiration and expiration respectively, d, is the particle aerodynamic diameter, and Q is the air flowrate with d:q having the dimensions of (~m)~(~rn)'/s. For rats, there is no formula available in the literature to predict head deposition. We have derived the following empirical expression for nasal deposition efficiency based upon the data reported by Raabe et al. T O MMAD, pm

7 C. P. Yu and G. B. Xu C... {H oil k MMAD, pm FIGURE 2. Total and regional depositions of diesel exhaust particles for humans at mouth breathing versus particle mass median diameter (MMAD) at (p = 0.3, a, = 4.5, and p = 1.5 g/cm3. The initial lung volume and respiratory conditions are the same as Figure 1. sel particles in a chosen region of the respiratory system is defined as the ratio of the mass of deposited particles in that region to the total mass of particles inhaled within one respiratory cycle, and the total deposition is the sum of deposition in all regions. It is seen that total deposition for nose breathing does not vary appreciably for MMAD between 0.1 and 0.3 pm. Increasing the MMAD leads to an increase in head deposition, but to decrease in tracheobronchial and alveolar depositions. At mouth breathing however, the rate of increase of head deposition with increasing MMAD is much smaller, and we find that total, tracheobronchial and alveolar depositions all decrease as MMAD increases. It is also observed that alveolar deposition in all cases varies within a small range from 7% to 1395, despite the fact that a wide range of particle sizes were considered. A good estimate of the average value of alveolar deposition for diesel exhaust particles is about 10%. FIGURE 3. Total and regional depositions of diesel exhaust particles for humans at mouth breathing versus particle geometrical standard deviation a, at (p = 0.3, MMAD = 0.2 pm, and p = 1.5 g/cm3. The initial lung volume and respiratory conditions are the same as Figure 1. The effect of a, on deposition is shown in Figure 3 for a particle size of 0.2 pm MMAD. It can been seen that increasing a, results in increases in all regional depositions due to the presence of additional large particles. Figure 4 shows the variation of deposition with particle mass density p. Depositions in the tracheobronchial and alveolar regions are found to be insensitive to mass density. This implies that deposition of diesel exhaust particles is governed primarily by Brownian diffusion. In Figure 5, the effect of the packing density on deposition is plotted for + varying from 0.2 to 0.5. There is a very weak dependence on + in this range. From electron micrographs of a typical diesel particle, + has an approximate value of 0.3 It is also known that the amount of particles deposited in the lung depends not only upon particle characteristics, but also upon the lung volume as well as the respiratory condition (i.e., respiratory frequency and

8 Deposition Models for Diesel Particulates FIGURE 4. Total and regional depositions of diesel exhaust particles for humans at mouth breathing versus particle mass density p = 0.3, MMAD = 0.2 pm, and a, = 4.5. The initial lung volume and respiratory conditions are the same as Figure 1. FIGURE 5. Total and regional depositions of diesel exhaust particles for humans at mouth breathing versus packing at MMAD = 0.2 pm, u, = 4.5, and p = 1.5 g/cm3. The initial lung volume and respiratory conditions are the same as Figure 1. Lung Volume, cm3 FIGURE 6. Total and regional depositions of diesel exhaust particles as a function of initial lung volume for humans at mouth breathing. Particle characteristics are + = 0.3, MMAD = 0.2 pm, a, = 4.5, and p = 1.5 g/cm3. Respiratory conditions are TV = 500 cm3 and RF = 13.7 minp'. tidal volume) of a subject (Yu and Diu, 1983). Figure 6 shows the effect of lung volume on deposition. It is seen that deposition decreases as lung volume increases and this decrease takes place primarily in the alveolar region. In Figures 7 and 8, the effects of respiratory frequency (RF) and tidal volume (TV) on deposition are depicted. While the respiratory frequency has only a small decreasing effect on all depositions, increasing the tidal volume will result in a considerable increase in total and alveolar deposition due to a deeper penetration of the particles into the alveolar region. As the tidal volume increases from 500 to 2100 cm3, whlch corresponds to a shift from quite breathing to heavy exercise, alveolar deposition increases from 12% to 20%. Up to now, there are no deposition data available for diesel particles in humans. Several experimental measurements have been conducted on rats with the use of diesel

9 C. P. Yu and G. B. Xu I.....I......I ( H RF, Vmin FIGURE 7. Total and regional depositions of diesel exhaust particles as a function of respiratory frequency for humans at mouth breathing. Particle characteristics are #J = 0.3, MMAD = 0.2 pm, a, = 4.5, and p = 1.5 g/cm3. Lung volume and tidal volume are, respectively, 3200 and 500 cm'. FIGURE 8. Total and regional depositions of diesel exhaust particles as a function of tidal volume for humans at mouth breathing. Particle characteristics are #J = 0.3, MMAD = 0.2 pm, ag = 4.5, and p = 1.5 g/cm3. Lung volume and respiratory frequency are, respectively, 3200 cm3 and 13.7 minpl. exhaust or similar particles. Using nearly monodisperse (a, = 1.5) fused aluminosilicate particles tagged with 169Yb, Raabe et al. (1977) reported a total deposition of % and an alveolar deposition of % in Long Evans rats which have an average breathing rate of 68 breaths/min with a tidal volume of 2.34 cm3. The mass median diameter of their particles was pm. Chan et al. (1981) did deposition measurements on male Fisher 344 rats using diesel exhaust particles with radioactive tracers of l3'ba and 14C. The particles were generated from a GM 5.7-liter diesel engine burning type 2D diesel fuel. The size of these particles was estimated to be pm mass median aerodynamic diameter (MMAD) with a, equal to They found the deposition efficiency in the lung to be 15%-17% with about 11% in the alveolar region. These results are much lower than those reported by Raabe et al. More recently, Wolff et al. (1984) reported deposition data of inhaled 67Ga,0, particles in Fisher 344 rats and other laboratory animals. The aggregates of 67Ga,03 used in their exposure study had a mass median diameter of 0.1 pm and a a, of 1.6. The measured respiratory frequency and tidal volume for the rats were 172 breaths/min and 1.6 cm3, respectively. Alveolar deposition of 10%-12% was also reported. Figures 9 to 11 compare calculated depositions with various experimental data using rats as subjects. In all cases, the agreement between theory and experiment is very good. It is clear from these figures that the discrepancies in the deposition data observed by various investigators is caused by the differences in subject respiratory conditions and particle characteristics during the exposure study. Since rats have been used frequently for the health effects study of diesel exhaust particles, it would be interesting to compare the lung particle dose between rats and humans under the same exposure conditions. Figure 12 shows total and regional deposi-

10 Deposition Models for Diesel Particulates Data (Wolff et al.) A A TB H MMAD, prn FIGURE 9. Comparison of theoretical and experimental deposition of aluminosilicate particles (+ = 0.3, a, = 1.5, p = 2.2 g/cm3) for Long Evans rats (TV = 2.34 cm3, RF = 68 min-i). The experimental data are by Raabe et al. (1977). FIGURE 10. Comparison of theoretical and experimental deposition of diesel exhaust particles (+ = 0.3, o, = 1.9, p = 1.5 g/cm3) for Fisher rats (TV = 1.6 cm3, RF = 98 min-i). The experimental data are by Chan et al. (1981) w Data (Chon et 01.1 H A TB MMAD, pm FIGURE 11. Comparison of theoretical and experimental deposition of Ga,03 particles (+ = 0.3, o, = 1.6, p = 1.5 g/cm3) for Fisher rats (TV = 1.6 cm3, RF = 172 min-i). The experimental data are by Wolff et al. (1984). FIGURE 12. Comparison of total and regional depositions of diesel exhaust particles between humans (lung volume = 3200 cm3, TV = 500 cm3, RF = 13.7 min- ') and Fisher rats (lung volume = 3.99 cm3, TV = 1.6 cm3, RF = 98 min-i) through nose with + = 0.3, o, = 1.9, and p = 1.5 g/cm Human --- Fisher Rat MMAD, prn MMAD, pm

11 C. P. Yu and G. B. Xu 'J Human I I I I I I I I I I I I Generation Number FIGURE 13. Comparison of deposition I \ pattern of diesel exhaust particles along airway generation between humans and \ Fisher rats at the normal respiratory condi- I tion through nose with + = 0.3, MMAD = 0.2pm,~,=1.9,andp=1.5~/crn~. I FIGURE 14. Minute dose of diesel exhaust particles per unit airway surface area for humans and Fisher rats with +=0.3,MMAD=0.2 pm, ug=1.9,and p = 1.5,g/cm3. The particle concentration is assumed to be 1 mg/m3. Generation Number

12 Deposition Models for Diesel Particulates 347 tion in one respiratory cycle for both rats and humans during normal breathing conditions through the nose. Despite the large differences in airway size and respiratory condition, deposition in the alveolar regions of the two species is remarkably similar. Figure 13 further shows this similarity at the airway generation level. In Figure 14, comparison is made between these two species with regard to the minute dose of diesel particles per unit airway surface when exposed to a particle concentration level of 1 mg/m3. The result for rats was found to be larger everywhere in the lung. In the alveolar region, the average value of minute dose per unit pulmonary surface for rats is x lo-' mg/cm2 min as compared to a value of X mg/cm2 min for humans. This result may have important implications in terms of extrapolating the observed clearance behavior of rats to humans because the rate of clearance appears to be dose dependent from experimental observations (Chan et al., 1984). CONCLUSIONS We have developed theoretical models for calculating the deposition of diesel exhaust particulates in the respiratory systems of humans and rats. Despite large variation in the physical characteristics of diesel exhaust particles, under normal breathing conditions, the percentage of deposition in the alveolar region for humans was found to be between 7% and 13% with an average value of 10%. The alveolar deposition for rats was found to have nearly the same magnitude and exhibited similar deposition patterns. However, the average particle dose per unit airway surface per unit time of exposure was also found to be significantly higher for rats. The effects of lung volume and respiratory conditions (tidal volume and respiratory frequency) on diesel particle deposition in humans were also investigated. The respiratory frequency was found to have a very small effect on deposition, whereas significant increases in alveolar deposition were found when either tidal volume was increased or lung volume was decreased. This research was supported by the Health Effects Institute under Agreement No We appreciate several technical discussions with Dr. L. Gradon during the course of this work. Technical assistance of B. Asgharian is also appreciated. REFERENCES Barth, D. S., and Blacker, S. M. (1978). J. Air Pollution Control Assoc. 28: Chan, T. L., Lee, P. S., and Hering, W. E. (1981). J. Appl. Toxicol. 1: Chan, T. L., Lee, P. S., and Hering, W. E. (1984). Fund. Appl. Toxicol. 4: Cheng, Y. S., Yeh, H. C., Manderly, J. L., and Mokler, B. V. (1984). Am. Ind. Hyg. Assoc. J. 45: Davies, C. N. (1945). Proc. Phys. Soc. 57:259. Diu, C. K., and Yu, C. P. (1983). Am. Ind. Hyg. Assoc. J. 44: Gross, K. B. (1980). In Health Effects of Diesel Engine Emissions (W. E. Pepelko, R. M. Danner, and N. A. Clarke, eds.). US EPA b, pp Harris, R. J., Jr., and Fraser, D. A. (1976). Am. Ind. Hyg. Assoc. J. 37: Kasper, G. (1982). Aerosol Sci. Technol. 1: Kittelson, D. B., and Dolan, D. F. (1980). In Generation of Aerosols and Facilities for Exposure Experiments (K. Willeke, ed.). Ann Arbor Science, pp Lee, P. S., Chan, T. L., and Hering, W. E. (1983). J. Toxicol. Enuiron. Heulth 12: Raabe, 0. G., Yeh, H. C., Newton, G. J., Phalen, R. F., and Velasquez, D. J. (1977). In Inhaled Particles IV (W. H. Walton, ed.). Pergamon Press, Oxford, England, pp Schum, M., and Yeh, H. C. (1980). Bull. Math. Biol. 42:l-15. Soderholm, S. C. (1981). Enuiron. Int. 5: Stahl, W. R. (1967). J. Appl. Physiol. 22: Waller, R. E. (1980), In Health Effects of Diesel Engine Emissions (W. E. Pepelko, R. M. Danner, and N. A. Clarke, eds.). US EPA-600/ b, pp Weibel, E. R. (1963). Morphometiy of the Human Lung. Academic Press, New York. Wolff, R. K., Kanapilly, G. M., Gray, R. H., and McClellan, R. 0. (1984). Am. Ind. Hvg. Assoc. J. 45: Yu, C. P. (1978). Powder Technol. 21: Yu, C. P., Diu, C. K., and Soong, T. T. (1981). Am. Iwd. Hyg. Assoc. J. 42: Yu, C. P. and Diu, C. K. (1983). J. Aerosol Sci. 5: Received 10 June 1985; accepted 5 February 1986