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Assessment of airborne particles
Aerosol Science, 1970, Vol. 1, pp. 369 to 378. Pergamon Press. Printed in Great Britain.
CONFERENCE
REPORTS
Assessment of airborne particles A conference was held in the Department of Radiation Biology and Biophysics of the University of Rochester, N.Y., on this subject, being the third of a series of international meetings in environmental toxicity. The Chairman was Professor T. Hatch and participation was by invitation. The first session dealt with aerosol fundamentals and opened with a paper by J. Pith of the Czechoslovak Academy of Sciences which was presented in absente. He dealt with the Wiegener effect, discovered in 1911, that the coagulation rate of small particles is increased in the presence of large particles. The new theory relates the rate of coagulation of a polydisperse colloid system to moments of its size distribution; the effect increases with Knudsen number in the case of aerosol particles in the slip flow region. The next paper, by G. A. Sehmel of Battelle Northwest gave experimental results for the deposition of particles between 6/~m and 14/zm diameter upon simulated grass 'in a turbulent wind tunnel. Concentration profiles of the uranium particles were constructed above the grass by sampling with transverse wires coated with a sticky film; the collection efficiency of the wires was determined by separate experiments. A somewhat complicated situation was revealed since the particles were not introduced uniformly over the tunnel section and a peak concentration tended to persist for considerable distances downwind. The velocity of deposition ranged from 5 to 8 cm/sec when the friction velocity varied between 70 and 144 cm/sec; over this range there was no dependence on particle size. K. R. Spumy, of the Czechoslovak Academy of Sciences, was also unable to attend but sent a paper on aerosol filtration by means of analytical pore filters. Starting with an account of how the filters are made, including latest method of bombarding membrane with fission fragments and etching the paths to produce cylindrical pores, he passed on to review pressure drop and filtration mechanisms. The latter include sieve action, inertial impaction on the face of the filter around each pore, diffusion to the surface around the pores, electrostatic action, direct interception within pores, diffusion within pores, sedimentation within pores and, possibly, thermophoresis. Experimental verification of the theories, use under extreme conditions, clogging and practical applications were also considered in this very comprehensive account. G. M. Hidy, of the California Institute of Technology, read a paper on the dynamics of aerosols in the lower troposphere. He did not consider that a self-preserving size distribution could be achieved because the residence times of particles were too short. For 0.1/~m particles it was reckoned that times of the order of one day in urban surface layers to 100 days in the middle troposphere were needed if a self preserving size distribution was to be established; such long life times were unlikely. A more likely explanation of the fourth power size distribution of particles above 0.1/~m was that this spectrum was source-dominated. The various contributions are in selfpreserving states already and produce the well-known form simply by mixing. 369
370
Confexence Reports
Particle growth processes and initial particle size distribution were discussed by J. R. Brock and Y. Boisdron of the University of Texas. They distinguished two principal mechanisms by which most of the sub-micron fractions of urban aerosols originate. One is physical nucleation, in which growth is initiated from supersaturated vapour. The other, chemical nucleation, has only recently been recognised. It is dependent on supersaturation but starts on free radicals or excited groups as nuclei and is propagated by free radical reactions and coagulation. A theory of the process in hydrocarbon flames, for which experimental data is available, was worked out. Another paper which was read in the author's absence was that by B. V. Derjaguin and A. I. Storozhilova of the Academy of Sciences of the U.S.S.R. This dealt with jet methods of investigating aerosols. A thin stream of aerosol, flanked by streams of filtered air, is directed towards a microscope objective. Lateral displacements of the jet can be measured with great accuracy and there is no need to use low concentrations of particles as is necessary with counters. Measurements of thermophoresis were described using oil droplets and particles of sodium chloride, the thermal conductivity of the latter being 300 times that of the former; in spite of this, very little difference was found in the velocity of thermophoresis as is required by some of the theories of the phenomenon. Use of the apparatus for studying diffusiophoresis and the activity of condensation nuclei was also described; ways of varying the activity were examined.
The second session of the conference covered aerosol production and measurement. M. Kerker, and his associates from Clarkson College, Potsdam, New York had a paper on the preparation of liquid aerosols and their particle size analysis by light scattering. A new generator was described using helium as the carrier gas and sodium chloride nuclei evaporated in an electric furnace. The parent aerosol material was run as a film down the inside wall of a vertical tube at the same speed as the helium, charged with nuclei, was flowing through. The whole system was enclosed in a jacket of hot oil. The gas emerged at the bottom through a straight, slightly tapered exit tube in which condensation took place. The apparatus was stable and gave an accurately reproducible aerosol, even after a period of disuse. The polarisation ratio of the aerosol was measured over a range of scattering angles, using a narrow beam of monochromatic light, and the resulting curve was compared with a series of curves which had been calculated by the Mie theory for particles of the same refractive index as the aerosol material but having different values for the peak particle size and standard deviation. When the best fit had been decided the aerosol was fully characterised. A check was imposed by repeating the measurements at several wavelengths; the scattering curves computed for each differed widely but the aerosol characteristic emerged each time. That this matching technique gives a unique definition was proved by plotting contours of the divergences between theory and experiment in the peak radii and in the spread against one another. A diagram of closed contours with a single central minimum was obtained. V. Prodi, of Bologna had made a condensation generator yielding solid particles. Highmelting paraffin wax (69-73°C) and carnauba wax (83--86°C) were dispersed; the latter gave lower values of crg, 1-02 to 1"08 compared with 1.04 to 1-13. Both sodium chloride and an osciUator-fed spark, between tin-coated electrodes, gave good nucleation. Full details of the performance were given together with shadowed electron micrographs of the
Conference Reports
371
aerosols. There was some discussion since many of the particles appeared to have fiat areas of surface. D. Sinclair and L. Hinchcliffe of the A.E.C. Health and Safety Laboratory, New York, employed a high frequency induction furnace to evaporate silver nuclei having diameters between 0.002 and 0.1/zm. A new counter has been developed using adiabatic condensation of alcohol to form a mist which is measured with a transmitted light beam, as in the Pollak counter. Discrepancies between PoUak and General Electric counters were observed. The fine silver particles were shown to penetrate a standard thermal precipitator. Some signs of evaporation of silver particles by the beam of the electron microscope are reported. A calibration is given for a filter used to determine particle size by diffusion. N. A. Fuchs of the Karpov Institute, Moscow, sent a paper, although he was unable to attend the meeting in person. It was titled "Some new methods and devices for aerosol studies" and mentioned a number of techniques with which he has been associated. A method for finding the particle size of fine, polydisperse aerosols is based on calculated penetration curves for a diffusion battery with lognormal aerosols, which can be compared with an experimental curve. Model fibrous filters can be used in a similar way, now that his theory of filtration is available; these have the advantage of being free from entry effect which affects diffusion tubes. Another useful technique for handling fine aerosols is the Kogan 'particle magnifier '. This produces a vapour of high boiling liquid which is nucleated by the particles so that counts can easily be obtained. The essential t~eature here is that no wall loss of particles can be permitted, such as occur in the Sinclair-La-Mer generator. The device can be used as a generator of monodisperse aerosols but droplet size is not adjustable over a wide range; the supersaturation and particle size are almost independent of the concentration of nuclei. A flow method of measuring high rates of coagulation uses a laminar stream through a metre long pipe. The central part of the flow is sucked from the pipe isokinetically. The barrier between clean air and dense aerosol becomes blurred by diffusion as it passes through the pipe and a nephelometric concentration contour, made at the outlet, enables the residence time (5-9 sec) of aerosol in the pipe to be determined. The decrease in number of particles due to coagulation was found with the aid of the ' particle magnifier '; before this could be applied a 500-1000 fold dilution was necessary. Coagulation constants for 25--45 A radius particles could be measured with a consistency of 10 9/o. A new disperser for powders, working with pulsation of air pressure, and a freezing technique for handling liquid droplets in an electron microscope were also described. The next session, on size analysis and shape factors, was opened by G. Zebel of the Silikose Forschungsinstitut, Bochum, Germany. His paper was entitled "The separation power of some methods of depositing aerosol particles according to their size". In it he developed a theory of resolution for a particle centrifuge and for a new kind of impactor. In the absence of diffusion the width of the deposition strip, or line width, is equal to the width of the inlet slit. Two particle sizes are regarded as being separated if the line width is equal to or less than the distance between the centres of the lines of deposition. Diffusion depends on the time of flight between inlet and deposition and the theory shows how the separating power depends on slit width and particle characteristics. In the new impactor the jet of aerosol is contained between two layers of clean air moving at the same velocity. Particle trajectories have been worked out and the results verified by experiments. There was some failing below the theoretical resolution due to turbulence.
372
Conference Reports
A detailed analysis of non-spherical particles was made by W. Sttber of Rochester. The dynamic shape factor is defined as the ratio of the drag of a particle to the drag of a sphere of the same volume. It is shown how it can also be expressed as the product of the drag coefficient of the particle multiplied by its density, multiplied by its equivalent volume diameter (the diameter of a sphere of the same volume) divided by a similar product for a sphere of unit density and the same a~rodynamic diameter. All the quantifies are susceptible of experimental measurement. An empirical expression is given, as well, which includes the Cunnlngham slip factor and is suitable for aerosol particles. Expressions are also derived for ellipsoidal particles, averaging over all orientations on account of Brownian motion; orientation, however, may have to be taken into account. Experiments were conduct(x1 in which the aerodynamic diameter was measured in a new type of aerosol centrifuge. Latex aerosols of uniform primary spheres were used and the aerodynamic diameters were determined for cluster aggregates of compact form containing up to 23 primary particles and also for the less common linear aggregates of up to 8 spheres. It was found that the dynamic shape factor of cluster aggregates containing more than 4 spheres was nearly constant at 1.233, decreasing to unity as the number of spheres fell to one. This means that the aerodynamic diameter of clusters of more than 4 spheres is proportional to the equivalent volume diameter. For hexagonal packing the theoretical minimal value for the dynamic shape factor is 1.127. Chains of spheres behave differently, the dynamic shape factor being slightly dependent upon the length. This means that the aerodynamic diameter is primaril~ determined by the diameter of the spheres and increases slowly with increasing length. It is suggested that straight chain aggregates settle with the axis horizontal. Some experiments with asbestos fibres confirmed this. Using the same aerosol centrifuge P. Kotrappa, of Rochester, determined shape factors for dusts of coal, uraniumoxide and thorium oxide. Projected area measurements were made by electron microscopy and the surface area was determined by gas adsorption. No appreciable changes of the volume shape factor were observed with particle size over the range of 0-5 to 5-0 #m aerodynamic diameter. Coal gave the largest dynamic shape factor of 2.0. A considerable mass of numerical data is presented. A possible method of providing automation for the Zeiss-Endter particle size analyser was put forward by B. H. Kaye of Laurentian University, Sudbury, Ontario. It was accompanied by an amusing and pertinent account of the right lines for systems analysis in general. The particle counting and sizing idea was to use bundles of glass fibres as optical sensors, these now being commercially available at a very low price and in sufficiently small diameter to give fairly good resolution. The output from the fibre bundles could be used to regenerate images of particles on a television receiver. V. Timbrell, of the Pneumoconiosis Research Unit, Penarth, Glamorgan, described how the study of asbestos fibres with his sedimentation aerosol spectrometer, indicated a probable link between the aerodynamic properties of the airborne fibres and the incidence of lung disease in South African asbestos miners. The rate of settlement under gravity is almost entirely governed by the diameter of fibres and not by their length; only fairly short fibres are able to penetrate to the lung alveoli, however, since wall interception catches larger ones before they can reach the deep parts of the lungs. Many other applications of the instrument are described in his paper.
Conference Reports
373
The fourth session of the conference consisted of five papers about the inhalation of aerosols. Following up his earlier work, J. M. Beeckmans of the University of Western Ontario, incorporated factors for spheroidal particles in random orientation in his computer programme for the deposition of spheres in the human lungs. Diffusion, sedimentation and impaction are allowed for in a simple plug-flow tube system based on the anatomy of Weibel. The effect of going from spheres to ellipsoids is dependent on the transport mechanism so that a single factor is of no use. Above 5/zm dia. an increase in the ratio of major to minor axis of the spheroid resulted in an increase in deposition; below about 3/zm the reverse was true. Values of the ratio up to 100 were used. C. N. Davies, of the Medical Research Council and the London School of Hygiene, described recent experiments with human subjects inhaling 0.5/zm aerosol particles. It was found that the deposition decreased if the lungs were expanded to a higher level during breathing, and vice versa. Both steady breathing and single breath experiments had been performed and from them it was concluded that particles remained airborne in the lungs during several breaths before depositing. R. G. Thomas, of the Lovelace Foundation, New Mexico, had developed a theoretical model which represented the processes of clearance, uptake and excretion of inhaled particles after deposition in the respiratory tract. By experimenting with the rate constants in various parts of the system an attempt to match experimental data can be made. The model comprises an upper respiratory compartment with rapid clearance, an intermediate one, and a deep, long term compartment. Transport to lymph nodes and to the blood is possible from each compartment and excretion is taken from the intermediate compartment (ciliary) and the blood. Curves of the burden in the lungs and the lymph nodes can be generated from the model. Some species differences were discussed. S. Laskin described experiments in which rats and hamsters were exposed to dust from polyurethane foam which was dispersed by grinding. One large bronchial carcinoma was produced and there were minor effects, especially in the rats. The pores of the material contain isocyanate which is released when it is dispersed. H. D. Landahl, of the University of California, was prevented by illness from attending but he sent a paper. This referred particularly to the factors governing infection by airborne virus particles. A condition of zero gravity was included as was absorption of water by soluble particles. It was concluded that the deposition in human lungs of particles from 2/~m to 6/~m in diameter would be much less in zero gravity than on the earth; at 20/~m nose retention is important. Inoculation of human subjects with three viruses were performed, drops being placed in the nose in some subjects and a 1.5/zm aerosol being sprayed for others to inhale. For Coxsackie A virus the amount of aerosol which was estimated to deposit in the nasal passages by breathing was about the same as the dose administered directly as drops. Infections in the lungs were recorded in the subjects who breathed aerosol. Rhinovirus should, on theory, have deposited about 6 times as much in the nose as the directly administered drops. Lung symptoms were again observed with aerosol but not with drops. Adenovirus, on the other hand, acted only on the lungs, as expected by theory, since
374
Conference Reports
almost all the aerosol penetrated the nose during breathing. The threshold dose appeared to be about three individual virus particles. The risk of infection by the latter virus is taken to be about two or three times greater on earth than in zero gravity. On the other hand a virus which was only effective in the nose, would be about 50 % more dangerous in zero gravity because particles which would have been fixed in the lungs when breathing on earth could deposit in the nose during exhalation.
Thefifth and last session was devoted to the assessment of hazardous aerosols. Morton Lippman, of New York University, had constructed a sampling device which split the sample of aerosol into small size fractions over a range of 1 #m to 10 ~m dia. It comprised six small cyclones in parallel sampling from a common inlet. Undersize particles from each are collected on filters. The etticiencies are independent of dust loading and long periods of running are possible. An experimental calibration was obtained with a radioactive aerosol. The collection characteristics of the individual cyclones were compared with experimental deposition curves for different regions of the human respiratory tract. These were obtained by inhaling radioactive aerosol and scanning the subject by a carefully worked out technique. The sampler was operated on the same aerosol which was inhaled by the subject. Data for nasal deposition, the first ever to be obtained during natural breathing, agreed well with the formula of Pattte, the amounts deposited being greater than those found by Hounam, Black and Walsh. Very large differences in tracheo-bronchial deposition were found between subjects. The method requires further development but looks very promising since the observed deposition data can be made to fit one of the cyclone curves, by adjusting the flow in the latter, and the other curves then relate to deposition in other regions of the lungs. An evaluation of the inhalation risk associated with urban aerosols was attempted by Morton Corn of the University of Pittsburgh. The possible effects in the three respiratory compartments, naso-pharyrgeal, tracheo-bronchial and pulmonary, were summarised for soluble and insoluble particles and long and short term effects for the latter were detailed. Dcmctable effects include changes in airway resistance and lung compliance. It was suggested that large ranges in the size of airways distal to the bronchioles might not be recognisable since the contribution of the small airways to the total resistance was small. Observations on aerosols in Pittsburgh, using an Andersen sampler, revealed large deviations from the 'serf preserving' size distribution and confirmed that there is no substitute for a good routine programme of air-sampling. The National Air-Sampling Network does not include particle size; to correct this ommission a special type of entry cover was designed for the high volume sampler and a run was made in parallel with the Andersen sampler. In terms of aerodynamic diameter, 28 % by weight of the material collected was finer than 0-5/~m, 25 9/o was between 0.5 and 5.7/,m and the rest was above 5.7 ~m. The fine fraction is significant because of its capacity for potentiating the action of irritant gases. It has perhaps been underrated in the past. A study was made of the shapes of particles collected. This empliasised the errors in judging ~ and weight which arise in visual microscopy. Fibres varied from 3 % to 13 % of the total number of particles. The density of particles averaged 2.2 g/crna with a range between 0-11 and 6.9 g/cm a.
Conference Reports
375
Surface areas were measured by gas adsorption. Many exotic organic compounds were detected but some 9 0 ~ of the organic content of urban aerosol is still unidentified; 8 5 ~ of the whole sample weight is unknown, although it is suspected that carbon accounts for a good deal of this. It is clear that a considerable analytical effort is needed before the toxicology can really be understood. K. R. May, of the Microbiological Research Establishment, Porton, spoke of the assessment of viable airborne particles. The problem differs from ordinary aerosol sampling in several ways. There may be loss of viability, a single particle may contain hundreds or thousands of units which may be returned as either one or individually, depending on the method of assessment, viability is not necessarily in step with infectivity, and some particles, for example viruses, can only grow in living cells. The range of sizes is roughly 1-100 t~m. There seems to be no natural mechanism for dispersing virus particles as individuals. Once sampled an immense magnification can be achieved; a single bacillus, for example, may yield a colony of many billions after overnight incubation. Two main types of sampler are in use. Type A deposits particles directly upon a nutrient surface; after incubation visible colonies are counted and it cannot be told how many viable units originally existed in the airborne particles. Type B samplers deposit particles in liquid which is diluted and plated out. This gives the total of viable units but not the number of airborne particles. The value of direct impingement has been much enhanced by coating the surface of the agar plate with oxyethylene docosyl ether to prevent drying. The recent microthread technique, using the threads produced by orb-spinning spiders, has made possible the ' captive aerosol '. Particles can be held for hours with negligible interference or shielding and subjected to all kinds of experimental treatment. It was this method which revealed the ' open air factor ', an unknown substance which causes rapid loss of viability out of doors, but not in closed rooms where it is presumably lost by adsorption on the walls. The kilting rate is inversely proportional to particle size, as would be anticipated on theoretical grounds. The last paper was by W. Walkenhorst of the Silikose Forschungsinstitut, Bochum, who described a new kind of fine filter made of aluminium oxide. The pores are 0.02 tLm in diameter and number 7.2 × 10x° per cmL They will stand temperatures up to 700°C and are good for electron microscopy. Unwanted coarse particles are kept away by sucking upwards into the filter through a vertical tube. The flow rate is 1 cm3/min for a diameter of 0.525 cm. These filters were used to study the size distribution of rock in coal dust. By incinerating at 550°C the coal was removed, after counting, and only rock remained. The results show the most probable diameter of rock particles was 0.1 t~m compared with 0.2 t~m for coal. 20 ~ of the surface area of rock dust was below 1 ~m diameter compared with 7.6 ~ for coal dust. It is considered that the fine dust below 1 t~m does not make a significant contribution to .the burden in coalminers' lungs. A book containing all papers and the discussions will be published in due course.
University of Essex, Department of Chemistry, Wivenhoe Park, Colchester.
C. N. DAVIES
CONFERENCE
REPORTS
Assessment of airborne particles A conference was held in the Department of Radiation Biology and Biophysics of the University of Rochester, N.Y., on this subject, being the third of a series of international meetings in environmental toxicity. The Chairman was Professor T. Hatch and participation was by invitation. The first session dealt with aerosol fundamentals and opened with a paper by J. Pith of the Czechoslovak Academy of Sciences which was presented in absente. He dealt with the Wiegener effect, discovered in 1911, that the coagulation rate of small particles is increased in the presence of large particles. The new theory relates the rate of coagulation of a polydisperse colloid system to moments of its size distribution; the effect increases with Knudsen number in the case of aerosol particles in the slip flow region. The next paper, by G. A. Sehmel of Battelle Northwest gave experimental results for the deposition of particles between 6/~m and 14/zm diameter upon simulated grass 'in a turbulent wind tunnel. Concentration profiles of the uranium particles were constructed above the grass by sampling with transverse wires coated with a sticky film; the collection efficiency of the wires was determined by separate experiments. A somewhat complicated situation was revealed since the particles were not introduced uniformly over the tunnel section and a peak concentration tended to persist for considerable distances downwind. The velocity of deposition ranged from 5 to 8 cm/sec when the friction velocity varied between 70 and 144 cm/sec; over this range there was no dependence on particle size. K. R. Spumy, of the Czechoslovak Academy of Sciences, was also unable to attend but sent a paper on aerosol filtration by means of analytical pore filters. Starting with an account of how the filters are made, including latest method of bombarding membrane with fission fragments and etching the paths to produce cylindrical pores, he passed on to review pressure drop and filtration mechanisms. The latter include sieve action, inertial impaction on the face of the filter around each pore, diffusion to the surface around the pores, electrostatic action, direct interception within pores, diffusion within pores, sedimentation within pores and, possibly, thermophoresis. Experimental verification of the theories, use under extreme conditions, clogging and practical applications were also considered in this very comprehensive account. G. M. Hidy, of the California Institute of Technology, read a paper on the dynamics of aerosols in the lower troposphere. He did not consider that a self-preserving size distribution could be achieved because the residence times of particles were too short. For 0.1/~m particles it was reckoned that times of the order of one day in urban surface layers to 100 days in the middle troposphere were needed if a self preserving size distribution was to be established; such long life times were unlikely. A more likely explanation of the fourth power size distribution of particles above 0.1/~m was that this spectrum was source-dominated. The various contributions are in selfpreserving states already and produce the well-known form simply by mixing. 369
370
Confexence Reports
Particle growth processes and initial particle size distribution were discussed by J. R. Brock and Y. Boisdron of the University of Texas. They distinguished two principal mechanisms by which most of the sub-micron fractions of urban aerosols originate. One is physical nucleation, in which growth is initiated from supersaturated vapour. The other, chemical nucleation, has only recently been recognised. It is dependent on supersaturation but starts on free radicals or excited groups as nuclei and is propagated by free radical reactions and coagulation. A theory of the process in hydrocarbon flames, for which experimental data is available, was worked out. Another paper which was read in the author's absence was that by B. V. Derjaguin and A. I. Storozhilova of the Academy of Sciences of the U.S.S.R. This dealt with jet methods of investigating aerosols. A thin stream of aerosol, flanked by streams of filtered air, is directed towards a microscope objective. Lateral displacements of the jet can be measured with great accuracy and there is no need to use low concentrations of particles as is necessary with counters. Measurements of thermophoresis were described using oil droplets and particles of sodium chloride, the thermal conductivity of the latter being 300 times that of the former; in spite of this, very little difference was found in the velocity of thermophoresis as is required by some of the theories of the phenomenon. Use of the apparatus for studying diffusiophoresis and the activity of condensation nuclei was also described; ways of varying the activity were examined.
The second session of the conference covered aerosol production and measurement. M. Kerker, and his associates from Clarkson College, Potsdam, New York had a paper on the preparation of liquid aerosols and their particle size analysis by light scattering. A new generator was described using helium as the carrier gas and sodium chloride nuclei evaporated in an electric furnace. The parent aerosol material was run as a film down the inside wall of a vertical tube at the same speed as the helium, charged with nuclei, was flowing through. The whole system was enclosed in a jacket of hot oil. The gas emerged at the bottom through a straight, slightly tapered exit tube in which condensation took place. The apparatus was stable and gave an accurately reproducible aerosol, even after a period of disuse. The polarisation ratio of the aerosol was measured over a range of scattering angles, using a narrow beam of monochromatic light, and the resulting curve was compared with a series of curves which had been calculated by the Mie theory for particles of the same refractive index as the aerosol material but having different values for the peak particle size and standard deviation. When the best fit had been decided the aerosol was fully characterised. A check was imposed by repeating the measurements at several wavelengths; the scattering curves computed for each differed widely but the aerosol characteristic emerged each time. That this matching technique gives a unique definition was proved by plotting contours of the divergences between theory and experiment in the peak radii and in the spread against one another. A diagram of closed contours with a single central minimum was obtained. V. Prodi, of Bologna had made a condensation generator yielding solid particles. Highmelting paraffin wax (69-73°C) and carnauba wax (83--86°C) were dispersed; the latter gave lower values of crg, 1-02 to 1"08 compared with 1.04 to 1-13. Both sodium chloride and an osciUator-fed spark, between tin-coated electrodes, gave good nucleation. Full details of the performance were given together with shadowed electron micrographs of the
Conference Reports
371
aerosols. There was some discussion since many of the particles appeared to have fiat areas of surface. D. Sinclair and L. Hinchcliffe of the A.E.C. Health and Safety Laboratory, New York, employed a high frequency induction furnace to evaporate silver nuclei having diameters between 0.002 and 0.1/zm. A new counter has been developed using adiabatic condensation of alcohol to form a mist which is measured with a transmitted light beam, as in the Pollak counter. Discrepancies between PoUak and General Electric counters were observed. The fine silver particles were shown to penetrate a standard thermal precipitator. Some signs of evaporation of silver particles by the beam of the electron microscope are reported. A calibration is given for a filter used to determine particle size by diffusion. N. A. Fuchs of the Karpov Institute, Moscow, sent a paper, although he was unable to attend the meeting in person. It was titled "Some new methods and devices for aerosol studies" and mentioned a number of techniques with which he has been associated. A method for finding the particle size of fine, polydisperse aerosols is based on calculated penetration curves for a diffusion battery with lognormal aerosols, which can be compared with an experimental curve. Model fibrous filters can be used in a similar way, now that his theory of filtration is available; these have the advantage of being free from entry effect which affects diffusion tubes. Another useful technique for handling fine aerosols is the Kogan 'particle magnifier '. This produces a vapour of high boiling liquid which is nucleated by the particles so that counts can easily be obtained. The essential t~eature here is that no wall loss of particles can be permitted, such as occur in the Sinclair-La-Mer generator. The device can be used as a generator of monodisperse aerosols but droplet size is not adjustable over a wide range; the supersaturation and particle size are almost independent of the concentration of nuclei. A flow method of measuring high rates of coagulation uses a laminar stream through a metre long pipe. The central part of the flow is sucked from the pipe isokinetically. The barrier between clean air and dense aerosol becomes blurred by diffusion as it passes through the pipe and a nephelometric concentration contour, made at the outlet, enables the residence time (5-9 sec) of aerosol in the pipe to be determined. The decrease in number of particles due to coagulation was found with the aid of the ' particle magnifier '; before this could be applied a 500-1000 fold dilution was necessary. Coagulation constants for 25--45 A radius particles could be measured with a consistency of 10 9/o. A new disperser for powders, working with pulsation of air pressure, and a freezing technique for handling liquid droplets in an electron microscope were also described. The next session, on size analysis and shape factors, was opened by G. Zebel of the Silikose Forschungsinstitut, Bochum, Germany. His paper was entitled "The separation power of some methods of depositing aerosol particles according to their size". In it he developed a theory of resolution for a particle centrifuge and for a new kind of impactor. In the absence of diffusion the width of the deposition strip, or line width, is equal to the width of the inlet slit. Two particle sizes are regarded as being separated if the line width is equal to or less than the distance between the centres of the lines of deposition. Diffusion depends on the time of flight between inlet and deposition and the theory shows how the separating power depends on slit width and particle characteristics. In the new impactor the jet of aerosol is contained between two layers of clean air moving at the same velocity. Particle trajectories have been worked out and the results verified by experiments. There was some failing below the theoretical resolution due to turbulence.
372
Conference Reports
A detailed analysis of non-spherical particles was made by W. Sttber of Rochester. The dynamic shape factor is defined as the ratio of the drag of a particle to the drag of a sphere of the same volume. It is shown how it can also be expressed as the product of the drag coefficient of the particle multiplied by its density, multiplied by its equivalent volume diameter (the diameter of a sphere of the same volume) divided by a similar product for a sphere of unit density and the same a~rodynamic diameter. All the quantifies are susceptible of experimental measurement. An empirical expression is given, as well, which includes the Cunnlngham slip factor and is suitable for aerosol particles. Expressions are also derived for ellipsoidal particles, averaging over all orientations on account of Brownian motion; orientation, however, may have to be taken into account. Experiments were conduct(x1 in which the aerodynamic diameter was measured in a new type of aerosol centrifuge. Latex aerosols of uniform primary spheres were used and the aerodynamic diameters were determined for cluster aggregates of compact form containing up to 23 primary particles and also for the less common linear aggregates of up to 8 spheres. It was found that the dynamic shape factor of cluster aggregates containing more than 4 spheres was nearly constant at 1.233, decreasing to unity as the number of spheres fell to one. This means that the aerodynamic diameter of clusters of more than 4 spheres is proportional to the equivalent volume diameter. For hexagonal packing the theoretical minimal value for the dynamic shape factor is 1.127. Chains of spheres behave differently, the dynamic shape factor being slightly dependent upon the length. This means that the aerodynamic diameter is primaril~ determined by the diameter of the spheres and increases slowly with increasing length. It is suggested that straight chain aggregates settle with the axis horizontal. Some experiments with asbestos fibres confirmed this. Using the same aerosol centrifuge P. Kotrappa, of Rochester, determined shape factors for dusts of coal, uraniumoxide and thorium oxide. Projected area measurements were made by electron microscopy and the surface area was determined by gas adsorption. No appreciable changes of the volume shape factor were observed with particle size over the range of 0-5 to 5-0 #m aerodynamic diameter. Coal gave the largest dynamic shape factor of 2.0. A considerable mass of numerical data is presented. A possible method of providing automation for the Zeiss-Endter particle size analyser was put forward by B. H. Kaye of Laurentian University, Sudbury, Ontario. It was accompanied by an amusing and pertinent account of the right lines for systems analysis in general. The particle counting and sizing idea was to use bundles of glass fibres as optical sensors, these now being commercially available at a very low price and in sufficiently small diameter to give fairly good resolution. The output from the fibre bundles could be used to regenerate images of particles on a television receiver. V. Timbrell, of the Pneumoconiosis Research Unit, Penarth, Glamorgan, described how the study of asbestos fibres with his sedimentation aerosol spectrometer, indicated a probable link between the aerodynamic properties of the airborne fibres and the incidence of lung disease in South African asbestos miners. The rate of settlement under gravity is almost entirely governed by the diameter of fibres and not by their length; only fairly short fibres are able to penetrate to the lung alveoli, however, since wall interception catches larger ones before they can reach the deep parts of the lungs. Many other applications of the instrument are described in his paper.
Conference Reports
373
The fourth session of the conference consisted of five papers about the inhalation of aerosols. Following up his earlier work, J. M. Beeckmans of the University of Western Ontario, incorporated factors for spheroidal particles in random orientation in his computer programme for the deposition of spheres in the human lungs. Diffusion, sedimentation and impaction are allowed for in a simple plug-flow tube system based on the anatomy of Weibel. The effect of going from spheres to ellipsoids is dependent on the transport mechanism so that a single factor is of no use. Above 5/zm dia. an increase in the ratio of major to minor axis of the spheroid resulted in an increase in deposition; below about 3/zm the reverse was true. Values of the ratio up to 100 were used. C. N. Davies, of the Medical Research Council and the London School of Hygiene, described recent experiments with human subjects inhaling 0.5/zm aerosol particles. It was found that the deposition decreased if the lungs were expanded to a higher level during breathing, and vice versa. Both steady breathing and single breath experiments had been performed and from them it was concluded that particles remained airborne in the lungs during several breaths before depositing. R. G. Thomas, of the Lovelace Foundation, New Mexico, had developed a theoretical model which represented the processes of clearance, uptake and excretion of inhaled particles after deposition in the respiratory tract. By experimenting with the rate constants in various parts of the system an attempt to match experimental data can be made. The model comprises an upper respiratory compartment with rapid clearance, an intermediate one, and a deep, long term compartment. Transport to lymph nodes and to the blood is possible from each compartment and excretion is taken from the intermediate compartment (ciliary) and the blood. Curves of the burden in the lungs and the lymph nodes can be generated from the model. Some species differences were discussed. S. Laskin described experiments in which rats and hamsters were exposed to dust from polyurethane foam which was dispersed by grinding. One large bronchial carcinoma was produced and there were minor effects, especially in the rats. The pores of the material contain isocyanate which is released when it is dispersed. H. D. Landahl, of the University of California, was prevented by illness from attending but he sent a paper. This referred particularly to the factors governing infection by airborne virus particles. A condition of zero gravity was included as was absorption of water by soluble particles. It was concluded that the deposition in human lungs of particles from 2/~m to 6/~m in diameter would be much less in zero gravity than on the earth; at 20/~m nose retention is important. Inoculation of human subjects with three viruses were performed, drops being placed in the nose in some subjects and a 1.5/zm aerosol being sprayed for others to inhale. For Coxsackie A virus the amount of aerosol which was estimated to deposit in the nasal passages by breathing was about the same as the dose administered directly as drops. Infections in the lungs were recorded in the subjects who breathed aerosol. Rhinovirus should, on theory, have deposited about 6 times as much in the nose as the directly administered drops. Lung symptoms were again observed with aerosol but not with drops. Adenovirus, on the other hand, acted only on the lungs, as expected by theory, since
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almost all the aerosol penetrated the nose during breathing. The threshold dose appeared to be about three individual virus particles. The risk of infection by the latter virus is taken to be about two or three times greater on earth than in zero gravity. On the other hand a virus which was only effective in the nose, would be about 50 % more dangerous in zero gravity because particles which would have been fixed in the lungs when breathing on earth could deposit in the nose during exhalation.
Thefifth and last session was devoted to the assessment of hazardous aerosols. Morton Lippman, of New York University, had constructed a sampling device which split the sample of aerosol into small size fractions over a range of 1 #m to 10 ~m dia. It comprised six small cyclones in parallel sampling from a common inlet. Undersize particles from each are collected on filters. The etticiencies are independent of dust loading and long periods of running are possible. An experimental calibration was obtained with a radioactive aerosol. The collection characteristics of the individual cyclones were compared with experimental deposition curves for different regions of the human respiratory tract. These were obtained by inhaling radioactive aerosol and scanning the subject by a carefully worked out technique. The sampler was operated on the same aerosol which was inhaled by the subject. Data for nasal deposition, the first ever to be obtained during natural breathing, agreed well with the formula of Pattte, the amounts deposited being greater than those found by Hounam, Black and Walsh. Very large differences in tracheo-bronchial deposition were found between subjects. The method requires further development but looks very promising since the observed deposition data can be made to fit one of the cyclone curves, by adjusting the flow in the latter, and the other curves then relate to deposition in other regions of the lungs. An evaluation of the inhalation risk associated with urban aerosols was attempted by Morton Corn of the University of Pittsburgh. The possible effects in the three respiratory compartments, naso-pharyrgeal, tracheo-bronchial and pulmonary, were summarised for soluble and insoluble particles and long and short term effects for the latter were detailed. Dcmctable effects include changes in airway resistance and lung compliance. It was suggested that large ranges in the size of airways distal to the bronchioles might not be recognisable since the contribution of the small airways to the total resistance was small. Observations on aerosols in Pittsburgh, using an Andersen sampler, revealed large deviations from the 'serf preserving' size distribution and confirmed that there is no substitute for a good routine programme of air-sampling. The National Air-Sampling Network does not include particle size; to correct this ommission a special type of entry cover was designed for the high volume sampler and a run was made in parallel with the Andersen sampler. In terms of aerodynamic diameter, 28 % by weight of the material collected was finer than 0-5/~m, 25 9/o was between 0.5 and 5.7/,m and the rest was above 5.7 ~m. The fine fraction is significant because of its capacity for potentiating the action of irritant gases. It has perhaps been underrated in the past. A study was made of the shapes of particles collected. This empliasised the errors in judging ~ and weight which arise in visual microscopy. Fibres varied from 3 % to 13 % of the total number of particles. The density of particles averaged 2.2 g/crna with a range between 0-11 and 6.9 g/cm a.
Conference Reports
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Surface areas were measured by gas adsorption. Many exotic organic compounds were detected but some 9 0 ~ of the organic content of urban aerosol is still unidentified; 8 5 ~ of the whole sample weight is unknown, although it is suspected that carbon accounts for a good deal of this. It is clear that a considerable analytical effort is needed before the toxicology can really be understood. K. R. May, of the Microbiological Research Establishment, Porton, spoke of the assessment of viable airborne particles. The problem differs from ordinary aerosol sampling in several ways. There may be loss of viability, a single particle may contain hundreds or thousands of units which may be returned as either one or individually, depending on the method of assessment, viability is not necessarily in step with infectivity, and some particles, for example viruses, can only grow in living cells. The range of sizes is roughly 1-100 t~m. There seems to be no natural mechanism for dispersing virus particles as individuals. Once sampled an immense magnification can be achieved; a single bacillus, for example, may yield a colony of many billions after overnight incubation. Two main types of sampler are in use. Type A deposits particles directly upon a nutrient surface; after incubation visible colonies are counted and it cannot be told how many viable units originally existed in the airborne particles. Type B samplers deposit particles in liquid which is diluted and plated out. This gives the total of viable units but not the number of airborne particles. The value of direct impingement has been much enhanced by coating the surface of the agar plate with oxyethylene docosyl ether to prevent drying. The recent microthread technique, using the threads produced by orb-spinning spiders, has made possible the ' captive aerosol '. Particles can be held for hours with negligible interference or shielding and subjected to all kinds of experimental treatment. It was this method which revealed the ' open air factor ', an unknown substance which causes rapid loss of viability out of doors, but not in closed rooms where it is presumably lost by adsorption on the walls. The kilting rate is inversely proportional to particle size, as would be anticipated on theoretical grounds. The last paper was by W. Walkenhorst of the Silikose Forschungsinstitut, Bochum, who described a new kind of fine filter made of aluminium oxide. The pores are 0.02 tLm in diameter and number 7.2 × 10x° per cmL They will stand temperatures up to 700°C and are good for electron microscopy. Unwanted coarse particles are kept away by sucking upwards into the filter through a vertical tube. The flow rate is 1 cm3/min for a diameter of 0.525 cm. These filters were used to study the size distribution of rock in coal dust. By incinerating at 550°C the coal was removed, after counting, and only rock remained. The results show the most probable diameter of rock particles was 0.1 t~m compared with 0.2 t~m for coal. 20 ~ of the surface area of rock dust was below 1 ~m diameter compared with 7.6 ~ for coal dust. It is considered that the fine dust below 1 t~m does not make a significant contribution to .the burden in coalminers' lungs. A book containing all papers and the discussions will be published in due course.
University of Essex, Department of Chemistry, Wivenhoe Park, Colchester.
C. N. DAVIES