ECOTOXICOLOGY
AND
ENVIRONMENTAL
The Relation
SAFETY
(1978)
I,429436
of Silica Dust to Accelerated
B. SAMIMI,~MORTONZISKIND,~*~ Section
Silicosisl
AND HANS WEILL~
of Pulmonary Diseases, Department of Medicine, and the Department oJ”Environmenta1 Engineering, Tulane University, New Orleans, Louisiana 70118
Received
May
12.1977
More than 130 cases of silicosis among sandblasters with an average exposure to free silica of 10 years have been studied in Louisiana. The mortality was approximately 25%. Examination of 180 gravimetric respirable dust samples from the breathing zones of sandblasters and other associated workers in two steel fabrication yards showed extensive dust exposure (up to 42.8 times the threshold limit value). The silica fraction of the respirable dust was determined either by X-ray diffraction or by a modified calorimetric technique based on that of Talvitie and Hyslop (Amer. Indust. Hyg. ASSOC. J. 19, 54-58, 1958). Sandblasters wearing non-air-supplied defective hoods were at the greatest risk. Their exposure to silica dust varied greatly depending on the type of hood, maintenance. proper fit, and atmospheric dust concentrations during nonblasting periods when they were unhooded. The development of so-called accelerated silicosis is related to ordinary and faulty characteristics of sandblasting: high free-silica content of sand, use of inadequate or faulty protective devices, carelessness. and incomplete safety training.
Silicosis, a variety of pneumoconiosis, caused by inhalation of free silica (cc-quartz) is a worldwide disease. The disease is prevalent in many industries and accounts for the largest number of pneumoconiotic deaths, either directly or by predisposing to tuberculosis. The incidence of silicosis has greatly increased in Louisiana since 1948, when offshore oil drilling was introduced. During the past 15 years more than 130 cases of silicosis in sandblasters, with a mortality of about 25%, have come to the attention of the investigators. The average duration of exposure for the sandblasters with fatal silicosis was approximately 10 years as compared with an average of 40 years before death for other occupations producing silicosis. Eight of the fatal cases lasted less than 3 years in enclosed spaces. Hunter (1962) states that silicosis with these same features is found among sandblasters throughout the world: a short period of employment and rapid course of disease. He writes that the average term of employment of sandblasters prior to death ’ This study was supported in part by USPHS Grant T 12 HE 05829-06. National Institutes of Health: Grant OH 00387-03. National Institute for Occupational Safety and Health: and NHLI Grant P17 HL 15092-02. * Associate Professor, Department of Occupational Health, School of Public Health, and Head, Division of Planning and Academic Affairs, Center for Coordination of Environmental Studies. University of Teheran, P.O. Box 13 10. Teheran, Iran. z Professor of Medicine, School of Medicine, Tulane University, New Orleans, Louisiana. 4 To whom requests for reprints should be addressed: 1700 Perdido Street, New Orleans. Louisiana 70112. 5 Professor of Medicine, School of Medicine, Tulane University. New Orleans, Louisiana. 429
014%6513/78/0014-0429 W2.0010 Copyright @ 1978 by Academic Press. Inc. All rights of reproduction in any form reserved. Printed in Great Britain
430
FIG. surface
SAMIMI,
1. Sandblasting. Projection for subsequent treatment.
of a stream
ZISKIND,
AND
WEILL
of silica sand by means of compressed
FIG. 2. Other workers in a sandblasting yard whose job sites are located sandblasting site are also exposed to fine suspended silica dust.
adjacent
air to prepare
or downwind
a clear
to the
SILICA
DUST
AND
SILICOSIS
431
from silicosis has been about 10 years. Drinker and Hatch (1954) and Bobear et al. (1962) quote from the literature that cases of silicosis among sandblasters have occurred after only 8 months of exposure to free-silica dust and that fatalities have followed exposures of only 1.5 years. Merewether (1965) surveyed the silicosis risk in sandblasters and shotblasters in Great Britain and showed that the average duration of employment of sandblasters who ultimately died of silicosis was 10.3 years, compared with 40.1 years for the average duration of employment in all cases of silicosis, irrespective of occupation. Sandblasting was introduced into industry in 1904 and is widely used in shipbuilding, manufacture and maintenance of oils rigs and platforms, and in other metal industries. It is a method by which a stream of silica sand is projected by compressed air to prepare a clean surface for subsequent treatment (Fig. 1). Large quantities of finely fragmented respirable particles of silica are created, which when inhaled are responsible for accelerated silicosis in sandblasters and associated workers. Exposure to respirable silica is intensified when blasting takes place in an enclosed area and is particularly dangerous for unprotected individuals. High risks for sandblasters are often the result of faulty or careless methods or the use of protective applicances that have become defective because of failure to provide the constant maintenance they require. Job sites such as those occupied by pot handlers, painters, welders, crane drivers, laborers, and other workers are hazardous when they are located adjacent or downwind to the sandblasting site (Fig. 2). An additional danger in sandblasting is the impact of the abrasive material as it rebounds off the blasting surface. Strong helmets, which protect head and shoulders, and adequate respiratory protection are needed to reduce the hazards of silica exposure. In this study which was undertaken in two steel fabrication yards near New Orleans, Louisiana, the causes and the environmental conditions that produce accelerated silicosis in this type of occupational environment, were investigated.
METHODS
AND
MATERIALS
Dust sampling. The MSA gravimetric dust sampler was used for collecting personal respirable dust samples. This instrument consists of a battery-powered air pump, a dualrate battery charger, and a cyclone assembly and filter holder which contained a preweighed Millipore 0.8~m-pore size hydrophobic filter 37 mm in diameter. In operation, the pump draws dust-laden air through the cyclone assembly at a preselected flow rate of 1.7 liters/min. The cyclone stage of the assembly discards larger nonrespirable (above 10 pm)-sized particles while finer particles are trapped on the filter. At the end of the sampling period, the weight of collected dust is determined by reweighing the filter, and dust concentration per cubic meter of air is calculated. This instrument simulates the human respiratory tract in its selectivity for the respirable fraction of dust particles. This personal sampler which can be attached in the breathing zone of the worker is suitable for continuous sampling during working periods of 8 hr. Sampling policy. One-hundred eighty gravimetric respirable dust samples were collected in the breathing zones of various groups of workers. For sandblasters, the samples were taken both outside and inside the various hoods in order to determine their
432
SAMIMI,
ZISKIND,
AND
WEILL
efficiency. The hoods studied were: (1) non-air-supplied, (2) Pulmosan6 air-supplied, (3) Bullard 77-D (also 77-DH) air-supplied, and (4) other older air-supplied models. Preparation of standard silica samples. A dust-generator chamber was designed and constructed to produce an evenly dispersed cloud of finely ground (5-50 pm in size) pure a-quartz dust. Gravimetric respirable dust samples were collected inside the dustgenerator chamber in essentially the same manner as used for collection of field samples. By varying the length of collection time, it was possible to obtain a series of standards ranging upward from 0.2 mg of quartz dust on the filter. The standards approximated the dust thickness distributions from the field samples. Determination of free-silica fraction in samples. The free-silica fraction within the respirable dust samples was determined either by a calorimetric method or an X-ray diffraction technique. The calorimetric method was a modification of that of Talvitie and Hyslop (1958). This technique was applied to 78% of the samples that were analyzed for free silica. The technique is based on solution of the collected dust in hydrofluoric acid, followed by calorimetric determination of yellow silicomolybdate or molybdenum blue after reduction with I-amino-2-naphthol-4-sulfuric acid. A Beckman Model DU spectrophotometer was used for transmission measurements. The degree of transmission was compared with a calibration curve that had been prepared by using the standard silica solutions; the quantity of silica on the filter was then determined. Since in sandblasting the blasting sand consists of almost 100% quartz, the interference of other silicates in the analysis was minimal. The instrument used for X-ray diffraction of the samples was a Norelco XRD-500 system that includes a vertical goniometer equipped with a graphite crystal monochromator and a sealed proportional counter. Calibration plots were made by using the data obtained from standard filters which had been analyzed according to the same operational parameters as had been used for the unknown samples.
RESULTS
The mean concentrations of respirable dust samplescollected from the breathing zones of various workers, the mean percentage of free silica in the samples,and the concentration of free silicain the breathing zones of the workers are presentedin Table 1. The average concentrations of respirable dust in the breathing zones of various workers were 6.98, 1.72, 1.66, 0.52, 1.01,0.61, and 0.91 mg/m3 of air for sandblasters, helpers, pot handlers, painters, welders, crane drivers, and other workers, respectively. It should be noted that the concentration of respirable dust for sandblastersdoes not represent the actual exposure of these workers. It is the overall average of samples collected inside and outside protective hoods. The average percentagesof free silica in the sampleswere 46.6,41.1, 32.9,41.0, 16.7, 9.9, and 13.6 for sandblasters, helpers, pot handlers, painters, welders, crane drivers, and other workers, respectively. The average concentrations of respirable free-silica particles in workers’ breathing zones were 4.77, 0.7 1, 0.30, 0.18, 0.20, 0.06, and 0.17 mg/m3 of sampled air for 6 Trade disapproval
names used in this paper are for identification of the devices.
only and do not constitute
either endorsement
or
SILICA
DUST
AND
433
SILICOSIS
sandblasters, helpers, pot handlers, painters, welders, crane drivers, and other workers, respectively. Considering the working ventilation (15 liters/min) and the length of a daily working period (8 hr), it was estimatedthat averagesof 50.3, 12.4, 12.0, 3.8, 7.3, 4.4, and 6.6 mg of respirable dust were inhaled daily by sandblasters, helpers, pot handlers, painters, welders, crane drivers, and workers in other job categories, respectively. Considering the percentage of free silica in the samplesof each working category, it was also estimatedthat the average massesof respirable free-silica particles inhaled daily by each worker were 34.3, 5.1, 2.1, 1.3. 1.4,0.5, and 0.8 mg for the abovementionedjobs, respectively. For all workers except sandblasters, the numbers listed above represent actual exposure to silica dust, becausetheseother workers do not usually wear any respiratory protective devices. TABLE MEAN MEAN
CONCENTRATIONS CONCENTRATIONS
OF RESPIRABLE DUST, MEAN PERCENTAGES OF FREE SILICA, AND OF FREE-SILICA PARTICLES IN SAMPLES COLLECTED AT WORKERS’ BREATHING ZONES
Number of
Occupation Sandblaster Helper Pot handler Painter Welder Crane driver Other workers ‘I Mean
1
samples 12 19 4 41 19 24
Respirable dust0 (mg/m3 of air)
Free silica in sample@ (%I
6.98 1.72 1.66 0.52 1.01 0.61 0.9 1
46.6 41.1 32.9 41.0 16.7 9.9 13.6
Free silica in breathing zone9 (mg/m3 of air) 4.77 0.7 1 0.30 0.18 0.20 0.06 0.17
values.
For sandblasterswho usually wear respiratory protective devices, the results listed above do not represent actual exposure to silica dust. They are the average concentrations of samples collected outside and inside various protective hoods. The actual exposure of sandblastersto respirable silica dust can be seenin Table 2. In this table, the average concentrations of dust in the sandblasters’breathing zones, under varying conditions, are listed. The actual exposure of sandblastersto silica dust was greatly dependent on the type of protective hood, maintenance and normal functioning of the hood, and proper precautions during unhooded intervals. In 23 samplescollected inside regular (non-air-supplied) hoods, a mean time-weighted concentration of 5.03 mg/m3 of respirable dust was obtained. The sandblasterswere supposedto wear dust respirators when using non-air-supplied hoods, but during the study it was observed that 35% of them either did not observe this precaution or did not have dust respirators. The average percentage of free silica in samplesto which thesemen were exposed was 47.6. The average concentration of respirable dust for thesemen exceededthe threshold limit value (TLV) by 42.8 times. The exposure of sandblasters who wore various types of old-model air-supplied hoods to silica dust varied greatly depending on (1) the type of hood, (2) hood
434
SAMIMI,
ZISKIND,
AND
WEILL
maintenance, (3) proper fit, and (4) concentration of dust in the air during nonblasting periods when the sandblasterswere unhooded. Samplescollected in the breathing zones of sandblasters wearing Pulmosan air-supplied-hoods indicated that the mean concentration of respirable dust collected continuously during working hours was 2.53 mg/m3. The mean percentage of free silica in these sampleswas 69.2. The average concentration of respirable dust for the sandblasterswearing Pulmosan hoods exceeded the TLV by 12.6 times. For sandblasters wearing various types of old-model airsupplied hoods, the degree of exposure was higher than for sandblasters wearing Pulmosan hoods, i.e., the average respirable dust concentration during the working day was 4.3 mg/m3, and the ratio of concentration/TLV was 38.2. It should be emphasized TABLE MEAN BREATHING CONCENTRATIONS MEAN TLV, AND MEAN RATIOS SAMPLES COLLECTED AT BREATHING
Typesof samples and/or Hoods Respirablecontinuously outsidehoods Respirablecontinuously wearingnon-air-supplied hoods Respirablecontinuously wearingPulmosanairsuppliedhoods Respirablecontinuously wearingold-modelairsuppliedhoods RespirablewearingBullard 77-D air-suppliedhoods During blastingonly During nonblasting Daily averageexposure u Mean
2
OF DUST, MEAN PERCENTAGES OF FREE SILICA. CONCENTRATION/TLV IN PERSONAL GRAVIMETRIC ZONES OF SANDBLASTERS WEARING VARIOUS TYPES OF PROTECTIVE HOODS OF
Numberof samples
Dust0 (wh3)
Silica” (%)
19
15.3
68.3
0.13
142.2
23
5.03
41.6
0.2
42.8
12
2.53
69.2
0.2
12.6
6
4.3
48.8
0.3
38.2
4
0.4 0.78 0.76
5.7 32.0 25.5
1.3 0.3 0.4
0.3 2.7 2.0
4 4
Concentration/ TLV”
TLV”
values.
that these sampleswere collected continuously during both blasting and nonblasting periods. Therefore, a proportion of the dust on the filter may have been picked up from the ambient air during nonblasting periods, due to presence of suspended dust transmitted from nearby sandblastingsites. Gravimetric respirable dust samples collected only during sandblasting under a modern and well-maintained air-supplied hood (Bullard 77-D) showed an average dust concentration of 0.4 mg/m3 of air. These samplescontained an average of free silica as low as 5.75% that causedthe concentration of respirable dust to fall to a ratio of almost one-third of the TLV. Dust samplescollected from the same sandblastersonly during nonblasting intervals showed an average concentration of 0.78 mg/m3 of air with an average free-silica content of 32.0%. The average dust concentration during nonblasting intervals was 2.7 times the TLV. Combining the weight of respirable dust samples,for
SILICA
DUST
AND
SILICOSIS
435
both blasting and nonblasting intervals, the overall exposure of sandblasters using modern and well-maintained air-supplied hoods became an average concentration of 0.76 mg/m3 of air, with an average free-silica content of 25.5% during the working day which was still 2. l-fold above the TLV. DISCUSSION
AND
CONCLUSION
The results presented in this paper should not lead to the conclusion that the airsupplied hoods are totally ineffective, but they do give information about the actual exposure of sandblastersand their associatedworkers to respirablesilica dust. From what has been investigated in the steelfabrication plants, it can be concluded that the following conditions which exist in most plants are responsiblefor producing accelerated silicosisin sandblastersand associateworkers. (2) Production of more respirable dust particles. Powerful collision of sand grains against a hard surface in sandblastingcausesthe silica sand grain to be fragmented into finer particles than in other dust-producing operations. Microscopic examination of dust samplesshowed that an average of 97.5% of the dust particles in the air were smaller than 10 m, which is known as the limiting size for penetration into human lower respiratory tract. (2) More intense dust at the working site. Production of a heavy dust cloud is peculiar to this type of industrial operation, i.e., an average number of 37 million dust particles (with a range of 14-77 million) lessthan 10 pm in diameter were counted at the blasting site. (3) Morefree-silica content. Since the silica sand contains almost 100% cc-quartz, the created dust at the blasting site contains a very high silica fraction which is responsible for the fibrosis of the lung tissues. (4) Use of worn-out and defective respiratory protective devices or lack of them. As was shown earlier in this paper, this is true for many sandblasterswhose protective equipment is not regularly inspected or well maintained. Other workers in the yards engaged in a variety of jobs such as helpers, pot handlers, painters, welders, crane drivers, and other jobs were exposed to various levels of silica dust depending on the proximity of their job sites to blasting sites and the wind direction. These workers did not have any respiratory protective devices. In many instances,where the job siteswere located close to or downwind from the blasting site, the workers were exposedto levels of respirable silica dust severaltimes greater than the TLV levels. (5) Faulty or careless operations and inadequate safety training. It was a normal practice for sandblastersto remove their respiratory protective devices while taking a break, although suspendeddust was still present in the air, or to stay downwind from the blasting site without wearing dust respirators. They often do not wear dust respirators when using non-air-supplied hoods. The majority of the workers were not adequately educated concerning the harmful effects of airborne silica dust on their lungs, the conditions of their working environment predisposing to silicosis, and the proper ways to protect themselves against suspendeddust that is blown through the yard. The points emphasizedin this discussionmay answerthe following question. Why is it that, in sandblasters,advanced fibrosis of the lungs develops four to five times faster than in workers engagedin other silicosis-producingindustries?
436
SAMIMI,
ZISKIND,
AND
WEILL
REFERENCES ALLEN, G., SAMIMI, B., ZISKIND, M., AND WEILL, H. (1974). X-Ray Diffraction determination of alphaquartz in respirable and total dust samples from sandblasting operations. Amer. Indust. Hyg. Assoc. J. 35,711-717. BOBEAR, J., HANEMANN, S., AND BEVEN, T. (1962). Silicosis in Louisiana: New or unrecognized industrial hazards. J. Ln. State Med. Sot. 114,39 l-398. CARLSON, A., AND BANKS, C. (1952). Spectrophotometric determination of siliceous atmospheric contaminations. Anal. Chem. 24,472-476. DRINKER, P., AND HATCH, T. (1954). Industrial Dust, 2nd ed. McGraw-Hill, New York. HUNTER, D. (1962). The Diseases ofOccupation, 3rd ed. McGraw-Hill, New York. LIPPMAN, M. (1970). Respirable dust sampling. Amer. Indust. H.vg. Assoc. J. 31, 138-159. MEREWETHER, E. (1965). Industrial Medicine. Butterworth. London. PATTY, A. (1948). Industrial Hygiene and Toxicology. Interscience, New York. SAMIMI, B. (1973). Silica Dust in Sandblasting Operation. Ph.D. Dissertation, Tulane University, New Orleans, Louisiana. SAMIMI, B., NEILSON, A., WEILL, H., AND ZISKIND, M. (1975). The efficiency of protective hoods used by sandblasters to reduce silica dust exposure. Amer. Ind. Hyg. Ass. J. 36,140-148. SAMIMI, B., WEILL, H., AND ZISKIND, M. (1974). Respirable silica dust exposure of sandblasters and associated workers in steel fabrication yards. Arch. Environ. He&h 29,61-66. TALVITIE, N., AND HYSLOP, F. (1958). Calorimetric determination of siliceous atmospheric contaminations. Amer. Indust. Hyg. Assoc. J. 19,54-58. WEIDNER, R. (1968). Personal Respirable Mass Sampling Procedure. National Center for Urban and Industrial Health, Cincinnati, Ohio.
AND
ENVIRONMENTAL
The Relation
SAFETY
(1978)
I,429436
of Silica Dust to Accelerated
B. SAMIMI,~MORTONZISKIND,~*~ Section
Silicosisl
AND HANS WEILL~
of Pulmonary Diseases, Department of Medicine, and the Department oJ”Environmenta1 Engineering, Tulane University, New Orleans, Louisiana 70118
Received
May
12.1977
More than 130 cases of silicosis among sandblasters with an average exposure to free silica of 10 years have been studied in Louisiana. The mortality was approximately 25%. Examination of 180 gravimetric respirable dust samples from the breathing zones of sandblasters and other associated workers in two steel fabrication yards showed extensive dust exposure (up to 42.8 times the threshold limit value). The silica fraction of the respirable dust was determined either by X-ray diffraction or by a modified calorimetric technique based on that of Talvitie and Hyslop (Amer. Indust. Hyg. ASSOC. J. 19, 54-58, 1958). Sandblasters wearing non-air-supplied defective hoods were at the greatest risk. Their exposure to silica dust varied greatly depending on the type of hood, maintenance. proper fit, and atmospheric dust concentrations during nonblasting periods when they were unhooded. The development of so-called accelerated silicosis is related to ordinary and faulty characteristics of sandblasting: high free-silica content of sand, use of inadequate or faulty protective devices, carelessness. and incomplete safety training.
Silicosis, a variety of pneumoconiosis, caused by inhalation of free silica (cc-quartz) is a worldwide disease. The disease is prevalent in many industries and accounts for the largest number of pneumoconiotic deaths, either directly or by predisposing to tuberculosis. The incidence of silicosis has greatly increased in Louisiana since 1948, when offshore oil drilling was introduced. During the past 15 years more than 130 cases of silicosis in sandblasters, with a mortality of about 25%, have come to the attention of the investigators. The average duration of exposure for the sandblasters with fatal silicosis was approximately 10 years as compared with an average of 40 years before death for other occupations producing silicosis. Eight of the fatal cases lasted less than 3 years in enclosed spaces. Hunter (1962) states that silicosis with these same features is found among sandblasters throughout the world: a short period of employment and rapid course of disease. He writes that the average term of employment of sandblasters prior to death ’ This study was supported in part by USPHS Grant T 12 HE 05829-06. National Institutes of Health: Grant OH 00387-03. National Institute for Occupational Safety and Health: and NHLI Grant P17 HL 15092-02. * Associate Professor, Department of Occupational Health, School of Public Health, and Head, Division of Planning and Academic Affairs, Center for Coordination of Environmental Studies. University of Teheran, P.O. Box 13 10. Teheran, Iran. z Professor of Medicine, School of Medicine, Tulane University, New Orleans, Louisiana. 4 To whom requests for reprints should be addressed: 1700 Perdido Street, New Orleans. Louisiana 70112. 5 Professor of Medicine, School of Medicine, Tulane University. New Orleans, Louisiana. 429
014%6513/78/0014-0429 W2.0010 Copyright @ 1978 by Academic Press. Inc. All rights of reproduction in any form reserved. Printed in Great Britain
430
FIG. surface
SAMIMI,
1. Sandblasting. Projection for subsequent treatment.
of a stream
ZISKIND,
AND
WEILL
of silica sand by means of compressed
FIG. 2. Other workers in a sandblasting yard whose job sites are located sandblasting site are also exposed to fine suspended silica dust.
adjacent
air to prepare
or downwind
a clear
to the
SILICA
DUST
AND
SILICOSIS
431
from silicosis has been about 10 years. Drinker and Hatch (1954) and Bobear et al. (1962) quote from the literature that cases of silicosis among sandblasters have occurred after only 8 months of exposure to free-silica dust and that fatalities have followed exposures of only 1.5 years. Merewether (1965) surveyed the silicosis risk in sandblasters and shotblasters in Great Britain and showed that the average duration of employment of sandblasters who ultimately died of silicosis was 10.3 years, compared with 40.1 years for the average duration of employment in all cases of silicosis, irrespective of occupation. Sandblasting was introduced into industry in 1904 and is widely used in shipbuilding, manufacture and maintenance of oils rigs and platforms, and in other metal industries. It is a method by which a stream of silica sand is projected by compressed air to prepare a clean surface for subsequent treatment (Fig. 1). Large quantities of finely fragmented respirable particles of silica are created, which when inhaled are responsible for accelerated silicosis in sandblasters and associated workers. Exposure to respirable silica is intensified when blasting takes place in an enclosed area and is particularly dangerous for unprotected individuals. High risks for sandblasters are often the result of faulty or careless methods or the use of protective applicances that have become defective because of failure to provide the constant maintenance they require. Job sites such as those occupied by pot handlers, painters, welders, crane drivers, laborers, and other workers are hazardous when they are located adjacent or downwind to the sandblasting site (Fig. 2). An additional danger in sandblasting is the impact of the abrasive material as it rebounds off the blasting surface. Strong helmets, which protect head and shoulders, and adequate respiratory protection are needed to reduce the hazards of silica exposure. In this study which was undertaken in two steel fabrication yards near New Orleans, Louisiana, the causes and the environmental conditions that produce accelerated silicosis in this type of occupational environment, were investigated.
METHODS
AND
MATERIALS
Dust sampling. The MSA gravimetric dust sampler was used for collecting personal respirable dust samples. This instrument consists of a battery-powered air pump, a dualrate battery charger, and a cyclone assembly and filter holder which contained a preweighed Millipore 0.8~m-pore size hydrophobic filter 37 mm in diameter. In operation, the pump draws dust-laden air through the cyclone assembly at a preselected flow rate of 1.7 liters/min. The cyclone stage of the assembly discards larger nonrespirable (above 10 pm)-sized particles while finer particles are trapped on the filter. At the end of the sampling period, the weight of collected dust is determined by reweighing the filter, and dust concentration per cubic meter of air is calculated. This instrument simulates the human respiratory tract in its selectivity for the respirable fraction of dust particles. This personal sampler which can be attached in the breathing zone of the worker is suitable for continuous sampling during working periods of 8 hr. Sampling policy. One-hundred eighty gravimetric respirable dust samples were collected in the breathing zones of various groups of workers. For sandblasters, the samples were taken both outside and inside the various hoods in order to determine their
432
SAMIMI,
ZISKIND,
AND
WEILL
efficiency. The hoods studied were: (1) non-air-supplied, (2) Pulmosan6 air-supplied, (3) Bullard 77-D (also 77-DH) air-supplied, and (4) other older air-supplied models. Preparation of standard silica samples. A dust-generator chamber was designed and constructed to produce an evenly dispersed cloud of finely ground (5-50 pm in size) pure a-quartz dust. Gravimetric respirable dust samples were collected inside the dustgenerator chamber in essentially the same manner as used for collection of field samples. By varying the length of collection time, it was possible to obtain a series of standards ranging upward from 0.2 mg of quartz dust on the filter. The standards approximated the dust thickness distributions from the field samples. Determination of free-silica fraction in samples. The free-silica fraction within the respirable dust samples was determined either by a calorimetric method or an X-ray diffraction technique. The calorimetric method was a modification of that of Talvitie and Hyslop (1958). This technique was applied to 78% of the samples that were analyzed for free silica. The technique is based on solution of the collected dust in hydrofluoric acid, followed by calorimetric determination of yellow silicomolybdate or molybdenum blue after reduction with I-amino-2-naphthol-4-sulfuric acid. A Beckman Model DU spectrophotometer was used for transmission measurements. The degree of transmission was compared with a calibration curve that had been prepared by using the standard silica solutions; the quantity of silica on the filter was then determined. Since in sandblasting the blasting sand consists of almost 100% quartz, the interference of other silicates in the analysis was minimal. The instrument used for X-ray diffraction of the samples was a Norelco XRD-500 system that includes a vertical goniometer equipped with a graphite crystal monochromator and a sealed proportional counter. Calibration plots were made by using the data obtained from standard filters which had been analyzed according to the same operational parameters as had been used for the unknown samples.
RESULTS
The mean concentrations of respirable dust samplescollected from the breathing zones of various workers, the mean percentage of free silica in the samples,and the concentration of free silicain the breathing zones of the workers are presentedin Table 1. The average concentrations of respirable dust in the breathing zones of various workers were 6.98, 1.72, 1.66, 0.52, 1.01,0.61, and 0.91 mg/m3 of air for sandblasters, helpers, pot handlers, painters, welders, crane drivers, and other workers, respectively. It should be noted that the concentration of respirable dust for sandblastersdoes not represent the actual exposure of these workers. It is the overall average of samples collected inside and outside protective hoods. The average percentagesof free silica in the sampleswere 46.6,41.1, 32.9,41.0, 16.7, 9.9, and 13.6 for sandblasters, helpers, pot handlers, painters, welders, crane drivers, and other workers, respectively. The average concentrations of respirable free-silica particles in workers’ breathing zones were 4.77, 0.7 1, 0.30, 0.18, 0.20, 0.06, and 0.17 mg/m3 of sampled air for 6 Trade disapproval
names used in this paper are for identification of the devices.
only and do not constitute
either endorsement
or
SILICA
DUST
AND
433
SILICOSIS
sandblasters, helpers, pot handlers, painters, welders, crane drivers, and other workers, respectively. Considering the working ventilation (15 liters/min) and the length of a daily working period (8 hr), it was estimatedthat averagesof 50.3, 12.4, 12.0, 3.8, 7.3, 4.4, and 6.6 mg of respirable dust were inhaled daily by sandblasters, helpers, pot handlers, painters, welders, crane drivers, and workers in other job categories, respectively. Considering the percentage of free silica in the samplesof each working category, it was also estimatedthat the average massesof respirable free-silica particles inhaled daily by each worker were 34.3, 5.1, 2.1, 1.3. 1.4,0.5, and 0.8 mg for the abovementionedjobs, respectively. For all workers except sandblasters, the numbers listed above represent actual exposure to silica dust, becausetheseother workers do not usually wear any respiratory protective devices. TABLE MEAN MEAN
CONCENTRATIONS CONCENTRATIONS
OF RESPIRABLE DUST, MEAN PERCENTAGES OF FREE SILICA, AND OF FREE-SILICA PARTICLES IN SAMPLES COLLECTED AT WORKERS’ BREATHING ZONES
Number of
Occupation Sandblaster Helper Pot handler Painter Welder Crane driver Other workers ‘I Mean
1
samples 12 19 4 41 19 24
Respirable dust0 (mg/m3 of air)
Free silica in sample@ (%I
6.98 1.72 1.66 0.52 1.01 0.61 0.9 1
46.6 41.1 32.9 41.0 16.7 9.9 13.6
Free silica in breathing zone9 (mg/m3 of air) 4.77 0.7 1 0.30 0.18 0.20 0.06 0.17
values.
For sandblasterswho usually wear respiratory protective devices, the results listed above do not represent actual exposure to silica dust. They are the average concentrations of samples collected outside and inside various protective hoods. The actual exposure of sandblastersto respirable silica dust can be seenin Table 2. In this table, the average concentrations of dust in the sandblasters’breathing zones, under varying conditions, are listed. The actual exposure of sandblastersto silica dust was greatly dependent on the type of protective hood, maintenance and normal functioning of the hood, and proper precautions during unhooded intervals. In 23 samplescollected inside regular (non-air-supplied) hoods, a mean time-weighted concentration of 5.03 mg/m3 of respirable dust was obtained. The sandblasterswere supposedto wear dust respirators when using non-air-supplied hoods, but during the study it was observed that 35% of them either did not observe this precaution or did not have dust respirators. The average percentage of free silica in samplesto which thesemen were exposed was 47.6. The average concentration of respirable dust for thesemen exceededthe threshold limit value (TLV) by 42.8 times. The exposure of sandblasters who wore various types of old-model air-supplied hoods to silica dust varied greatly depending on (1) the type of hood, (2) hood
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maintenance, (3) proper fit, and (4) concentration of dust in the air during nonblasting periods when the sandblasterswere unhooded. Samplescollected in the breathing zones of sandblasters wearing Pulmosan air-supplied-hoods indicated that the mean concentration of respirable dust collected continuously during working hours was 2.53 mg/m3. The mean percentage of free silica in these sampleswas 69.2. The average concentration of respirable dust for the sandblasterswearing Pulmosan hoods exceeded the TLV by 12.6 times. For sandblasters wearing various types of old-model airsupplied hoods, the degree of exposure was higher than for sandblasters wearing Pulmosan hoods, i.e., the average respirable dust concentration during the working day was 4.3 mg/m3, and the ratio of concentration/TLV was 38.2. It should be emphasized TABLE MEAN BREATHING CONCENTRATIONS MEAN TLV, AND MEAN RATIOS SAMPLES COLLECTED AT BREATHING
Typesof samples and/or Hoods Respirablecontinuously outsidehoods Respirablecontinuously wearingnon-air-supplied hoods Respirablecontinuously wearingPulmosanairsuppliedhoods Respirablecontinuously wearingold-modelairsuppliedhoods RespirablewearingBullard 77-D air-suppliedhoods During blastingonly During nonblasting Daily averageexposure u Mean
2
OF DUST, MEAN PERCENTAGES OF FREE SILICA. CONCENTRATION/TLV IN PERSONAL GRAVIMETRIC ZONES OF SANDBLASTERS WEARING VARIOUS TYPES OF PROTECTIVE HOODS OF
Numberof samples
Dust0 (wh3)
Silica” (%)
19
15.3
68.3
0.13
142.2
23
5.03
41.6
0.2
42.8
12
2.53
69.2
0.2
12.6
6
4.3
48.8
0.3
38.2
4
0.4 0.78 0.76
5.7 32.0 25.5
1.3 0.3 0.4
0.3 2.7 2.0
4 4
Concentration/ TLV”
TLV”
values.
that these sampleswere collected continuously during both blasting and nonblasting periods. Therefore, a proportion of the dust on the filter may have been picked up from the ambient air during nonblasting periods, due to presence of suspended dust transmitted from nearby sandblastingsites. Gravimetric respirable dust samples collected only during sandblasting under a modern and well-maintained air-supplied hood (Bullard 77-D) showed an average dust concentration of 0.4 mg/m3 of air. These samplescontained an average of free silica as low as 5.75% that causedthe concentration of respirable dust to fall to a ratio of almost one-third of the TLV. Dust samplescollected from the same sandblastersonly during nonblasting intervals showed an average concentration of 0.78 mg/m3 of air with an average free-silica content of 32.0%. The average dust concentration during nonblasting intervals was 2.7 times the TLV. Combining the weight of respirable dust samples,for
SILICA
DUST
AND
SILICOSIS
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both blasting and nonblasting intervals, the overall exposure of sandblasters using modern and well-maintained air-supplied hoods became an average concentration of 0.76 mg/m3 of air, with an average free-silica content of 25.5% during the working day which was still 2. l-fold above the TLV. DISCUSSION
AND
CONCLUSION
The results presented in this paper should not lead to the conclusion that the airsupplied hoods are totally ineffective, but they do give information about the actual exposure of sandblastersand their associatedworkers to respirablesilica dust. From what has been investigated in the steelfabrication plants, it can be concluded that the following conditions which exist in most plants are responsiblefor producing accelerated silicosisin sandblastersand associateworkers. (2) Production of more respirable dust particles. Powerful collision of sand grains against a hard surface in sandblastingcausesthe silica sand grain to be fragmented into finer particles than in other dust-producing operations. Microscopic examination of dust samplesshowed that an average of 97.5% of the dust particles in the air were smaller than 10 m, which is known as the limiting size for penetration into human lower respiratory tract. (2) More intense dust at the working site. Production of a heavy dust cloud is peculiar to this type of industrial operation, i.e., an average number of 37 million dust particles (with a range of 14-77 million) lessthan 10 pm in diameter were counted at the blasting site. (3) Morefree-silica content. Since the silica sand contains almost 100% cc-quartz, the created dust at the blasting site contains a very high silica fraction which is responsible for the fibrosis of the lung tissues. (4) Use of worn-out and defective respiratory protective devices or lack of them. As was shown earlier in this paper, this is true for many sandblasterswhose protective equipment is not regularly inspected or well maintained. Other workers in the yards engaged in a variety of jobs such as helpers, pot handlers, painters, welders, crane drivers, and other jobs were exposed to various levels of silica dust depending on the proximity of their job sites to blasting sites and the wind direction. These workers did not have any respiratory protective devices. In many instances,where the job siteswere located close to or downwind from the blasting site, the workers were exposedto levels of respirable silica dust severaltimes greater than the TLV levels. (5) Faulty or careless operations and inadequate safety training. It was a normal practice for sandblastersto remove their respiratory protective devices while taking a break, although suspendeddust was still present in the air, or to stay downwind from the blasting site without wearing dust respirators. They often do not wear dust respirators when using non-air-supplied hoods. The majority of the workers were not adequately educated concerning the harmful effects of airborne silica dust on their lungs, the conditions of their working environment predisposing to silicosis, and the proper ways to protect themselves against suspendeddust that is blown through the yard. The points emphasizedin this discussionmay answerthe following question. Why is it that, in sandblasters,advanced fibrosis of the lungs develops four to five times faster than in workers engagedin other silicosis-producingindustries?
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REFERENCES ALLEN, G., SAMIMI, B., ZISKIND, M., AND WEILL, H. (1974). X-Ray Diffraction determination of alphaquartz in respirable and total dust samples from sandblasting operations. Amer. Indust. Hyg. Assoc. J. 35,711-717. BOBEAR, J., HANEMANN, S., AND BEVEN, T. (1962). Silicosis in Louisiana: New or unrecognized industrial hazards. J. Ln. State Med. Sot. 114,39 l-398. CARLSON, A., AND BANKS, C. (1952). Spectrophotometric determination of siliceous atmospheric contaminations. Anal. Chem. 24,472-476. DRINKER, P., AND HATCH, T. (1954). Industrial Dust, 2nd ed. McGraw-Hill, New York. HUNTER, D. (1962). The Diseases ofOccupation, 3rd ed. McGraw-Hill, New York. LIPPMAN, M. (1970). Respirable dust sampling. Amer. Indust. H.vg. Assoc. J. 31, 138-159. MEREWETHER, E. (1965). Industrial Medicine. Butterworth. London. PATTY, A. (1948). Industrial Hygiene and Toxicology. Interscience, New York. SAMIMI, B. (1973). Silica Dust in Sandblasting Operation. Ph.D. Dissertation, Tulane University, New Orleans, Louisiana. SAMIMI, B., NEILSON, A., WEILL, H., AND ZISKIND, M. (1975). The efficiency of protective hoods used by sandblasters to reduce silica dust exposure. Amer. Ind. Hyg. Ass. J. 36,140-148. SAMIMI, B., WEILL, H., AND ZISKIND, M. (1974). Respirable silica dust exposure of sandblasters and associated workers in steel fabrication yards. Arch. Environ. He&h 29,61-66. TALVITIE, N., AND HYSLOP, F. (1958). Calorimetric determination of siliceous atmospheric contaminations. Amer. Indust. Hyg. Assoc. J. 19,54-58. WEIDNER, R. (1968). Personal Respirable Mass Sampling Procedure. National Center for Urban and Industrial Health, Cincinnati, Ohio.
