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ever, that with adequate environmental controls, silicosis might be prevented or its development delayed considerably beyond the 20-year exposure period. A third unexpected opportunity for further study presented itself in December 1956 when the Committee on Education and Labor, House of Representatives, held its major hearing on bills introduced in the 84th Congress relating to inspections and investigations in metallic and nonmetallic mines and quarries.8 A substantial part of the hearing was devoted to the silicosis problem and testimony was presented by the Public Health Service and the Bureau of Mines. As an outgrowth of the hearing, the Congress appropriated funds to the Public Health Service and the Bureau of Mines to revaluate the silicosis problem in the metal mining industry. METHODOLOGY Following the appropriation of funds by the Congress, an Inter- agency Technical Committee composed of three representatives and an alternate each from the Bureau of Mines and the •Public Health Service was appointed in September 1958 to organize and direct a joint study of silicosis in the metal mining industry. A major re- sponsibility of the Committee was to determine objectives, policies, and procedures to be used in the study. The objective of the study was defined as the determination of the prevalence of silicosis and assessment of the present day environmental conditions in the metal mining industry. The study was designed to answer three important questions. These were :(1) Are the cases presently occurring the result of pre-control exposure, in view of the long latent period for the development of silicosis? (2) Are they accounted for by the failure to apply controls universally? (3) Are the silicosis cases occurring because of the inadequacy of standards in use since 1935? The study was limited to active, underground • metal mines em- ploying more than 20 persons underground, but it was broad enough to include all major commodities in the various mining areas of the United States. The medical examinations consisted of a 14- by 17- inch chest roentgenogram, an occupational and medical history, and simple pulmonary function tests of employees of mines included in • the study. Through engineering studies, environmental conditions were eval- uated by determining dust concentrations and existing dust control measures. Dust evaluations included the determination of the con- centration, particle size, and composition of the dust. Airborne dust and dust-source material were analyzed by X-ray diffraction 4 I ) for free silica and by spectrographic methods for chemical com- position. Ventilation rates were measured. Weighted dust expo- sures _ were determined for each major underground occupation. Mine air samples were collected in sufficient numbers to permit the evaluation of exposures to carbon monoxide, carbon dioxide, and oxides of nitrogen. Because previous studies had utilized the impinger and the refer- ence standard was based on this instrument, the Committee agreed that it would be the basic sampling instrument in the environmen- tal study. However, it was felt that it would be well to supplement the impinger with the thermal precipitator, the electrostatic pre- cipitator, and a filter paper sampler. For various technical rea- sons, the thermal precipitator was used for only a portion of the routine study and use of the electrostatic precipitator was limited largely to a special supplemental investigation. The filter paper technique was continued throughout the study, but the sample was only used for particle sizing. For particle-size determination, both j the optical and the electron microscope were used. The Committee, recognizing the importance of exploring various dust quantitation techniques other than the impinger method, agreed that special dust sampling studies would be conducted in laboratories of the Public Health Service and the Bureau of Mines, and in selected mines. These studies were performed to compare the results yielded by a variety of dust sampling and quantitation techniques with those obtained by the standard impinger method as used in the routine surveys. Very early in its deliberations, the Committee concluded that the success of the study would depend upon keeping and presenting the data in such a manner that the identity of the mines and individ- uals would not be disclosed. Such procedures are also in accord- ance with the policies of the Department of the Interior and the De- partment of Health, Education, and Welfare. It was, therefore, decided that neither engineering nor medical data as they related to an individual or a rpining company would be revealed to State officials, management, labor, or others outside the Public Health Service or Bureau of Mines. However, following each mine sur- vey, the Bureau of Mines conferred with mine management to re- port general findings. The companies were also advised by letter of the free silica content of settled dust samples from its mine and the spectrographic analysis of a composited ore sample. For medical data, an exception was to be made only when the X- ray film revealed a condition that needed immediate medical at- tention, such as suspected cancer, tuberculosis, or heart disease. In such cases, the employee's personal physician was notified by the Public Health Service, if so authorized by the employee. 5

To assure high diagnostic standards, the roentgenographic films were first screened for quality and unusual pathology by physicians of the Division of Occupational Health, Public Health Service, and then read by a panel of outstanding radiologists in accordance with the International Classification of Radiographs of the Pneumoco- nioses (International Labour Office, Geneva, 1958). A major task of the Interagency Technical Committee was to in- form industry, labor unions, and o$icial agencies of the study plans. To assure acceptance at the national level, meetings were held with officials and representatives of the American Mining Congress and the major national labor unions involved. The labor unions readily agreed to the limitations which were placed on the dissemination of information. Through their communication channels, the na- tional labor unions advised their locals of the proposed investigation. Following these discussions, most of the States in which the studies were to be conducted were visited to discuss the proposal with safety and health officials as well as representatives of the State mining associations. Most of the State mining associations contacted in- dicated willingness to cooperate. Individual contacts were then made with mining companies by a representative of either the Public Health Service or the Bureau of Mines, and in many cases, by a joint approach. The cooperation of the operating companies was generally quite good, although it varied to some extent among the various mining districts. For instance, in the Western States, no company declined to participate in the study. In other parts of the country, a few companies would permit only the environmental investigation, being ~ apprehensive that the medical study might cause some concern among their employees and reopen the que$tion of compensation. i However, since the environmental data would be of limited value unless accompanied by the corresponding medical information, these mines were eliminated from the sample. A small group of mines objected to the conduct of either the medical or the environ- mental examinations. Figure 1.1 shows the States in which the survey was conducted. As an outgrowth of the study, a group of mines in the Lake Super- ~ ior district volunteered to make available to the Public Health Service and the Bureau of Mines the medical and engineering data i obtained in their silicosis control program, which began in 1933. Through the Saranac Laboratory, which conducted the medical pro- gram, serial films on about 5,000 miners were available for study, some covering a period of almost 30 years. These data permitted a detailed analysis of present day and past conditions and were in- valuable in relating the development and progression of silicosis to a dust control program. 6

Other companies volunteered their mines for more detailed en- I vironmental studies on particle size, effectiveness of various en- ~ gineering control methods, the relationship of airborne silica to the ~ silica content of the settled dust, and medical data which were needed to supplement the present study. The 67 underground mines in the environmental study included '; 14 iron mines with 4,231 employees, 11 copper miries with 7,260 em- i ployees, 22 lead-zinc-silver mines with 4,281 employees, a miscel- laneous group of 12 mines with 4,365 employees, and 8 uranium j mines with 373 employees. Since it was necessary to limit the mag- nitude of the study, open pit mines were not included. r REFERENCES.• 1. Cummings, D. E. Dusts in Atmosphere: Methods of Estimation and Si9nifi- cance. Second Saranac Symposium on Silicosis. B. E. Kuechle, ed. Wausau, Wis. : Employers Mutual Liability Insurance Co., 1935. 2. National Silicosis Conference. Summary Reports Submitted to the Secretary of Labor by Conference Committees, Feb. 3, 1937. U.S. Department of Labor Buli. No. 13. Washington: U.S. Government Printing Office, 1937. (Out of print. ) 3. Doyle, H. N., and others. Accomplishments in the Epidemiologic Study of Silicosis in the Unite$ States, A.M.A. Archives of Industrial Health 12: 48-55, July 1955. 4. Trasko, Victoria M. Some Facts on the Prevalence of Silicosis in the United States, A.M.A. Archives of Industrial Health 14 : 379-386, October 1956. 5. Russell, A. E., and others. The Health of Workers 'in Dusty Trades. 2. Exposure to Siliceous Dust (Granite Industry). Public Health Bull. No. 187. Washington: U.S. Government Printing Office, 1929. (Out of print. ) 6. Russell, A. E. The Health of Workers in Dusty Trades. 7. Restudy of a Group of Granite Workers. Public Health Bull. No. 269. Washington: U.S. Government Printing Office, 1941. (Out of print.) 7. Hosey, A. D., and others. Control of Silicosis in Vermont Granite Industry- Progress Report. Public Health Service Pub. No. 557. Washington : U.S. Government Printing Office, 1957. 8. Mine Safety (Metallic and Nonmetallic Mines) : Hearings (and Report) before the Committee on Education and Labor, Souse of Representatives, 84th Congress. Washington: U.S. Government Printing Office, 1956. 1 A u I 9 8

r CHAPTER II = Summary, Conclusions, and Recommendations SUMMARY A revaluation of the prevalence of silicosis in the metal mining industry of the United States was carried out by the Division of Occupational Health, Public Health Service, Department of Health, Education, and Welfare, and the Bureau of Mines, Department of the Interior, between March 1958 and September 1961. The study was an outgrowth of hearings before the Committee on Education and Labor, House of Representatives, 84th Congress, Washington, D.C., December 1956. - At the turn of the century there was a growing awareness of the problem of silicosis and frequently associated tuberculosis among workers in the dusty trades in this country. During subsequent years a series of studies was initiated to define the problem and institute control programs. Included in this series was a number of studies of silicosis in the metal mines. These metal mine stud- ies showed that massive dust exposures often were encountered and that scarcely any of the employees were free of harmful dust expo- sure. When the dust had a high free silica content, employees ex- posed to massive levels developed severe silicosis within a few years. It was common to find silicosis in 30 to 80 percent of the employees of specific mines studied. In these early studies, up to 60 percent of the employees with silicosis also had tuberculosis. In the mid-1930's a large part of the metal mining industry insti- tuted major dust control practices. World War II, however, im- posed major difficulties in the followup and development of these practices. The war also curtailed the attention given to research and studies on silicosis in metal mines. This and other postwar problems resulted in a dearth of information on the subject. In 1954 the Public Health Service began a revaluation of silico- sis as an occupational disease problem. This consisted first of a study of compensation and other records of official agencies to de- termine the magnitude of the silicosis problem. During a 5-year period, 1950-54, 10,362 cases of silicosis had been compensated or 11 11 I

reported in 22 States from all industries. The silicotic population was primarily an older group with 75 percent of the cases being 50 years and older. Of 3,455 persons for whom reasonably adequate employment histories were available, only 10 percent allegedly re- ceived their entire dust exposure after 1935. The total mining in- dustry contributed two-thirds of all the cases; metal mining accounted for 24.5 percent of these cases. A revaluation of the granite cutting industry in Vermont in 1.956 revealed that the dust concentrations at the time of the survey were well below 5 million particles per cubic foot of air, and that records revealed no cases of silicosis among the granite workers whose initial exposure fol- lowed the installation of dust controls in the mid-1930's. Consequently, at the start of this study (1958-61), it was not known whether the present prevalence of silicosis in the metal mines resulted from a reservoir of miners still working who had significant exposures before dust control practices were instituted, or was due to the lack of application of dust control practices or to inadequate standards. The medical phase of the study was conducted by the Division of Occupational Health of the Public Health Service and the environ- mental phase by the Bureau of Mines. The environmental study included 67 underground mines employing approximately 20,500 persons-14,000 of whom worked underground and 6,500 in surface occupations. At the time of the study this group represented more than 50 percent of the working population of underground metal mines in the United States. The medical study included employees from 50 of the above 67 metal mines and a large number of uranium mines. The mines included in the study represented virtually all metals mined in commer, cially significant quantities in the United States and represented all principal mining methods. Only under- ground mines employing 20 or more men were studied. The study was the most extensive thus far undertaken in the metal mining industry of the United States. ENVIRONMENTAL~,S.TUDY In the environmental study particular emphasis was placed on evaluation of airborne dust in mine working areas. Observations were made also of pertinent factors such as dust control methods, ventilation, methods of working, and air quality. Mine dust must have three characteristics to be capable of pro- ducing silicosis: (1) it must contain crystalline silicon dioxide, such as quartz, (2) it must be in the respirable particle-size range, and 12 (3) it must be present in sufficient concentration. Thus, in a mine program for prevention of silicosis, the variable which lends itself to control is (3) -the concentration of dust. Determination of the alpha-quartz content of the host rock in the various mining areas studied indicated a range from less than 1 per- cent to 95 percent. Dust exposures were evaluated for the mines studied on the basis of the quartz content of 234 samples of settled dust collected from mine working areas. Quartz in the settled dust ranged from less than 2 percent to 95 percent. In 55 percent of the mines the settled dust contained less than 20 percent quartz; 39 per- cent of the mines were in the range of 20-50 percent quartz; and 6 percent of the mines were above 50 percent quartz. Particle-size characteristics of 481 samples of airborne dust were determined by optical microscopy, using the oil immersion technique. Median particle diameter was 0.36 micron. Comparison of particle- size characteristics on split samples using both optical and electron microscopy indicated that there was not a preponderance of sub- micron particles below the range detectable by optical microscopy using the oil immersion technique. Little difference was found in size properties of airborne dust produced in the various mining op-~ erations. All determinations indicated ranges of size of particles capable of significant retention in the alveolar spaces of the lungs. The impinger was used as the principal instrument for sampling airborne dust throughout the environmental study. The threshold limit value for industrial dust currently recognized in the United States is based upon determination. of airborne dust by use of this instrument. Furthermore, the impinger has been employed as the dust-assessing instrument in previous studies, dating back more than 25 years, in which the health status of workers has been cor- related with their occupational exposure to dust. Thus, its use in this study permitted comparison of overall findings with results of previous studies. In discussing results of the study, the threshold limit value for siliceous dust adopted by the American Conference of Governmental Industrial Hygienists' as acceptable for occupational exposure was used as a guideline for evaluating the concentrations of airborne dust as determined in the study. The threshold limit values in effect during the period of the study, 1958-61, had been recognized since the mid-1940's. In 1962, however, the American Conference of Gov- ernmental Industrial Hygienists (ACGIH) adopted a new threshold limit value for dusts containing silica. Results of the study were considered in respect to both sets of threshold limit values. Neither of these sets of values were used, however, as sharp lines of dis- tinction between safe and unsafe conditions, but merely as base lines or reference points for comparison of observed conditions. 13 t

The threshold limit value is applicable to the interpretation of daylong integrated, or "weighted average," exposures. Although the threshold limit value is not applicable to single samples, it is convenient to have some means for their classification. Individual samples of airborne dust that could contribute significantly to weighted average exposures that would exceed 1962 ACGIH thresh- old limit values are considered as containing "excessive dust." It is emphasized that the working environment as assessed, repre- sented conditions existing only at the time of the sampling; the re- sults may not be interpreted as representative of past or future conditions. A total of 14,480 impinger samples was collected in underground working places. Of the samples, 75.6 percent were in the range of 0-5 million particles per cubic foot of air (mppcf) ; 19.3 percent in the 5-20 mppcf range; 3.9 percent in the 20-50 mppcf range; and 1.2 percent over 50 mppcf. An additional 357 samples were collected in intake and return airways not considered as occupied working places. Of the grand total 14,837 impinger samples collected under- ground, 1,440, or 9.7 percent, contained excessive concentrations of dust. During the study 789 full-shift weighted average exposures were determined to provide an evaluation of specific underground opera- tions. These determinations applied only to operations, as such, and were not intended to classify the total underground mining popu- lation into various degrees of dust exposure. Based upon the thresh- old limit value that was in effect during the study, 44, or 5.6 percent, of the weighted average exposures exceeded the limit; whereas, on the basis of the 1962 threshold limit value, 104, or 13.2 percent of the weighted averages, were above the threshold limit value. Distribu- tion of weighted averages that exceeded the threshold limit value was not uniform among the 67 mines studied. In relation to the threshold limit value in effect between 1958-61, none of the weighted averages determined in 46 mines exceeded the limit. In relation to the 1962 threshold limit value, 30 mines had no weighted averages above the limit. Among the other mines the number of weighted averages above the threshold limit value var- ied from one per mine to five or more per mine. It was notable that a few mines contributed a major portion of the weighted aver- age exposures that exceeded the limit, indicating need for more overall attention to the dust control programs at these mines. It is emphasized also that in every mine studied some individual im- pinger samples contained excessive dust, indicating need for im- proved dust control at these particular locations. A comparison of dust concentrations obtained at a group of lead- zinc mines in the Western States with dust concentrations obtained 14 in Utah lead-zinc mines in 1939, indicated very substantial improve- ment in dust control during the years intervening between the stud- ies. Dust concentrations for comparable occupations underground had been reduced at least 80 percent, and in some instances as much as 90 percent. Dust concentrations for comparable occupations on the surface had been reduced a minimum of 50 percent, and in some cases as much as 90 percent. MEDICAL STUDY Medical examinations, including medical histories and symptoms, occupational histories and chest roentgenograms were completed on a total of 14,076 currently employed metal mine workers. Two simple pulmonary function tests were performed by each partici- pant unless maximal respiratory exertion was thoughtt by the team physician to be contraindicated. Participation in the survey was on a voluntary basis, but every effort was made by the Public Health Service working with com- pany and union officials to examine all mine employees. The re- sponse varied widely from 50 percent to 100 percent, the overall average being 77 percent for the 50 nonuranium mines included in the study. In nine mines 90 percent or more of the employees participated and, of these, three small mines had 100-percent participation. - Of the 14,076 chest roentgenograms taken, 476 or 3.4 percent were classified as consistent with a diagnosis of silicosis. Of these, 305 were classified as simple silicosis and 171 were classified as com- plicated silicosis. Although the overall prevalence rate for the study was 3.4 percent, the prevalence varied greatly, ranging from 12.9 percent in one mine to zero in seven mines. Thirteen mines had a prevalence. rate of less than 1 precent silicosis while five mines had more than 7 percent. This overall silicosis prevalence rate of 3.4 percent was in marked contrast with rates revealed by earlier studies of silicosis among metal mine workers conducted in various areas of the country be- tween the years of 1914 and 1935. In some of these early studies more than 60 percent of the workers were found to have roentgen- ologic evidence of silicosis, and the prevalence rate seldom was less than 25 percent. Silicosis in the 1958-61 study, for the most part, was confined to the older miners with more than 15 years of metal mining experi- ence. Silicosis was not observed in the chest film of - any miner under 35 years of age, and only seven cases, or 0.4 percent, were found in the 35-39-year age group. Beginning with the 40-44-year 15 707-103 0-6 -3

age group with a prevalence rate of 2.4 percent, there was a mod- erate increase in the rate with each succeeding age group until it tended to level off at about 12 percent for men from 55 to 64 years of age. Of 63 men examined who were 65 years or older, about one-third were silicotic. In relating silicosis to years of work at the mines, no cases occurred with less than 5 years of exposure. Seven cases, or 0.2 percent, oc- curred in workers with 5-9 years of exposure. Thirty-five cases, or 1.4 percent, occurred with 10-14 years of exposure and 58 cases, or 3.0 percent, occurred with 15-19 years of exposure. After 20 years of exposure the prevalence rates rose rapidly in 5-year incre- ments from 7.6 and 12.1 percent up to an average rate of about 17 percent for the four exposure groups working 30, 35, 40 and 45 years and over. A past history of pulmonary tuberculosis was reported quite in- frequently by this employed mining population, being well under 1 percent in the large nonsilicotic population and reaching only 3.8 percent in the silicotic groupings. An evaluation of the case histories and a recheck of the related X-ray film interpretations, moreover, showed that some of these histories could not be proven, as rela- tively few showed definite evidence of past pulmonary infection. On the other hand, actual X-ray evidence of past tuberculous infection as shown by review of all the chest films was found in 0.6 percent of the total nonsilicotic and 5.3 percent of the silicotic population, a low rate as compared with earlier studies of silicosis and tuberculosis. Information on other illnesses was solicited in the medical history which was obtained from each worker examined. A history of pneu- monia was reported by about one-fifth to one-fourth of all employees, increasing slightly with age. Pleurisy was reported somewhat less frequently than pneumonia, especially among the nonsilicotic em- ployees. Bronchitis was reported in a small percentage of all em- ployees, increasing very slightly with age and showed only a small increase in prevalence among the silicotic workers. Asthma was reported in about 4 to 6 percent of all employees in both the silicotic and nonsilicotic groups. Shortness of breath was reported by less than 5 percent of the non- silicotic miners under 35 years of age, but gradually increased to 18.4 percent among miners 55 years of age and over; for the same age groups, shortness of breath was reported twice as often in the silicotic population. It was more prevalent among persons with complicated than those with simple silicosis. There were only slight differences in the prevalence of shortness of breath at the various elevations of the mines. 16 A history of lead poisoning and mercurial poisoning was reported very infrequently as compared with previous studies. If only men working at mines producing lead in the present study are considered, there were 26 workers or 0.7 percent who reported lead poisoning at any time in the past. In the entire study 23 cases of mercurial poisoning were reported by the miners interviewed. Among 309 employees at mercury mines, 7 said they had been affected at some time with mercurial poisoning. So far as possible, each employee was classified according to his principal occupation. This generally was the broad occupational group in which he had spent more than half of his working life in the metal mining industry, regardless of his present occupation which might have changed in recent years.* All men, however, who had spent 10 years or more at the working face of the mines were classified as "faceworkers." Over one-half of all silicosis cases occurred among men who were classified as faceworkers. With 10-19 years of mining employment they showed a silicosis prevalence of 3 percent which rose to 19 per- cent among men working 20 years or longer. Smaller, but signifi- cant silicosis rates were also found among employees with more than 20 years of employment in other underground operations, sur- face maintenance and construction work, and surface mill operations. Surface transportation and miscellaneous surface operations showed very few cases of silicosis. Silicosis was often found at the time of the survey among older surface workers who had previously spent many years underground, but had been transferred to surface opera- tions for various reasons. The mines were divided according to the size of their working population and the prevalence of silicosis in each mine was expressed as the percentage of employees so affected. There was little rela- tionship between the size of the mines and the prevalence of silicosis. Attention was also directed toward the question as to whether the silicosis found at a metal mine was attributable solely to employ- ment at that mine or to• a combination of work experience at several mines. A comparison of silicosis rates for groups of employees who had experience in one mine only, and for groups of employees with experience in two or more mines, showed little difference when similar occupations and periods of exposure were compared. The prevalence of silicosis was slightly greater among 'employees with experience in two or more mines. *This tabulation does not include uranium miners, many of whom could not be classified by principal occupation, and workers at seven iron and lead-zinc mines situated in low free silica limestone formation, who were found to have a negligible prevalence of silicosis in all occupations. 17

The prevalence of silicosis among workers in mines producing iron, lead-zinc, copper, uranium, and miscellaneous commodities was determined by each commodity. There was no great differ- ence in the pattern of silicosis prevalence which could be attributed to the difference in commodities. Men having had 5 or more years of exposure in other dusty trades in addition to the metal mining industry, were excluded from the study group because of the possibility that the prevalence of silico- sis in such a mixed exposure group would be unduly influenced by other dusty employment. Among this mixed exposure group it was noted that, when the total duration of employment in dusty trades was approximately the same, the silicosis prevalence rates were very similar to that of metal mine workers included in the study. It thus appears that the exclusion of 671 workers from the study group because of previous exposure in other dusty trades did not appreciably alter the results of the study. In general, great advances have been made by the metal mining industry in controlling the silicosis hazard, beginning about the mid- 1930's. The effects of these dust control measures would not have been evident until many years later, because of the reservoir of miners exposed to dust prior to this period. Fortunately the 1939 Study of Non-Ferrous Metal Mine Workers in Utah presented data which may be contrasted with data from a group of 12 western lead-zinc mines investigated during the present survey (1958-61). The overall prevalence of silicosis was found to be 40 percent lower than in the earlier study. Even more striking was the reduction of 80 percent in the silicosis rate among persons employed in the mines less than 10 years and 73 percent for the group employed 10-19 years. The environmental data of the 1958- 61 survey also showed a very favorable trend in reduction from the at- mospheric dust levels found in the 1939 Utah survey. An analysis was also made of comparative prevalence rates within the 1958-61 study for silicosis among metal mine workers who had worked in the industry only since 1935 or later, and those who had some portion of their employment before 1935 as well as later, excluding those at seven mines located in low free silica limestone formations. This permits some comparison of silicosis prevalence rates among workers within this study who had substantial expo- _ sure before dust control measures became widely used, and those employed only during the subsequent 25 years or so. Among the relatively small group of miners with some mining experience be- fore 1935, but who had worked in metal mines a total of only 10-14 years, the silicosis rate was 6.1 percent; a group of 1,818 miners who had worked the same number of years but only in 1935 or later had a rate of 1.5 percent. Figures for persons with 15-19 years in metal mines showed 8.3 percent with silicosis in the pre-1935 group and 3.3 percent with silicosis in the after-1935 group. After 20-24 years in metal mining, men with experience prior to 1935 had a silicosis prevalence of 12.7 percent compared with 7.2 percent for miners with experience during or after 1935. This is a trend similar to that shown in comparing the present study with the 1939 Utah study. A special study was made of the records of a group of metal mine workers from one iron mine which had a continuous silicosis con- trol program underway since 1933. Beginning with the records of that year it was possible to examine X-ray films of all workers then employed and others as they were hired and to follow the entire group year by year as serial X-ray films were taken throughout the 28-year period. Complete work records were also . available for analysis. Among the 1,293 men included in this study, 410 had worked be- fore 1933, and thus had been exposed before the improvement in the mine environment. Silicosis was found at the time of first X- ray examinations in 1933 in 83 men, and 16 men who were negative in 1933 developed silicosis later. Among the 883 men who were first employed after the control program began in 1933, there was not a single case of silicosis which developed even in a substantial group with more than 20 years of exposure. Only 6 percent of the men with silicosis showed any progressive change in the disease as they continued mining employment. CONCLUSIONS Prior to the beginning of this study it was known that in recent years substantial numbers of men who had been employed in the metal mining industry were awarded disability compensation for silicosis. With silicosis, thus known to occur in the industry the study was designed to determine the prevalence of silicosis among the work force of the industry, to define present day environmental conditions, and to seek answers to the questions : 1. Are the cases presently occurring the result of pre-control exposure, in view of the long latent period for the develop- ment of silicosis ? 2. Are they the result of failure to apply dust controls univer- sally 1 3. Are cases occurring because of inadequacy of standards for acceptable levels of dustiness in use since 1935 ? 18 1 19

THE WORKING ENVIRONMENT 1. Each mining company should maintain a dust monitoring program conducted or supervised by a person competent in the techniques of dust sampling and interpretation of results. a. For determining levels of exposure, dust samples should be taken in the breathing zones of workmen. b. The program should be conducted in such a manner that it will detect changes in environmental condi- tions and promptly locate conditions in need of correction. c. Accurate and complete records of dust conditions should be kept. These should be tabulated, analyzed, and reported to a responsible level of management at regular intervals. 2. Proper methods of dust control should be initiated promptly when the need is discovered. a. Adequate ventilation by mechanical means should be provided at all working places. b. Recirculation of air should be held to a minimum consistent with good mining practice. c. All ore and broken rock should be thoroughly wetted to reduce dust during subsequent handling operations. d. All dust control devices and materials handling equipment, both underground and on surface, should be frequently inspected and maintained in proper working condition to limit to the lowest prac- ticable level the generation or dispersion of dust. e. Men should not be permitted to reenter a workplace after blasting until sufficient time has elapsed' for dust and gases to be reduced to a safe level. 3. Workers should be informed of the dust hazards associated with their job, the methods employed for the control of dust exposure, and instructed in good work procedures to minimize dust dispersion and in the proper use of equip- ment. All employees should give their full cooperation in helping to maintain an effective dust control program. 4. Mining companies should request, whenever necessary, the assistance of the Bureau of Mines or other qualified agen- cies in instituting and evaluating dust monitoring and dust control programs. MEDICAL SERVICES 1. Medical examinations a. All men entering the metal mining industry should have a preplacement physical examination includ- ing a technically satisfactory X-ray film of the chest, b. Periodic physical examinations including an X-ray chest film should be performed annually on under- ground workers, and biennially on surface workers in order to detect early silicotic changes, evidence of active pulmonary tuberculosis, or other pulmo- nary disorders. c. No worker should be denied employment for which he is trained because of simple silicosis but rather he should be permitted to work in an environment with effective dust control that would be safe both for him and his fellow workers. d. Any employee found to have active pulmonary tuber- culosis should be placed under treatment and should not be permitted to resume employment at a dusty occupation: Workers with minimal, arrested, or healed reinfection tuberculosis should be allowed to continue to work, but should observe the same precautions as the man with simple silicosis. Healed primary tuberculosis does not seem to be a con- traindication for employment in a dusty trade. 2. Health supervision and practices : Although the following recommendations were not developed directly as a result of the 1958-61 silicosis study, they represent standards of good practice and for the most part result from previous studies of the Public ublic Health Service and the Bureau of Mines. a. Close medical supervision is desirable for all em- ployees in order to prevent or control ordinary res- piratory infections, and other common illnesses. During the course of medical examinations and day- to-day visits to the hospital or clinic, employees should be advised on various aspects of hygiene and preventive medicine. b. Employees returning to work after an absence due to an injury or illness should be cleared through the medical service to insure their being physi- cally fit for their jobs, thereby protecting their own and their coworkers' health and safety. 23

c. The mine physician, whether full or part time, should be familiar with the various mining operations and their potential health hazards and should make pe- riodic sanitary inspections. The official health de- partment should be informed of any conditions requiring technical or specialized assistance. Also, the physician should be in close communication with plant safety and dust control personnel. d. All mines should provide conveniently located change houses with facilities for hot and cold shower baths, and lockers and drying rooms for work and street clothing. e. Records of all absenteeism due to illness or injury should be kept by the mine medical department indi- cating the course, nature, duration, and outcome of such disability. These records should be tabulated and analyzed in a monthly report to serve as a basis for study and corrective measures to promote em- ployee health and minimize such absenteeism. I 24 1 25