Silicosis among Foundry Workers

Transcription

1 American Journal of Epidemiology Copyright O 1996 by The John3 Hopkins University School of Hygiene and Public Health All rights reserved Vol. 144, No. 9 Printed in U.S.A Silicosis among Foundry Workers Implication for the Need to Revise the OSHA Standard Kenneth D. Rosenman, 1 Mary Jo Reilly, 1 Carol Rice, 2 Vicki Hertzberg, 2 Chih-Yu Tseng, 2 and Henry A. Anderson 3 To evaluate the risk of pneumoconiosis among workers in a Midwestern automotive foundry, medical records and silica sand exposure data were analyzed for 1,72 current and retired employees with at least 5 years of employment as of June Approximately half of these employees had worked at the foundry for 2 or more years. Sixty workers were found to have radiographic evidence of pneumoconiosis. Twenty-eight workers had radiographs consistent with silicosis, of which 25 were consistent with simple silicosis and three with progressive massive fibrosis. The prevalence of radiographic changes consistent with silicosis increased with: number of years worked at the foundry (6 for 2-29 years and 12 for 3 or more years); cigarette smoking (12.2 among smokers with high silica exposure vs. 4.4 among never smokers with high silica exposure); work area within the foundry (cleaning room, core room, mold area, core knockout); and quantitative silica exposure ( of workers at the current Occupational Safety and Health Administration (OSHA) standard and of workers above the OSHA standard). In addition, the odds of developing radiographic changes consistent with silicosis were Increased for African Americans (odds ratio = 2.14, 95 confidence interval 5-5.6) in comparison with whites. (The risk was similar when silica exposure was equal, but African-American workers on average had greater exposure to silica, despite having a similar duration of work as white workers.) Another eight workers had radiographic evidence of asbestosis, and 24 had pleural plaques. These asbestos-related changes were not associated with increasing exposure to silica but rather were associated with being In the maintenance department and performing repair work. After controlling for cigarette smoking, race, and exposure to silica at another job besides the foundry, the authors found a 1.45 increased risk of developing a radiograph consistent with silicosis after 2 years of work at the current OSHA standard, and a 2.1 increased risk after 4 years of work at the current OSHA standard. On the basis of these findings, the authors recommend maintaining silica air levels no higher than the exposure level of.5 mg/m 3 recommended by the National Institute for Occupational Safety and Health. Am J Epidemiol 1996; 144:89-9. asbestosis; occupational exposure; occupational hearth; pleurisy; silicosis; smoking; tuberculosis Lung disease in workers secondary to exposure to silica has been recognized since antiquity (1). Despite awareness of the condition and its cause, individuals continue to develop silicosis, an interstitial fibrosis of lung parenchyma caused by exposure to silica. Overexposure to silica continues to be documented in American industry, particularly foundries (2, 3). Received for publication October 13,1995, and in final form June 1, Abbreviations: Cl, confidence interval; NIOSH, National Institute for Occupational Safety and Health; OR, odds ratio; OSHA, Occupational Safety and Health Administration. 1 Department of Medicine, College of Human Medicine, Michigan State University, East Lansing, Ml. 2 Department of Environmental Hearth, College of Medicine, Unlversrt of Cincinnati, Cincinnati, OH. 3 Environmental and Occupational Health, Division of Health, Wisconsin Department of Hearth and Social Sciences, Madison, Wl. There is a known exposure-response relation between cumulative exposure to silica and the development of radiographic lung changes (4). Previous exposure-response analyses, however, have not been able to completely explain who will develop silicosis (5, 6). Other factors suggested to be important in the development of the disease include cigarette smoking, other lung disease, immunologic changes, and genetic background. The association between silicosis and tuberculosis is well documented in the medical literature (7). In the present study, exposure estimates were used to identify silica levels that increased the risk of developing the radiographic changes associated with silicosis. The study was carried out in a Midwestern gray iron foundry that has been producing automotive engine blocks since

2 Silicosis among Foundry Workers 891 MATERIALS AND METHODS The cohort comprised both current and retired employees. Current employees were defined as persons who were current hourly employees as of June 4, 1991, and who began working at the foundry prior to January 1,1986. Current salaried employees who were former hourly employees were also included. Retirees were defined as former hourly employees who began working at the foundry before January 1, 1986, had retired from the foundry, and were receiving a pension as of July 2, Retired foundry employees are eligible to receive a pension after 25 years of employment or at the age of 6 with at least 5 years of employment, or if they have a medical disability. Where it was available, information on work history, medical history, current health, and cigarette smoking status was collected for each member of the cohort. The most recent chest radiograph was also obtained. Furthermore, environmental sampling data, maps, and other relevant historical exposure information was collected. Data sources included medical and personnel records, management and union industrial hygiene files, and personal interviews with employees in the cohort. Information abstracted from personnel records included a personal identifier, beginning and ending dates for each job (i.e., task), department code, job classification number and title, plant, and employment history prior to employment at the foundry. Information on usual shift worked was not available. Current employees were interviewed at work on paid time by two interviewers. Telephone interviews were conducted by three medical school students with retirees and with current employees who could not be reached at work. Three federally certified "B" readers interpreted chest radiographs independently of one another and were unaware of previous interpretations. When no chest radiograph was available e.g., when there was no radiograph at the foundry, a radiograph was only available on 16-mm microfilm, a radiograph was taken after less than 1 years' duration of work at the foundry, or the available radiograph was unreadable an attempt was made to locate a current chest radiograph elsewhere. The standard form (form CDC/ NIOSH (M) 2.8, revised 4/8) for recording radiographic interpretations was used. Furthermore, radiographs were not segregated according to any severity grouping when sent to each "B" reader for interpretation. Two of the three "B" readers each interpreted all of the chest radiographs. The third reader interpreted all retiree radiographs and the radiographs of current and salaried employees when either of the other two readers had interpreted a radiograph as positive. For a radiograph to be considered positive, at least two of the three "B" readers must have indicated a positive film with similar positive findings. A radiograph was considered normal if the only pleural abnormality detected was a blunted costophrenic angle. All available medical information for individuals with a consensus interpretation of pneumoconiosis was reviewed. Generally, this consisted of the questionnaire completed by the employee. One individual with a consensus interpretation consistent with pneumoconiosis had a lung biopsy showing sarcoidosis; this person was considered in the analyses to have a normal chest radiograph not showing pneumoconiosis. Through the use of historical industrial hygiene information and the results of detailed interviews with five long-term employees in primary operations (cores, foundry, cleaning), the plant's layout, raw materials, production process, production volume, work practices, environmental control systems, numbers of employees, and descriptions of job titles and assignments were documented over time. Management and union personnel reviewed the timeline, and their comments were incorporated into the final version. Industrial hygiene data collected over four decades were linked to the foundry's job codes after being grouped into exposure zones based on the physical location of the operations and the similarity of tasks performed (8). Respirators were used sporadically by some employees. During the years when most of the subjects' exposure occurred, there was no enforced use of fittested paniculate respirators; therefore, exposures were calculated on the basis of the air sampling data, unadjusted for any reduction due to the use of a respirator. Early silica exposure data obtained using midget impingers provided estimates of dustiness at a specific task (i.e., mold-making) or in a certain area (i.e., grinding room). To calculate the weighted daily exposure estimate, we identified the proportion of time spent in a task/area and used the sum over the entire day of the product of each task/area of exposure and duration. We converted weighted total dust exposure from impinger data to an estimate of silica exposure by multiplying it by the average percentage of quartz in bulk samples; we obtained an estimate of exposure in mass units (mg/m 3 ) by multiplying by.9 (1 million particles per cubic foot (mppcf) =.9 mg/m 3 ) (9). For each cohort member, cumulative exposure was calculated by multiplying the duration from first employment to last employment in each foundry job (in days) by the estimated exposure, summing over all jobs. Average exposure was derived by dividing cumulative exposure by total number of days employed.

3 892 Rosenman et al. A natural logarithmic transformation was made before data were used in analyses with continuous variables. For each categorical analysis, we planned (a priori) to form categories for which the upper bound of each group was 3-4 times the lower bound, and to require that at least 5 percent of the study subjects be in the category with the highest exposure (9, 1). In addition, the mean value of each group had to be statistically different {p <.1) from the other means, as tested by analysis of variance. Asbestos had been used in curtains and gloves and as pipe lagging and boiler covering. There were no reports or documentation of asbestos contaminating raw materials or being used in the cupola, core room, or other production processes. No samples with which to quantify airborne asbestos exposure were found in the industrial hygiene records. Removal of asbestos from mechanical piping and fan housings was carried out by contractors between 1987 and Contingency table methods with chi-square statistics were used to examine radiologic abnormalities (11). Logistic regression modeling was carried out in a stepwise, hierarchical fashion using the PROC LOGISTIC procedure in SAS (SAS Institute, Inc., Cary, North Carolina). Variables entered into the model included race, pack-years of cigarette smoking, silica exposure outside of the foundry, and a variety of exposure indices, as well as interactions among the main effects. The initial variables selected for consideration were those showing univariate significance associated with the outcome. The exposure variable resulting from this stepwise modeling was cumulative exposure to silica up to the time of the first positive or latest negative radiograph. Logistic regression analysis does not allow for a fitted probability of zero. In the population under study, there was no individual included who had zero exposure. RESULTS The total number of individuals in the cohort was 1,72: 549 current production workers, 497 retired production workers, and 26 current salaried but former production workers. The majority of the workers were African-American (57.7 percent). Almost all of them were male (96.8 percent), and their average age in 1992 was 58.9 years. The employees had worked an average of 19.2 years at the foundry, and, on average, 28.3 years had elapsed since they were first hired at the foundry. Almost all of the current workers had radiographs (96.5 percent); fewer retirees had them (79.9 percent). The percentages of current employees and retirees who completed the questionnaire were similar (response rates were 77 percent and 72 percent, respectively). Approximately 8 percent of workers for whom questionnaires or medical records were available had smoked cigarettes. Among those for whom questionnaires were available, 33.8 percent of current workers, 23.3 percent of retirees, and 29.5 percent of the entire group were still smoking cigarettes. Because of the incompleteness of available medical records, it was possible that an additional 17.7 percent of current workers, 6 percent of retirees, and 12.9 percent of the entire group who had ever smoked cigarettes could still have been smoking. In the 71 workers contributing to the final logistic regression analysis, the mean number of pack-years of smoking was 25.3 (standard deviation, 27.2), with a minimum of pack-years and a maximum of Approximately 5 percent of the subjects had ever been told that they had tuberculosis. Another 3.2 percent had had a positive skin test for tuberculosis. An additional 4. percent of the workers reported that someone in their family had had tuberculosis. A slightly higher percentage of the retirees had ever had tuberculosis (5.1 percent vs. 4.1 percent), a positive skin test (3.7 percent vs. 2.9 percent), or a family member with tuberculosis (4.8 percent vs. 3.3 percent) in comparison with the current employees. Approximately 65 percent (64.6 percent) of the cohort had been employed at another job where they might have been exposed to asbestos and/or silica. A larger percentage of the retiree group had probable previous exposure to asbestos and/or silica than the current employee group (85.2 percent vs percent). Overall, 59.3 percent of the cohort had had other possible silica exposure in addition to that incurred at the foundry, and 19.2 percent had had possible additional asbestos exposure outside of the foundry. For the 395 (73.9 percent) employees with available data on duration of exposure to silica outside of the foundry, the median duration of outside exposure was less than 3 years, and it was less than 1 percent of the total number of years of possible exposure to silica. No radiograph was available for 17 individuals. For another 13 individuals, the radiograph was considered unreadable by two or more "B" readers. Among the remaining 952 workers who had chest radiographs that could be interpreted, 6 people had radiographs that were interpreted by at least two "B" readers as showing evidence of pneumoconiosis (table 1). Eight had radiographs with linear opacities in the lower lobes, which is typical of asbestosis (profusion category 1), and 24 had radiographs with pleural thickening without parenchymal changes, which is consistent with asbestos exposure. Two people with radio-

4 Silicosis among Foundry Workers 893 TABLE 1. Consensus Interpretations (by at least two "B" readers) of chest radiographs for both parenchymal and pleural abnormalities considered secondary to asbestos or silica exposure, 1991 Current employees* No. No. Retirees No. Parenchymal abnormafities Profusion category 5/1 (no pleural changes) Profusion category 1/ Rounded opacities Basilar linear opacities Pleura) abnormalities only (1t) (2) (1.8) (.4) (18) (6) (4.5) (1.5) (28) (8) (2.9) (*) of radtographs Includes 26 salaried employees who were former hourly employees. One of the 26 salaried Individuals had rounded parenchymal opacities. f Numbers in parentheses, subentries. $ One current employee and one retired employee had both parenchymal and pleural abnormalities. Twenty chest radiographs were unavailable or unreadable for current hourly employees, and 1 chest radiographs were unavailable or unreadable for retirees. graphs with linear opacities in the lower lobes also had pleural thickening, and they were included in the asbestosis group for the analyses (table 1). Among the 28 people with rounded opacities in the upper lobes, which is the typical radiographic finding of silicosis, 25 had radiographic evidence of simple silicosis and three had radiographic evidence of progressive massive fibrosis. Among those with radiographs consistent with simple silicosis, 2 workers had category 1 profusion, three had category 2 profusion, and two had category 3 profusion. In profusion category 1, 1 workers were in profusion category 1/, nine were in 1/1, and one was in 1/2. For 23 of the 28 (82 percent) radiographs that were consistent with silicosis, three of the eight (37.5 percent) radiographs that were consistent with asbestosis, and nine of the 24 (37.5 percent) that were consistent with pleural plaques, all three "B"-readers thought that the radiographs were abnormal. As per the protocol, for the other abnormal radiographs, at least two of the three "B"-readers thought they were abnormal. Table 2 shows that most of the individuals with an abnormal chest radiograph had first been hired 2 or more years previously and had worked at the foundry for 2 or more years. There was a greater percentage of asbestos-related radiographic changes than silicarelated changes among the workers with a shorter duration of work at the foundry. Four of eight (5 percent) workers with radiographs consistent with asbestosis, nine of 24 (37.5 percent) with radiographs suggesting pleural plaques, and none of the workers with radiographs suggesting silicosis had worked 5 percent or more of their time in maintenance. The remaining four workers with radiographs consistent with asbestosis (5 percent), another nine TABLE 2. Consensus interpretations (by at least two "B" readers) of chest radiographs for both parenchymal and pleural abnormalities considered secondary to asbestos or silica exposure, by latency period from date of first hire and duration of employment at the foundry under study, 1991 Latency (years) Duration (years) of participants of participants with radiographs viewed as positive Silicosis (^1/ rounded opacities) Asbestosis ( 1/ linear opacities) Pleural plaques f 756 * s do not equal 952 because of incomplete work histories, t One individual had an incomplete work history, and thus duration could not be calculated. (37.5 percent) with suggested pleural plaques, and 15 of 28 (53.6 percent) with suggested silicosis had ever worked in maintenance. All but one of the workers with a radiograph consistent with silicosis had worked 5 percent or more of their time in a high-silica-use area: 13 (46.4 percent) in the clean room; eight (28.6 percent) in the core room; four (14.3 percent) in the mold area; and two (7.1 percent) in the core knockout

5 894 Rosenman et al. area. The remaining worker with a radiograph consistent with silicosis had primarily been a truck driver. Table 3 shows the four silica exposure categories formed a priori for cumulative and average exposure estimates. The estimated daily average for cumulative exposure category 2 approximates the recommended exposure limit from the National Institute for Occupational Safety and Health (NIOSH) of.5 mg/m 3 (12); and the average exposure for cumulative exposure category 3 is slightly greater than the current Occupational Safety and Health Administration (OSHA) limit of.1 mg/m 3 (13) for dust composed of pure silica. For the average silica exposure data, the category 2 mean exposure level is.9 mg/m 3, which is approximately equal to the OSHA standard. The percentage of individuals with radiographs consistent with silicosis was greater among the workers with higher mean and cumulative silica exposures (table 4). No association was found with radiographs consistent with asbestosis and/or pleural plaques. The increased prevalence of radiographs suggesting silicosis with increased exposure was similar and significant when analysis was restricted to workers who had had no exposure to silica other than that incurred at the foundry being studied. Approximately 7 percent of all workers who had ever smoked cigarettes during their lifetime had evidence of pneumoconiosis on their most recent chest radiograph. Those who had developed radiographic changes consistent with pneumoconiosis comprised almost 6 percent of the workers who were current smokers, 8.1 percent of the workers who were former smokers, 7.3 percent of those who had ever smoked, and 4.2 percent of those who had never smoked. Table 5 shows that after controlling for duration of work, more smokers than nonsmokers had radiographic evidence consistent with asbestosis or silicosis. Smokers and nonsmokers had similar prevalences of pleural plaques. The results for latency period since first being hired were similar to those for duration (data not shown). When smoking status was controlled, the percentage of individuals with radiographs consistent with silicosis continued to be greater among the workers with the highest silica exposures (table 6). Of the 32 individuals who had ever been told that they had tuberculosis and had a radiograph available, 15.6 percent had radiographs consistent with silicosis, none had radiographs suggesting asbestosis, and 6.3 percent had radiographs suggesting pleural plaques, as compared with 2.8 percent,.9 percent, and 2.1 percent, respectively, of the 75 individuals who had not had tuberculosis. Twelve of the workers with chest radiographs consistent with pneumoconiosis had an unknown tuberculosis history. Of the three individuals with progressive massive fibrosis, one had been told that he had tuberculosis, one had had a recent negative tuberculin skin test, and one had never been tested. The individuals with radiographic changes consistent with silicosis had a sixfold increased risk of tuberculosis in comparison with the rest of the workers (odds ratio (OR) = 6.34, 95 percent confidence interval (CI) ). Table 7 indicates an increased prevalence of chest radiographs consistent with silicosis among workers with a history of tuberculosis, controlling for mean and cumulative silica exposure. White workers had a higher prevalence than African Americans of radiographs consistent with asbestosis (1.5 percent vs..4 percent) and pleural plaques (3.5 percent vs. 1.8 percent), and African-American workers had a higher prevalence of radiographs consistent with silicosis (3.8 percent vs. 1.8 percent). The differences remained after controlling for latency (for a period of >2 years from first hire, the prevalence of radiographs suggesting silicosis was 6.2 percent among blacks vs. 2.9 percent among whites) or duration of foundry employment (for >2 years worked, TABLE 3. Numbers of participants, mean values, and daily averages for silica exposure categories formed a priori, 1991 Exposure category Range No. of participants Mean Cumulative silica exposure (mg-days/mj) > >72-2, , >2, ,183.8 Dally average* Average silica exposure (mg/m 3 ) > > > * Calculated assuming 286 workdays per year over a 4-year working lifetime.

6 Silicosis among Foundry Workers 895 TABLE 4. Consensus Interpretations (by at lent two "B" readers) of chest radiographs for both parenchymaj and pleural abnormalities considered secondary to asbestos or silica exposure, by mean and cumulative silica exposures, 1991 Mean exposure < > >.45 (mg/m») oj participants of participants wtth radiographs viewed as positive Silicosis Asbestosls (;>1A) rounded opacities) Dnear opacities) Pleural plaques *.976 TABLE 5. CConsensus lnterpreuiuuii> \uy ai RWI mu "B" readers) of chest radiographs consistent with pneumoconiosla, b 1 and smoking status, 1991 Duration (years) o-e ^3 Totatt Cumulative exposure (mg-days/m*) < >72-2,16 >2,16 One Individual had an incomplete work history, and thus duration could not be calculated. Smokers. T * SIBcosis Nonsmokers Smoke re * number of participants. t Percentage of participants with radiographs viewed as positive. t Smoking status was unknown for 96 individuals. the prevalence of radiographs suggesting silicosis was 8.3 percent among blacks vs. 4. percent among whites). Durations of work for African-American and white workers are shown in figure 1, and mean silica exposures for African-American and white workers are shown in figure 2 (21.5 percent of black workers vs. 6 percent of white workers (OR = 4.3, 95 percent CI ) were in the highest exposure category). After controlling for exposure to silica, the prevalences of radiographs consistent with silicosis were similar among African-American and white workers. For cumulative exposures greater than 2,16 mg-days/ m 3, the prevalence of radiographs consistent with silicosis was 13 percent among white workers versus 9.5 percent among African-American workers (OR = Asbestoeis Nonsmokere Smokers ».9725 Pleural plaques Nonsmokers , 95 percent CI ). For mean exposures greater than.45 mg/m 3, the prevalence of silicosis was 8.7 percent among white workers versus 5.9 percent among African-American workers (OR = 1.5, 95 percent CI.-8.85). Table 8 presents the odds ratios for developing a radiograph consistent with silicosis according to different silica exposure levels, controlling for packyears of cigarette smoking, race, and exposure to silica at any facility other than the foundry being studied. The average, minimum, and maximum levels of exposure entered into the model were 1,297,.72, and 7,776 mg-days/m 3, respectively. Since all individuals had some nonzero estimated level of exposure, extrapolation beyond the range of the data led to an inaccu-

7 896 Rosenman et al. TABLE 6. Consensus Interpretations (by at least two M B" cumulative silica exposure and smoking status, 1991 readers) of chest radiographs consistent with pneumoconlosia, by Mean exposure (mg/m3) < > >.45 * * Smokers. t Silicosis Nonsmokars Smokers.3274 Asbestosis Nonsmokers Smokers Pleura! plaques Nonsmokers Cumulative exposure (mg-days/m 3 ) < >72-2,16 >2,16 * * number of participants. t Percentage of participants with radiographs viewed as positive. t Smoking status was unknown for 96 individuals. TABLE 7. Consensus Interpretations (by at least two "B" readers) of chest radiographs consistent with slllcosis, by mean and cumulative silica exposure and history of tuberculosis, 1991 Mean exposure (mg/m») < > >.45 Cumulative exposure (mg-days/m*) < >72O-2,16 >2,16 History of tuberculosis Tota " I t No history of tuberculosis * number of participants. t Percentage of participants with radiographs viewed as positive. rate estimate of.99 percent at zero exposure. Within the range of the data, the model fitted well. Among the observations in the model, there was concordance of percent between predicted response and observed response. DISCUSSION This study evaluated the effect of long-term work in a foundry on the development of radiographic lung changes consistent with pneumoconiosis. Foundries are known to be high-risk areas for the development of silicosis. We found that approximately 6 percent of the workers with 2-29 years of work in the foundry and 12 percent of the workers with 3 or more years of work had chest radiographs consistent with silicosis (table 2). A study conducted by the US Public Health Service in 1948 and 1949 among 19 ferrous foundries in Illinois found that 9.2 percent of the workers had pulmonary fibrosis overall and that 25.8 percent had pulmonary fibrosis after 2 or more years of work (14). More recent results from a gray iron foundry in Pennsylvania found radiographs consistent with silicosis in eight of 15 workers (53 percent) with 2 or more years of work who had had radiographs taken between 1977 and 198 (15). The higher rate of radiographic changes in the Pennsylvania foundry may have been secondary to the use of silica flour to speed the release of cast iron pipe from the mold. In contrast, our results show a greater prevalence of radiographs suggesting silicosis than a report from Canada (16), where less than 2 percent of Ontario foundry workers with 4 years of exposure were reported to have de-

8 Silicosis among Foundry Workers 897 e o Duration (years) ^H African American Hi White FIGURE 1. Duration of work for African-American and white workers at a US gray iron foundry as of June * < > >.45 Mean Silica Exposure (mg/m 3 ) Hi African American Hi White FIGURE 2. Mean silica exposures for African-American and white workers at a US gray Iron foundry as of June veloped radiographic changes. The Ontario foundry data were derived from a province-wide surveillance system for silicosis for the years We were unable to identify any other recent surveys of foundry workers in the medical literature. The risk of having radiographic changes consistent with silicosis was not equally distributed among the jobs and departments within the foundry. The primary job assignments of workers whose radiographs suggested silicosis were 1) working in the cleaning room removing sand from the metal products, 2) making cores, 3) making molds, and 4) performing core knockout. African-American workers had twice the risk of having radiographs consistent with silicosis in comparison with white workers. This effect was secondary to more African-American workers' having had greater exposure to silica despite a similar duration of work in the foundry (figures 1 and 2). When exposure to silica was controlled for in the analysis, African- American and white workers had similar prevalences of radiographs consistent with silicosis.

9 898 Rosenman et al. TABLE 8. Odds ratios for silicosia according to varying silica exposure levels (mg/m 3 ) after controlling for pack-years of cigarette smoking, race, and silica exposure at another Job besides the foundry under study, 1991* Time-weighted average of ftftlm (TnYifflirn (mflfm*) ! I2.3.25! > ' C).O4 2-year exposuret 95 CIS ^» OR year exposure^ 95 Cl Logistic regression coefficient: p = 6.46 x 1-* ±1.31 x 1-4 (standard error); p<.1. t Logistic regression analysis based on cumulative silica exposure, assuming a 2-year work exposure, j Logistic regression analysis based on cumulative silica exposure, assuming a 4-year work exposure. OR, odds ratio; Cl, confidence interval. At the NIOSH recommended exposure limit of.5 mg/m 3, there was a.3- percent prevalence of radiographs consistent with silicosis (table 4). At the OSHA standard of.1 mg/m 3, there was a percent prevalence of radiographs consistent with silicosis (table 4). At levels above the current OSHA standard, prevalence rates of radiographs consistent with silicosis were percent (table 4). Similar results were found when individuals who might have been exposed to silica at other workplaces besides the foundry under study were excluded from the analyses; this indicates that the lack of exposure information for other employment sites did not seriously bias the exposure-response relation. After controlling for pack-years of cigarette smoking, race, and silica exposure at another job besides the foundry, working for 2 years at the current OSHA standard increased the risk of developing a radiograph consistent with silicosis 1.45 times. At 4 years, the risk was increased 2.1-fold. This would be equivalent to approximately 2 percent of the workforce developing radiographs consistent with silicosis after 2 years and 3 percent after 4 years. Reducing the allowable silica level to the NIOSH recommended exposure limit showed a 1.2 increased risk of radiographs consistent with silicosis for 2 years of silica exposure and a 1.45 increased risk for 4 years of exposure (table 8). This would be equivalent to approximately 1 percent of the workforce developing radiographic changes consistent with silicosis in 2 years and 2 percent in 4 years. Besides the general uncertainty of developing exposure indices from incomplete historical data, we are aware of two factors that might suggest the silica exposure indices in our analysis of exposure-response to be overestimates. Exposure was calculated to the last day of work or the time at which the radiograph was taken, whichever was earlier. However, we did not know the year in which a worker's lungs first showed silica-related abnormalities, and this date could have been earlier than the date of the radiograph used for interpretation. Additionally, respirator use, which would have reduced exposure, was not included in the exposure metrices. Controlling for exposure, we found an association between cigarette smoking and silicosis (tables 5 and 6). One previous report of an absence of an association between cigarette smoking and the development of radiographic changes consistent with silicosis did not control for the level of silica exposure (15). Another study found no association between smoking and the progression of silica-related radiographic changes in workers already diagnosed with silicosis (17). Still another found no association between the degree of profusion of opacities and years of cigarette smoking (6). Both the Hughes et al. (17) and Theriault et al. (6) studies found an increased prevalence of radiographs consistent with silicosis among cigarette smokers, as we did in our study; however, neither of those studies controlled for level of silica exposure. Rice et al. (18) showed that a history of cigarette smoking decreased the latency time to disease development. Data from the province-wide Ontario surveillance system for silicosis reported that, after controlling for year of first exposure, smokers were 1.45 times more likely to develop radiographs consistent with silicosis than

10 Silicosis among Foundry Workers 899 nonsmokers (16). We suggest that, similar to the case with asbestos exposure (19), cigarette smokers are more likely to manifest the radiographic changes associated with silicosis. A similar association between radiographic changes and smoking has not been found among coal- or clay-exposed workers (19). Controlling for exposure, we found an association between radiographic changes consistent with silicosis and a history of tuberculosis (table 7). Individuals with silicosis are known to be at increased risk of tuberculosis (7). In addition to the radiographic changes of silicosis, there were 32 individuals with radiographic changes consistent with asbestos exposure. There was no association between basilar linear or pleural changes and either increased duration of work at the foundry (table 2) or measures of silica exposure (table 4). The determination of whether changes on the 6 radiographs suggestive of pneumoconiosis were consistent with silica exposure or asbestos exposure was made without any knowledge of the 6 workers' exposure history. The absence of an association between measures of silica exposure and basilar linear changes and/or pleural changes is consistent with the classical descriptions of silicosis and asbestosis. In addition, all of the workers with basilar linear changes and all but six of the workers with pleural thickening had their primary assignment in maintenance or had worked in maintenance, where asbestos exposure was likely. There has been one report on silica-exposed workers in the Vermont granite sheds that has described linear radiographic changes (2). However, that report did not account for concomitant asbestos exposure. Our data are consistent with those from other studies of workers in industrial facilities where asbestos has been used as pipe insulation (21). The incidence of asbestos-related radiographic changes, predominantly pleural disease, was increased, particularly among maintenance personnel. There was no evidence to suggest that the linear parenchymal changes and/or pleural changes seen among these foundry workers were secondary to silica exposure. In summary, our data show that the current OSHA standard is not sufficiently low to protect workers against the development of radiologic evidence of silicosis. Other findings include an association between cigarette smoking and radiologic evidence of silicosis and an association between tuberculosis and radiographic changes consistent with silicosis. Pleural radiographic changes and basilar linear parenchymal changes were associated with employment in jobs likely to involve exposure to asbestos and not likely to be related to silica exposure. ACKNOWLEDGMENTS This project was wholly funded by the United Auto Workers-Chrysler Corporation National Joint Committee on Health and Safety. 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