Respiratory Health and Silica Exposure of Stone

Respiratory Health and Silica Exposure of Stone Carvers in Thailand TANONGSAK YINGRATANASUK, MPH, MS, NOAH SEIXAS, PHD, SCOTT BARNHART, MD, MPH, DREW BRODKIN, MD, MPH

A cross-sectional study of dust exposures and health outcomes was conducted in a stone-carving company in Thailand. 147 respirable dust samples were collected and 97 subjects participated. Exposure indices were constructed and health outcomes, including respiratory symptoms, pulmonary functions, and chest radiographs, were assessed. Severities of employees’ current exposures to quartz were 0.5–8.8 times the ACGIH-TLV, depending on job and site. Durations of exposures ranged from 4 months to 30 years. The prevalence of silicosis (profusion grade ≥ 1/0) was 2%. Pulmonary tuberculosis was also detected in 4%. Linear regression analyses revealed decreased lung function in workers with longer work durations (p < 0.05), regardless of age, sex, height, and smoking status. No clear association was seen between cumulative exposure metrics and indicators of silicosis. Elevated silica exposure levels indicate an ongoing risk of silicosis in this industry. Exposures were increased by the use of grinding tools with no ventilation and by proximity to other workers. However, because the number of workers with dust-exposure histories was limited, exposure measurements were confined to current conditions. Key words: exposure assessment; exposure–response relationship; silicosis; stone work. I N T J OC C U P E N V I R O N H E A LT H 2 0 0 2 ; 8 : 3 0 1 – 30 8

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he prevalence of silicosis in Thailand is well documented.1–4 However, data on exposure assessment are still lacking. Exposure data are crucial tools in disease prevention.5 They can provide scientific information essential to identify which types of jobs or specific worker populations are at risk of developing diseases. Stone carving is one of 12 industries targeted by the silicosis monitoring program implemented by the Ministry of Public Health in Thailand.6 Stone carving is a cottage industry, involving Received from the Faculty of Public Health, Burapha University, Chonburi, Thailand (TY); and the Departments of Medicine and Environmental Health, University of Washington (NS, SB, DB). Supported by the Fogarty International Scholars in Occpational and Environmental Health, Project NIH #D43W/ES00642-06. Address correspondence and reprint requests to: Tanongsak Yingratanasuk, MPH, MS, Assistant to the President for International Relations, Burapha University, Bang-Saen, Chonburi 20131, Thailand; telephone: (66-38) 745900 Ext. 1027; fax:(66-38) 390047; mobile phone (in Thailand): 01-6866432.

groups of 10–20 workers at a single site working in close proximity. These are usually migrant workers, often without health insurance, employment records, or social security benefits. This study is the first reported attempt to characterize the magnitude of the workers’ exposures to crystalline silica, to determine the effect of these exposures on their respiratory health, and to characterize the exposure–response relationship among workers in a stone carving company in eastern Thailand.

MATERIALS AND METHODS Design, Sites, and Population A cross-sectional epidemiologic study was conducted in a stone-carving company in Chonburi province during November 1999 to October 2000. The company’s operations were conducted at three work sites. It manufactured various types of stone sculptures, including tombstones and kitchen utensils such as rice mill grinders, pestles and mortars, pots, and bowls. The type of stone used was mainly granite. The 97 subjects who participated in the study were classified into job categories, carvers, pestle makers, and mortar makers.

Exposure Assessment A full shift (eight hours) of personal dust sampling was performed on a subset of subjects randomly selected by jobs and sites. A total of 148 dust samples were collected during March 11, 2000, to October 22, 2000. The respirable dust mass of each sample was determined gravimetrically at the Division of Occupational Health, MOPH, Thailand. A subset of the dust samples was analyzed for crystalline silica content by IR spectrophotometry at the Department of Environmental Health Laboratory, University of Washington, Seattle, Washington. Severity of exposure is determined by the ratio of average current quartz exposure to the value indicated by the regulatory standard (the Thai permissible exposure limit; Thai-PEL7 ) and by the recommended exposure limit (the American Conferences of Governmental Industrial Hygienists–Threshold Limit Value; ACGIH-TLV 8). From the occupational history data of each worker and the dust concentrations taken

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from the subjects by jobs and sites, three exposure metrics were constructed as follows: 1. Years in Trade Years in trade is the number of years from the time first hired in the stone-carving industry until January 1 of 2000, the year in which the study took place, and is defined by: Years in trade = 2000 – the year at hire 2. Exposure-years Exposure-years is the summation of the overall exposure time (months per year) that a subject has worked in any stone-working jobs. For example, subject A works 6 months per year from 1991 to 2000, Exposure-years for subject A will be 10 3 6/12 = 5 years. Exposure-year can be expressed as: Exposure-years = S Tj 3. Jahr’s cumulative quartz exposure9 Jahr’s cumulative quartz exposure is an exposureweighing method for quartz and is defined as: T

Jahr’s Quartz Exposure = S CQ j (T – T t + 0.5) Tt

where CQ j is current quartz exposure by job, T is the year 2000 in which the study took place, and Tt is the year of exposure.

Health Assessment Respiratory symptoms, pulmonary function and chest radiographs of 97 subjects were assessed during November 1999–March 2000. Respiratory symptoms were assessed by the investigators using the translated ATS questionnaire.10 Five measures of respiratory symptoms were derived, which included cough, phlegm, wheeze, dsypnea, and chronic bronchitis. Spirometry was performed on each subject using the ATS guidelines. Quality assurance protocols included daily calibration of the equipment prior to testing, measurement procedure (at least three acceptable forced expiratory volume curves), and reproducibility (variabilities of the two best FVC and FEV1 were within 5%). The largest FVC and FEV1 valueswere selected as test results. Prediction equations for normal lung function were based on standard predicted values (Knudson) with a correction factor of 0.85 for Asian population (Intermountain Thoracic Society; ITS11) and the Thai equations.12 Posteroanterior chest radiographs were obtained from the participating subjects. All radiography was performed at Burapha Unversity Hospital. Four readers read the films independent of each other and were blinded to the clinical status of the participating subjects. All readers had experience in reading films for silicosis patients and one reader was a NIOSH certified B reader. The interpretation of chest radiographs followed guidelines given by the ILO.13 The findings of all four readers in regard to nodular profusion scores and the presence of tuberculosis were aver-

The close proximity of these workers increases their risk of respiratory problems.

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TABLE 1. Population Characteristics of the Thai Stone Workers in the Study Site I

Site II

Site III

All

Subjects

68 (70.1%)

19 (19.6%)

10 (10.3%)

97

Gender Male Female

39 (57.4%) 29 (42.6%)

16 (84.2%) 3 (15.8%)

9 (90.0%) 1 (10.0%)

64 (66.0%) 33 (34.0%)

35.8 years (9.6)

28.7 years (3.4)

24.8 years (4.37)

33.2 years (9.2)

42 (61.8%) 0 26 (38.2%)

0 19 (100.0%) 0

0 10 (100.0%) 0

42 (43.3%) 29 (29.9%) 25 (25.8%)

16.0 years (8.0)

7.1 years (4.8)

6.8 years (4.2)

13.4 years (8.2)

Work Location Indoor Outdoor Partial enclosure

0 29 (42.6%) 39 (57.4%)

0 0 19 (100.0%)

0 0 10 (100.0%)

0 29 (29.9%) 68 (70.1%)

Fan use Never Sometimes Always

40 (58.8%) 19 (27.9%) 9 (13.2%)

1 (5.3%) 7 (36.8%) 11 (57.9%)

6 (60.0%) 3 (30.0%) 1 (10.0%)

47 (48.5%) 29 (29.9%) 21 (21.6%)

1.8 years (2.1)

3.5 years (3.2)

4.0 years (1.7)

3.0 years (2.5)

Smokers Current Ex Never

24 (35.3%) 9 (13.2%) 35 (51.5%)

11 (57.9%) 0 8 (42.1%)

5 (50.0%) 2 (20.0%) 3 (30.0%)

41 (42.3%) 12 (12.4%) 44 (45.4%)

Pack-years, mean (SD)

6.77 (11.95)

3.51 (4.16)

1.45 (2.85)

5.52 (10.3)

Age,mean (SD) Current job Carvers Mortar makers Pestle makers Time in trade,mean (SD)

Duration of work in previous dusty jobs, mean (SD)

aged. A chest radiograph was considered positive for silicosis if the average of its profusion scores from four readers was 1/0 or greater. Profusion scores were treated as a continuous variable on a 12-point scale. Tuberculosis score was not included in the dose–response analysis.

Statistical Analysis Simple descriptive analyses of population characteristics, job histories, dust exposures, and health outcome information were conducted by job, by site, and as a whole. The t-test of independent variables was used to compare mean values. In the determination of an exposure–response relationship, exposure metrics were regressed on outcomes, controlling for potential confounders such as age, height, and smoking history. Linear models were developed for pulmonary function results, while logistic models were used for respiratory symptoms and chest radiographic results.

RESULTS Population Characteristics Table 1 presents the characteristics of the study population. There were 97 subjects (96% of eligible) from three sites participating in the study.

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Exposure Results Severity of exposure. As shown in Table 2, severity of exposure was highest among the mortar makers at Site III. For carvers at Site I the mean current quartz exposure was below the Thai-PEL but the 95th percentile was 1.4 times higher. The mean quartz exposures for pestle makers at Site I and mortar makers at Site II were much lower than the Thai-PEL and still lower than the ACGIH-TLV. In most cases, severity of exposure was higher than 1 based on the ACGIH standard. Exposure metrics. Table 3 shows the arithmetic means and standard deviations of the exposure metrics by job and site. Carvers at Site I had the highest exposure levels in all three metrics. Among mortar makers, those at Site II had a substantially higher Jahr’s cumulative quartz exposure valuescompare with the other sites.

Health-assessment Results Respiratory symptoms. As shown in Table 4, the most common respiratory symptom in the study population was phlegm (26.8%), followed by cough, dyspnea, chronic bronchitis, and wheeze (16.5%, 12.4%, 6.2%, and 4.1%, respectively). Mortar makers at Sites II and III reported high prevalences of phlegm (42.1% and 50%, respectively).

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TABLE 2. Severity* of Workers’ Exposures to Quartz (mg/m3 ) by Job and Site Current 95th Exposure Percentile

PEL

Severity __________________ 95th Mean Percentile

Severity __________________ 95th Mean Percentile

TLV

Site I Carvers Pestle makers

0.22 0.05

0.70 0.16

0.49 0.68

0.45 0.07

1.43 0.23

0.1 0.1

2.2 0.5

7.0 1.6

Site II Mortar makers

0.05

0.13

0.72

0.07

1.8

0.1

0.5

1.3

Site III Mortar makers

0.88

2.12

0.72

1.29

2.9

0.1

8.8

21.2

*Severity is defined by the ratio of the average quartz exposure to the values indicated by the standards.

TABLE 3. Exposure Metrics by Job and Site Exposure Metrics; mean (SD) ________________________________________________________________________ Exposure-Year Years in Trade Jahr’s Quartz Exposure (Years) (Years) (mg/m3–year) Carvers Pestle makers Mortar makers, Site 2 Mortar makers, Site 3 All

13.74 (7.39) 12.41 (8.22) 6.38 (4.61) 3.3 (2.12) 10.87 (7.71)

17.2 (8.0) 14.2 (7.6) 7.1 (4.8) 6.8 (4.2) 13.32 (8.2)

Pulmonary functions. The participation rate for pulmonary function testing was 95%. Among those who could produce at least two blows, 73.9% met the ATS reproducibility criteria for FVC and 75%, for FEV1 (the values were within 5% of each other). Several studies indicated that inability to meet reproducible criteria was related to poor health and that it was better to keep sicker persons in the study to avoid underestimation of risk and the healthy-worker effect.14,15 Therefore, all spirometric values of the 92 subjects were included in the analyses. The population-means predicted values for the spirometric results using the Knudson equation and the Thai equation were almost the same (Table 5). The mean predicted FEV1 and FVC for smokers were higher than those for subjects who had never smoked. However, smoking intensity in our population was low, only about 6 pack-years. Chest radiograph. In regard to the nodular profusion score, the correlations of readings between individual pairs of readers were low, as shown in Table 6. Reader

34.98 (30.98) 9.48 (14.72) 2.17 (2.46) 14.82 (16.16) 19.64 (26.19)

1 agreed more with readers 2 and 3, and least with reader 4. Reader 3 and reader 4 agreed to each other better than to the other two readers. Reader 2 agreed minimally with readers 3 and 4. Although there was high variability in the readings of different readers, there existed a trend towards a dose–response relationship between average profusion score and years in trade. This indicates that averaging the scores given by all readers seems to predict the profusion risk as shown in Figure 1. However, there were only four subjects who had been working for > 30 years in the industry. Of the 96 subjects who underwent chest x-rays, two were judged to have profusion grade 1/0 according to the ILO system. Four subjects were identified as tuberculosis cases, one of whom had silico-tuberculosis.

Exposure–Response Relationship Respiratory symptoms. As shown in Table 6, there was a marginally significant relationship between exposure-

TABLE 4. Respiratory Symptom Pr evalences by Job and Site Site I ____________________________ Carvers Pestle Makers n = 42 n = 26 No.(%) No.(%) Cough Phlegm Wheeze Dsypnea Chronic bronchitis

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8 (19.0) 12 (28.6) 1 (2.4) 2 (4.8) 3 (7.1)

4 (15.4) 1 (3.8) 1 (3.8) 8 (30.8) 0 (0)

Site II ______________ Mortar Makers n = 19 No.(%)

Site III ______________ Mortar Makers n = 10 No.(%)

All n = 97 No.(%)

3 (15.8) 8 (42.1) 2 (10.5) 1 (5.3) 2 (10.5)

1 (10.0) 5 (50.0) 0 (0) 1 (10.0) 1 (10.0)

16 (16.5) 26 (26.8) 4 (4.1) 12 (12.4) 6 (6.2)


TABLE 5. Percentages of Predicted Spirometry Value by Smoking Status Smoking Status _____________________________________________________________________________ Never Former Current All n = 43 n = 12 n = 41 n = 96 Mean % (SD) Mean % (SD) Mean % (SD) Mean % (SD) FEV1 (Knudson) FEV1 (Thai)

97.95 (12.20) 99.66 (11.45)

102.75 (12.26) 102.08 (12.29)

103.24 (15.63) 103.86 (14.66)

100.79 (13.84) 101.7 (12.98)

FVC (Knudson) FVC (Thai)

91.02 (11.72) 92.80 (10.95)

91.75 (11.72) 91.59 (12.23)

94.82 (16.61) 94.91(16.57)

92.68 (13.93) 93.50 (13.59)

108.34 (8.53)

112.92 (5.84)

110.10 (8.76)

109.68 (8.39)

FEV1/FVC

years and cough. The odds ratio for cough was 1.12 times for a year increase in exposure (exposure-years). No relation of exposure-years to any other symptom was found. Current dust and average dust metrics indicated a risk for phlegm, but this was not statistically significant. There were substantially higher risks for wheeze in women and in current smokers, but not statistically significant ones. Pulmonary functions. The results of the analyses show significant relationships (p < 0.05) between exposureyears and FVC and FEV1 but not FEV1/FVC ratio (Table 7). This relationship indicates that for every additional year exposed to dust, FVC decreases by 19 mL, and FEV1 decreases by 18 mL. However, there was no significant relationship between other exposure metrics and pulmonary functions. Chest radiograph. Linear regression results indicate no significant relationship between exposure metrics and mean profusion scores.

DISCUSSION AND CONCLUSION The Magnitude of Workers’ Exposures to Crystalline Silica These employees’ current exposures to quartz were 0.2 to 3.0 times the Thai-PEL and 0.5 to 8.8 times the ACGIH-TLV, which indicates that immediate action should be undertaken to protect workers from silicosis risk in this company. The results of the study indicate that workers on the same job were exposed to substantially different dust levels. Mortar makers at Site III had a mean exposure level 17 times higher than that of mortar makers at Site II. From our observation, close

worker proximity may contribute to high levels of exposures to dust. As a result, dust exposures may be substantially reduced by worker proximity re-design.

The Prevalence of Silicosis Our study indicated that the prevalence of silicosis was 2%. This prevalence, as in all cross-sectional studies, depends on many factors. Most importantly, this prevalence may not be comparable to those in other studies of the same design. The prevalence figure may underestimate the actual disease risk. Disease prevalence captures only the number of cases that are detected at a specific point in time. For example, the study by Metadilogkul and colleagues in 1988 in pestle and mortar makers in northern Thailand found a prevalence of 21%,1 far higher than that in the present study. The disparity can be explained by the differences in levels of detail in film reading, the numbers of readers who read the films, and, most importantly, the study populations. In this study, we evaluated only active workers in the facilities. Those who are sick might not continue to work, resulting in a lower prevalence rate (healthy-worker effect), whereas in the Metadilogkul study, all subjects in the community were recruited (including the very sick), perhaps resulting in a higher prevalence.

Exposure–Response Relationship Relation to respiratory symptoms. The study findings indicated a marginally significant association between 2 1.5

TABLE 6. Variability Analysis of Readers’ Findings of Nodular Profusion* Reader 1 Reader 2 Reader 3 Reader 4 Reader 1 Reader 2 Reader 3 Reader 4

1.000 0.342† 0.320† 0.265†

1.000 0.168 0.140

*Kendall’s correlation coefficient. †Significance level p < 0.01.


1 0.5 0

1.000 0.369†

30

Years in Trade Figure 1. Average nodular profusion scores by years in trade.


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TABLE 7. Linear Regression Models for Pulmonary Function Variables by Exposure-Years (n = 91)

r2 Constant Coefficient Standard error Age (years) Coefficient Standard error Sex (0 = male,1 = female) Coefficient Standard error Height (cm) Coefficient Standard error Current smoker (0 = no,1 = yes) Coefficient Standard error Ex-smoker (0 = no,1 = yes) Coefficient Standard error Exposure-years (year) Coefficient Standard error

exposure-years and cough, controlling for age, smoking status, and fan use (p = 0.049). The risk for cough was 1.12 times higher for an additional year of exposure (exposure-years). In other words, the regression coefficient predicted a threefold increase in cough for every ten years of exposure. No relationship of exposure-years to any other symptom was revealed. Since results from several studies indicate that radiographically detectable fibrosis is an important factor in developing clinically significant changes in respiratory symptoms,15 it is not surprising that our population, with a relatively mild radiographic profile and a low prevalence of smoking, should report a low incidence of symptoms.

Relation to Pulmonary Functions The results of the study suggest that workers with longer work durations had worse lung function. As workers continued to work in this industry, FVC declined at an average rate of –19 mL/year of work, and FEV1 at –18 mL/year worked. This finding was similar to that observed in a cross-sectional study of quartz exposures among Norwegian men, in that FEV1 declined in an association with each year of quartz dust or stone dust exposure (–4.3 mL/year among non-smokers and –6.9

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FVC (L)

FEV1 (L) (L)

FEV1/FVC (%)

0.659

0.730

0.130

–3.644 1.365 (p < 0.01)

–2.075 1.078 (p < 0.1)

140.735 18.494 (p < 0.001)

–0.0034 0.008 (p < 0.1)

–0.0121 0.006 (p < 0.1)

–0.200 0.110 (p < 0.1)

–0.557 0.167 (p < 0.01)

–0.568 0.132 (p < 0.001)

–3.169 2.207 (p > 0.1)

0.0457 0.008 (p < 0.001)

0.0361 0.006 (p < 0.001)

–0.246 0.110 (p < 0.05)

0.0850 0.144 (p > 0.5)

0.050 0.113 (p > 0.6)

–0.681 1.935 (p > 0.7)

–0.112 0.189 (p > 0.5)

–0.040 0.149 (p > 0.7)

1.286 2.554 (p > 0.6)

–0.0188 0.009 (p < 0.05)

–0.0175 0.007 (p < 0.05)

–0.0431 0.126 (p > 0.7)

mL/year among those with 1 pack-year of cigarette smoking).16 Similar results were also found among silica exposed gemstone workers in Hong Kong.17 The decreases in FVC and FEV1 were significantly related to increasing years of employment, after adjusting for the effects of age, height, and smoking. In comparison with studies by others of the effects of smoking on lung function in workers exposed to quartz dust,17–21 our study did not show a significant relationship between smoking and lung-function impairment. This discrepancy may be explained by the fact that smoking intensity in our population was low, only about 6 pack-years. Although exposure-year was the only significant predictor in this study, the use of exposure-year as a surrogate for true dose may contribute to biased results. The use of exposure-year (in this study) or years in trade (by other studies) as exposure or dose in an epidemiologic study can be accurate if all exposed subjects have uniform exposures at each time period.22 According to our findings of the variability of current dust exposures, we believe that stone-carving workers are exposed to various levels of dust concentration over time. As a result, we urge that industrial hygiene monitoring data by job and site in relation to time-intensity of dust exposure be emphasized for exposure-estimate studies.


Stone car vers at work.The worker in the bottom left photo is grinding mortars. Those at top left and lower right are mortar makers using cutting tools.

Relation to Radiographic Results Interestingly, years in trade shows a trend towards a dose–response relationship with average nodular profusion score (although no significant relationship was established). Additionally, all profusion-positive cases had worked for more than ten years and all were from Site I, which had lower exposures. This implies that duration of exposure has a causative relationship to nodular profusion. Despite having a higher level of current quartz exposures, the mortar makers at Site III


had fewer years in the trade; therefore, they had lower cumulative exposures, and no profusion was detected.

RECOMMENDATIONS This study has provided information important to the silicosis surveillance and control program in two ways. First, worker exposures to respirable quartz dust in the stone-carving company were explicitly high. More concern should be given to protecting the health of this underserved population. Second, prevalence data may


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underestimate the true disease risk. Since the study population had worked for only about 13 years in this trade, silicosis cases were detected in only 2%. Given the long latency period of silicosis from exposure to the expressed disease state, we would expect to find a higher prevalence of the disease with continued observation of this exposed population. References 1. Metadilogkul O, Limpakarnjanarat K, Ittiravivongs A, Rattanapornpan N, Pimiam S. Silicosis among mortar and pestle workers in northern Thailand: Cross-sectional study. J Med Assoc Thai 1988;71:533-6. 2. Metadilogkul O, Siriwattananukul P. Occupational silicosis among workers in an ore mill. In: Abstract of Communications, VIIth International Pneumoconiosis Conference, August 23–26, 1988; Pittsburgh, PA: 45. 3. Obhasi N. Silicosis occurrence among workers in potential risk industries. Division of Occupational Health, Ministry of Public Health, Thailand. Unpublished report, 1993. 4. Aungkasuvapala N, Jiamjarasrangsri W, Obhasi N. Silicosis and pulmonary tuberculosis in stone-grinding factories in Saraburi, Thailand. J Med Assoc Thai. 1995;78:662-9. 5. Lemen RA. Role of exposure databases in disease surveillance and occupational epidemiology. Appl Occup Environ Hyg. 1995;10:400-1. 6. Division of Occupational Health, Ministry of Public Health, Thailand, 1993. 7. Proclamation of the Ministry of the Interior, Thailand, 1977 8. American Conference of Governmental Industrial Hygienists (ACGIH). 2000 TLV® and BEI. Threshold limit values for chemical substances and physical agents, biological exposure indices. Cincinnati, OH: ACGIH, 2000.

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