Respiratory health and silica exposure of stone carvers in Thailand

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 > or = 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.     

Silicosis in finishing workers in quartz conglomerates processing

Introduction:Outbreaks of silicosis have bene recently reported in artificial stone workers.Aim:To describe the features of silicosis in quartz conglomerate workers in North-Eastern Italy.Methods:Active search of pneumoconiosis was performed in 11 companies of North-Eastern Italy involved in the fabrication of quartz conglomerate countertops. Occupational history, lung function tests, chest X-ray and high resolution computed tomography (HRCT) were performed. In selected cases, trans-bronchial biopsies were taken for histological evaluation and identification of silica crystals in the tissue. Cumulative exposure to crystalline silica was estimated.Results:We recruited 45 workers and 24 cases of silicosis were diagnosed. Mean age at diagnosis was 43 years and duration of exposure to quartz conglomerate dust was 3.5 to 20 years. The average silica cumulative exposure was 4.3 mg/m³/y. Abnormal findings were detected in 42% of chest X-rays, in 33% of spirometry and 50% of carbon monoxide lung diffusion (DLco). HRCTs were abnormal in all cases showing well-defined rounded opacities, irregular/linear intralobular opacities and bilateral enlarged mediastinal lymph-nodes. Histological findings consistent with silicosis were observed in 24 cases. Numerous silica particles (diameter 0.1-5 μm) were identified in lung tissue.Conclusions:We reported an unexpected high incidence of silicosis in Italian workers exposed to quartz conglomerate dust. The results suggest that chest HRCT is indicated for screening of workers with high exposure to silica and DLco should be added to spirometry in health surveillance. More rigorous application of safety regulations and more effective preventive interventions at work are necessary.Key words: Silica dust, pneumoconiosis, quartz conglomerates, occupational exposure, artificial stones     

8-Hydroxydeoxyguanosine in leukocyte DNA and urine of quartz-exposed workers and patients with silicosis

Objective: To examine radical-induced DNA damage and its elimination in workers exposed to quartz and in patients with silicosis, and to assess the relationship of these effects to lung function. Methods: Blood and spontaneous urine samples were obtained from active, quartz-exposed workers without silicosis (n=63), and from retired workers with silicosis (n=42). Levels of 8-hydroxydeoxyguanosine (8-OHdG) were determined in peripheral blood leukocyte DNA and urine, by the use of high-performance liquid chromatography coupled with ultra violet- (UV) and electrochemical detection. Results: No significant differences in the mean levels of 8-OHdG in leukocyte DNA and of urinary excretion of 8-OHdG were found between silicosis patients and quartz-exposed healthy workers. However, in the group of silicosis patients with increased oxidative DNA damage the urinary excretion of 8-OHdG was lower than in the corresponding group of active workers without silicosis. In the case of silicosis, urinary 8-OHdG correlated positively, and 8-OHdG in DNA correlated negatively, with forced expiratory volume in one second (FEV₁) and forced vital capacity (FVC). Healthy workers with a personally estimated high dust exposure in the workplace showed higher levels of 8-OHdG in DNA than did workers with moderate dust exposure. No association of 8-OHdG formation and/or elimination with duration of employment, field of activity, smoking or age was found. Conclusion: Our findings suggest that a less effective repair of 8-OHdG is associated with a higher degree of pulmonary airway obstruction in patients with silicosis. Received: 13 August 1999 / Accepted: 5 January 2000     

Silicosis in a grey iron foundry. The persistence of an ancient disease

An industrial hygiene and medical survey was conducted in an iron foundry to study the occurrence of silicosis. Breathing zone exposures to respirable crystalline silica had been very high in 1977 [1 045 micrograms/m3 (geometric mean) for coremakers and 198 micrograms/m3 for fettlers]; exposures in 1980 and 1982 were substantially lower. A radiographic evaluation of 188 workers revealed silicosis in 18 (9.6%). Eight had category 1 profusion of small rounded pulmonary lesions (by the 1980 classification of the International Labour Office); two had category 2; and eight had category 3. Two had progressive massive fibrosis. Four workers without silicosis in 1977 had developed lesions by 1980. The prevalence increased from 1.5% among workers employed less than 20 years to 53% among longer term workers. No association was found between the prevalence of silicosis and cigarette smoking. Chronic cough was more common in workers with heavy current dust exposure than in those with light exposure, more common in smokers than in nonsmokers, and more common in silicotics than in nonsilicotics. A multiplicative interaction existed between dust exposure and smoking in the etiology of cough. Silicosis continues to exist in American foundries. Cigarette smoking does not contribute to the causation of silicosis, but it aggravates the attendant respiratory symptoms.     

Risk of silicosis in a Colorado mining community

We investigated exposure-response relations for silicosis among 134 men over age 40 who had been identified in a previous community-based random sample study in a mining town. Thirty-two percent of the 100 dust-exposed subjects had radiologic profusions of small opacities of 1/0 or greater at a mean time since first silica exposure of 36.1 years. Of miners with cumulative silica exposures of 2 mg/m³-years or less, 20% had silicosis; of miners accumulating > 2 mg/m³-years, 63% had silicosis. Average silica exposure was also strongly associated with silicosis prevalence rates, with 13% silicoties among those with average exposure of 0.025–0.05 mg/m³, 34% among those with exposures of > 0.05–0.1 mg/m³, and 75% among those with average exposures > 0.1 mg/m³. Logistic regression models demonstrated that time since last silica exposure and either cumulative silica exposure or a combination of average silica exposure and duration of exposure predicted silicosis risk. Exposure-response relations were substantially higher using measured silica exposures than using estimated silica exposures based on measured dust exposures assuming a constant silica proportion of dust, consistent with less exposure misclassification. The risk of silicosis found in this study is higher than has been found in workforce studies having no follow-up of those leaving the mining industry and in studies without job title-specific silica measurements, but comparable to several recent studies of dust exposure-response relationships which suggest that a permissible exposure limit of 0.1 mg/m³ for silica does not protect against radiologic silicosis. © 1996 Wiley-Liss, Inc.     

Vermont granite mortality study: an update with an emphasis on lung cancer

This mortality study extends the period of observation of an article published in 1988 of 5414 workers in Vermont granite sheds and quarries to assess whether previously reported reductions in silicosis and tuberculosis mortality were maintained. The relationship between lung cancer and quartz exposure is also examined by comparing mortality in workers hired before and after 1940, when dust controls were introduced and exposures were reduced by 80% to 90%. Before 1940, general stone shed air contained 20 million particles/cubic foot (mppcf) (approximately equivalent to 0.2 mg/m of quartz), and pneumatic chisel workers were exposed on average to 60 mppcf (approximately equivalent to 0.6 mg/m of quartz). Other workers had variable exposures. After 1940, a period of decline occurred in dust levels and then stabilized in approximately 1955, when average dust levels were 5 to 6 mppcf (equivalent to 0.05-.06 mg/m of quartz). Dust exposures in the Vermont industry is considered to be free of confounding occupational substances such as arsenic, although cigarette smoking was common. By the end of 1996, 2539 workers, or 46.9% of the cohort, had died. There were no silicosis deaths in workers hired after 1940 who were exposed only in the Vermont granite industry, illustrating the effect of lowering quartz exposures. Tuberculosis caused 2 deaths in those hired after 1940 (standardized mortality ratio [SMR] = 0.52; not significant). Overall lung cancer mortality was elevated in shed workers who had been exposed both to high levels of quartz before 1940 and to the lower levels prevailing after 1940 (SMR = 1.32; P < 0.01). Quarry workers did not show an excess of lung cancer (SMR = 0.73; not significant). When shed workers with high and low exposure histories (before and after 1940) but with comparable latency and tenure were contrasted, lung cancer mortality was similar. Differing levels of quartz exposure, which resulted in large differences in the mortality experience from silicosis, did not result in differences in lung cancer mortality. The results do not support the hypothesis that granite dust exposure has a causal association with lung cancer.     

Prevalence of silicosis in the Dutch fine-ceramic industry

Summary In the nineteen-seventies a cross-sectional survey was conducted in the Dutch fine-ceramic industry. Workers employed in the ceramic industry (n = 3258) were examined for the presence of silicosis. In this article the results are reported for the area of Gouda and Maastricht. In Gouda the fine-ceramic industry consists of small workshops. The Maastricht working population comprises workers of two large, mechanized companies. The survey indicated that silicosis is still commonly present in Gouda (total prevalence of 13.3%), but is relatively rare in Maastricht (total prevalence of 1.7%). A clear dose-response relationship was found in both areas between duration of exposure to quartz-containing dust and the prevalence of silicosis. Furthermore it was noted that smoking was a risk factor for silicosis. However, this was restricted to workers who were heavy smokers and had had an occupational history of 20 years or more of exposure to quartz-containing dust. In this exposure category the prevalence of silicosis among heavy smokers was 50% higher than in light smokers and non-smokers.     

Respiratory health effects of occupational exposure to charcoal dust in Namibia

Background: Charcoal processing activities can increase the risk of adverse respiratory outcomes.

Objective: To determine dose–response relationships between occupational exposure to charcoal dust, respiratory symptoms and lung function among charcoal-processing workers in Namibia.

Methods: A cross-sectional study was conducted with 307 workers from charcoal factories in Namibia. All respondents completed interviewer-administered questionnaires. Spirometry was performed, ambient and respirable dust levels were assessed in different work sections. Multiple logistic regression analysis estimated the overall effect of charcoal dust exposure on respiratory outcomes, while linear regression estimated the exposure-related effect on lung function. Workers were stratified according to cumulative dust exposure category.

Results: Exposure to respirable charcoal dust levels was above occupational exposure limits in most sectors, with packing and weighing having the highest dust exposure levels (median 27.7 mg/m³, range: 0.2–33.0 for the 8-h time-weighted average). The high cumulative dust exposure category was significantly associated with usual cough (OR: 2.1; 95% CI: 1.1–4.0), usual phlegm (OR: 2.1; 95% CI: 1.1–4.1), episodes of phlegm and cough (OR: 2.8; 95% CI: 1.1–6.1), and shortness of breath. A non-statistically significant lower adjusted mean-predicted % FEV1 was observed (98.1% for male and 95.5% for female) among workers with greater exposure.

Conclusions: Charcoal dust levels exceeded the US OSHA recommended limit of 3.5 mg/m³ for carbon-black-containing material and study participants presented with exposure-related adverse respiratory outcomes in a dose–response manner. Our findings suggest that the Namibian Ministry of Labour introduce stronger enforcement strategies of existing national health and safety regulations within the industry.     

Risk of silicosis in cohorts of Chinese tin and tungsten miners, and pottery workers (I): an epidemiological study

BACKGROUND: Epidemiological evaluations of the risk of silicosis in relation to exposure to crystalline silica have raised the question of whether different types of silica dust exposures vary with respect to their ability to cause silicosis. The aim of this study is to compare the risk of silicosis among cohorts of silica dust-exposed Chinese tin miners, tungsten miners, and pottery workers and to assess whether gravimetric measurements of respirable silica dust sufficiently determine the risk of silicosis or whether other factors of exposure may play a significant role. METHODS: Cohorts were selected from 20 Chinese mines and potteries. Inclusion criteria were starting employment after January 1, 1950 and being employed for at least 1 year during 1960-1974 in one of the selected workplaces. Radiological follow-up for silicosis onset was from January 1, 1950 through December 31, 1994. Silicosis was assessed according to the Chinese radiological criteria for diagnosis of pneumoconiosis (as suspect, Stage I, II, or III). Exposure-response relationships were estimated for silicosis of Stage I or higher. Silica dust exposure was estimated in terms of cumulative total dust exposure, calculated from a workplace, job title, and calendar year exposure matrix, and individual occupational histories. Cumulative total dust exposure was converted in two steps into cumulative respirable dust exposure and cumulative respirable silica dust exposure using conversion factors estimated from side-by-side measurements conducted in 1988-89. RESULTS: The male cohorts included 4,028 tin miners, 14,427 tungsten miners, and 4,547 pottery workers who had similar onset of employment and duration of follow-up. For a given exposure level, the risk of silicosis was higher for the tin and tungsten than the pottery workers. CONCLUSION: The observed differences in the risk of silicosis among the three cohorts suggest that silica dust characteristics, in addition to cumulative respirable silica dust exposure, may affect the risk of silicosis.     

不同煤种呼吸性粉尘与尘肺病患病风险的剂量-反应关系分析

目的 探讨无烟煤、烟煤、褐煤呼吸性粉尘与尘肺病累计患病率的剂量-反应关系,为科学制定职业卫生标准提供依据。方法 对9处国有煤矿采煤工人的接尘和尘肺病患病情况进行调查研究,以寿命表法分别计算出无烟煤、烟煤、褐煤呼尘累计接尘量对应的累计患病率,利用线性回归方程进行单侧区间统计控制,得到呼尘接触浓度限值。结果 9处煤矿共21 000名采煤工人纳入研究,其中无烟煤、烟煤、褐煤煤矿采煤工人尘肺病检出率分别为11.27%、21.32%、6.00%,平均接尘工龄为20.12、22.88、25.21年。无烟煤、烟煤、褐煤煤矿采煤工人呼尘累计接尘量与尘肺病累计患病率的剂量-反应关系分别为线性回归方程y=5.788x-16.043（R²=0.949）、y=5.679x-16.837（R²=0.904）、y=6.465x-19.573（R²=0.944）。当接尘30年尘肺累计患病率≤1%时,以安全系数为1.2计算,三者呼尘接触浓度限值分别为1.7、2.3、3.9 mg/m³。结论 不同煤种的尘肺病检出率、平均接尘工龄、呼尘接触浓度限值均不同,在煤尘累计接尘量相似情况下,煤的挥发分越低（含碳量越高）,累计患病率越高（无烟煤 > 烟煤 > 褐煤）。建议按照不同煤种分别制定国家职业卫生煤尘标准。     

A 40-year follow-up of whetstone cutters on silicosis

The present study, consisted of two separate surveys, was initiated to clarify the development of clinical pictures of silicosis after termination of dust exposure. The first survey was a 40-year follow-up of radiographic pictures of the chest among 200 male whetstone cutters (Group I workers). The second survey was conducted in 75 male recipients (Group II workers) of disability compensation for silicosis due to whetstone dust exposure. The study on Group I workers made it clear that the proportion of those free of radiographic findings in the chest pictures decreased during a 40-year follow-up period from 84% in the 1st health examination in 1952-6 to 36% in the 3rd examination in 1995. The rate of progression of the disease from Category 1 to 3 (after ILO-guided classification) to higher categories in a 15-year period was as high as >50%. Longer service duration and higher category of chest radiography at the previous health examination were the influential factors in determining the rapid progression of silicosis. The latter observation was confirmed also through a similar analysis on Group II workers. Whetstone preparations contained SiO2 by about 50%. No industrial hygiene data were available for both groups of stone cutters, but the exposure of Group I workers was estimated to be about 1 mg/m3, or well in excess of the current occupational exposure limit.     

Respiratory health status of gilsonite workers

Gilsonite, a solidified hydrocarbon used in the manufacture of automotive body seam sealers, is mined only in the Uinta Basin of Eastern Utah and Western Colorado. Health effects of gilsonite dust exposure have not previously been published and exposure to gilsonite dust is not regulated. To examine potential respiratory health effects associated with gilsonite dust exposures, this cross-sectional study surveyed the 100 current male employees who had been exposed to gilsonite dust at 3 existing gilsonite companies. Total dust exposures up to 28 times the nuisance dust standard were found, and 5 of 99 (5%) workers had chest radiographs consistent with pneumoconiosis of low profusion. Increased prevalences of cough and phlegm were found in workers with high-exposure jobs, but no evidence for dust-related pulmonary function impairment was noted. To prevent pulmonary health effects, we recommend reducing dust exposures for those workers in jobs currently characterized by relatively high dust exposures.     

Agricultural Exposures in Patients with COPD in Health Systems Serving Rural Areas

ABSTRACT

Background: Agricultural exposure is a risk factor for the development of chronic obstructive pulmonary disease (COPD). However, there are no good estimates of the number of COPD patients with a history of agricultural exposure. Methods: We conducted a telephone interview of subjects with COPD identified by reviewing all pulmonary function tests at the Omaha Veterans Administration Hospital between November 2004 and March 2005. Obstructive lung disease was defined as a FEV₁/FVC ratio of less than 70%. The survey detailed demographic data, smoking history, pulmonary symptoms, and history of agricultural exposures. Results: Participants included 150 veterans (mean age 68.2 years ±10.8). A history of agricultural exposure was elicited in 68% of subjects. Of those who had worked in agriculture, the types of exposures varied, with 14% in hog confinement barns, 20% on dairy farms, 8% on poultry farms, and 87% exposed to grain dust. There was a trend of diminishing FEV₁ with increasing years of agricultural exposure. Conclusions: In health systems that serve rural areas, patients with COPD commonly have a history of agricultural exposures that may contribute to the development of COPD. Health care workers in these areas should include agricultural exposures as an important part of the social/occupational history in these patients.     

Change of exposure response over time and long-term risk of silicosis among a cohort of Chinese pottery workers

An analysis was conducted on a cohort of Chinese pottery workers to estimate the exposure-response relationship between respirable crystalline silica dust exposure and the incidence of radiographically diagnosed silicosis, and to estimate the long-term risk of developing silicosis until the age of 65. The cohort comprised 3,250 employees with a median follow-up duration of around 37 years. Incident cases of silicosis were identified via silicosis registries (Chinese X-ray stage I, similar to International Labor Organisation classification scheme profusion category 1/1). Individual exposure to respirable crystalline silica dust was estimated based on over 100,000 historical dust measurements. The association between dust exposure, incidence and long-time risk of silicosis was quantified by Poisson regression analysis adjusted for age and smoking. The risk of silicosis depended not only on the cumulative respirable crystalline silica dust exposures, but also on the time-dependent respirable crystalline silica dust exposure pattern (long-term average concentration, highest annual concentration ever experienced and time since first exposure). A long-term "excess" risk of silicosis of approximately 1.5/1,000 was estimated among workers with all annual respirable crystalline silica dust concentration estimates less than 0.1 mg/m(3), using the German measurement strategy. This study indicates the importance of proper consideration of exposure information in risk quantification in epidemiological studies.     

The relation of silica dust to accelerated silicosis

More than 130 cases of silicosis among sandblasters with an average exposure to free silica of 10 years have been studied in Louisiana. The mortality was approximately 25%. Examination of 180 gravimetric respirable dust samples from the breathing zones of sandblasters and other associated workers in two steel fabrication yards showed extensive dust exposure (up to 42.8 times the threshold limit value). The silica fraction of the respirable dust was determined either by X-ray diffraction or by a modified colorimetric technique based on that of Talvitie and Hyslop (Amer. Indust. Hyg. Assoc. J. 19, 54–58, 1958). Sandblasters wearing non-air-supplied defective hoods were at the greatest risk. Their exposure to silica dust varied greatly depending on the type of hood, maintenance, proper fit, and atmospheric dust concentrations during nonblasting periods when they were unhooded. The development of so-called accelerated silicosis is related to ordinary and faulty characteristics of sandblasting: high free-silica content of sand, use of inadequate or faulty protective devices, carelessness, and incomplete safety training.     

Outbreak of silicosis in Spanish quartz conglomerate workers

Abstract

Objectives: To describe the epidemiological and clinical characteristics of an outbreak of occupational silicosis and the associated working conditions.

Methods: Cases were defined as men working in the stone cutting, shaping, and finishing industry in the province of Cádiz, diagnosed with silicosis between July 2009 and May 2012, and were identified and diagnosed by the department of pulmonology of the University Hospital of Puerto Real (Cádiz). A census of workplaces using quartz conglomerates was carried out to determine total numbers of potentially exposed workers. A patient telephone survey on occupational exposures and a review of medical records for all participants were conducted.

Results: Silicosis was diagnosed in 46 men with a median age of 33 years and a median of 11 years working in the manufacturing of countertops. Of these cases, 91·3% were diagnosed with simple chronic silicosis, with an abnormal high-resolution computerized tomography (HRCT) scan. One patient died during the study period. Employer non-compliance in prevention and control measures was frequently reported, as were environmental and individual protection failures.

Conclusions: The use of new construction materials such as quartz conglomerates has increased silicosis incidence due to intensive occupational exposures, in the context of high demand fuelled by the housing boom. This widespread exposure poses a risk if appropriate preventive measures are not undertaken.     

Silicosis prevalence and exposure-response relations in South African goldminers

Aims: To measure the prevalence of silicosis among black migrant contract workers on a South African goldmine and to investigate exposure-response relations with silica dust.

Methods: In a cross sectional study, 520 black goldminers (aged >37 years) were interviewed and had chest radiographs taken. Silicosis was defined as International Labour Organisation Classification radiological profusion of 1/1 or greater.

Results: Mean length of service was 21.8 years (range 6.3–34.5). The mean intensity of respirable dust exposure was 0.37 mg/m³ (range 0–0.70) and of quartz 0.053 mg/m³ (range 0–0.095). The prevalence of silicosis was 18.3–19.9% depending on reader. Significant trends were found between the prevalence of silicosis and length of service, mean intensity of exposure, and cumulative exposure.

Conclusion: Results confirm a large burden of silicosis among older black workers in the South African goldmining industry, which is likely to worsen as such miners spend longer periods in continuous employment in dusty jobs. An urgent need for improved dust control in the industry is indicated. If the assumption of stability of average dust concentrations on this mine over the working life of this group of workers is correct, these workers developed silicosis while exposed to a quartz concentration below the recommended occupational exposure limit (OEL) of 0.1 mg/m³. This accords with a mounting body of evidence that an OEL of 0.1 mg/m³ is not protective against silicosis.

     

Total dust and endotoxin in poultry operations: comparison between cage and floor housing and respiratory effects in workers

OBJECTIVE: The objective of this study was to assess respiratory outcomes and environmental exposure levels of workers in cage-housed and floor-housed poultry operations. METHODS: Poultry operations were evaluated for total dust, endotoxin, and ammonia, and respiratory symptoms and lung function tests of workers were conducted. RESULTS: Workers in floor-housed poultry operations had significantly greater exposures to total dust and ammonia, whereas workers from cage-housed poultry operations reported greater frequency of current and chronic symptoms overall and significantly greater current and chronic phlegm (39% vs 18% and 40% vs 11%, respectively). Endotoxin concentration (EU/mg) was a significant predictor (P = 0.05) of chronic phlegm for all poultry workers. CONCLUSIONS: Greater endotoxin concentration in the presence of significantly lower total dust, in conjunction with greater respiratory symptoms in workers from cage-housed poultry operations, as compared with workers from floor-housed poultry operations, appears to indicate that differences in environmental exposures may impact respiratory outcomes of workers.     

Exposure to crystalline silica, silicosis, and lung disease other than cancer in diatomaceous earth industry workers: a quantitative risk assessment

Objectives: To estimate excess lifetime risk of (a) mortality from lung disease other than cancer (LDOC), and, (b) onset of radiographic silicosis, arising from occupational exposure to respirable crystalline silica dust.

Methods: Data from a cohort of California diatomaceous earth mining and processing workers exposed to crystalline silica dust (mainly as cristobalite) were reanalyzed with Poisson regression methods with internal and external adjustments for potential confounding by calendar time, age, smoking, Hispanic ethnicity, and time since first observation. Model fit was evaluated by comparing deviances and fitting cubic spline models. Lifetime risks of death from LDOC and radiographic silicosis were estimated up to age 85 with an actuarial approach accounting for competing causes of death.

Results: For deaths due to LDOC, a linear relative rate model gave the best fit in Poisson regression analyses. At the mean cumulative exposure of LDOC cases to silica, after adjustment for smoking, the estimated rate ratio was 4.2 (p<0.0001); at the maximum cumulative exposure of cases, the rate ratio was 18.4. The excess lifetime risk for white men exposed to respirable cristobalite dust for 45 years at the current permissible exposure limit (PEL; about 0.05 mg/m³) of the Occupational Safety and Health Administration was 54/1000 (95% confidence interval (95% CI) 17 to 150). For 70 incident cases of radiographic silicosis largely manifest before the end of employment, the best fit was also the linear relative rate model, predicting a rate ratio of 25.6 for silicosis at the mean cumulative exposure of the cases (p<0.0001). The excess lifetime risk for silicosis at the current PEL was 75/1000.

Conclusion: Current occupational health standards for crystalline silica permit risks of lung disease other than cancer far in excess of what is usually considered acceptable by the Occupational Safety and Health Administration (a lifetime risk of less than one in a thousand deaths).

     

Mortality from non-malignant respiratory diseases among people with silicosis in Hong Kong: exposure-response analyses for exposure to silica dust

Objectives

To examine the exposure–response relationships between various indices of exposure to silica dust and the mortality from non‐malignant respiratory diseases (NMRDs) or chronic obstructive pulmonary diseases (COPDs) among a cohort of workers with silicosis in Hong Kong.

Methods

The concentrations of respirable silica dust were assigned to each industry and job task according to historical industrial hygiene measurements documented previously in Hong Kong. Exposure indices included cumulative dust exposure (CDE) and mean dust concentration (MDC). Penalised smoothing spline models were used as a preliminary step to detect outliers and guide further analyses. Multiple Cox''s proportional hazard models were used to estimate the dust effects on the risk of mortality from NMRDs or COPDs after truncating the highest exposures.

Results

371 of the 853 (43.49%) deaths occurring among 2789 workers with silicosis during 1981–99 were from NMRDs, and 101 (27.22%) NMRDs were COPDs. Multiple Cox''s proportional hazard models showed that CDE (p = 0.009) and MDC (p<0.001) were significantly associated only with NMRD mortality. Subgroup analysis showed that deaths from NMRDs (p<0.01) and COPDs (p<0.05) were significantly associated with both CDE and MDC among underground caisson workers and among those ever employed in other occupations with high exposure to silica dust. No exposure–response relationship was observed for surface construction workers with low exposures. A clear upward trend for both NMRDs and COPDs mortality was found with increasing severity of radiological silicosis.

Conclusion

This study documented an exposure–response relationship between exposure to silica dust and the risk of death from NMRDs or COPDs among workers with silicosis, except for surface construction workers with low exposures. The risk of mortality from NMRDs increased significantly with the progression of International Labor Organization categories, independent of dust effects.Excessive mortality from non‐malignant respiratory diseases (NMRDs) including chronic obstructive pulmonary disease (COPD) has been reported among cohorts exposed to silica dust or among cohorts with disease silicosis.^{1,2,3,4,5,6,7} However, inconsistent or even conflicting evidence existed on the exposure–response relationship between exposure to silica dust and death from NMRDs or COPD.^{1,3,4,5,6,7,8,9} Besides the potential differences in methods or toxicity of quartz polymorphs and exposure levels, discrepancies between studies might reflect the differences in biological effectiveness of various exposure indices.^3,10 The potential limitations of cumulative dust exposure (CDE) had been emphasised by Smith,¹¹ who said that CDE might be a poor dose index in examining the association with lung diseases by using a pharmacokinetic model. Hughes et al¹² found a substantially steeper relationship with silicosis among diatomaceous earth workers exposed at the highest concentrations of crystalline silica. More recently, Buchanan et al¹³ provided supportive evidence on this issue and suggested that quantifying the risks of silicosis should take into account the variations in quartz exposure intensity, particularly for concentrations >1 or 2 mg/m³, even if exposures were for relatively short periods. Studies comparing the possible effect of cumulative exposure to silica dust and mean dust concentration (MDC) on the risk of NMRD or COPD remain sparse and in demand. We report here the exposure–response analyses between various indices of exposure to silica dust and mortality from NMRDs or COPD among a cohort of 2789 workers with silicosis in Hong Kong, taking into consideration the effect of cigarette smoking.