Lung cancer mortality among silicotic workers in Hong Kong--no evidence for a link

Association of spirometric restriction with mortality in the silicotics: a cohort study

BACKGROUND

Restrictive spirometry pattern (RSP), defined as reduced forced vital capacity (FVC) in absence of airflow obstruction (AFO), is associated with increased risk of mortality in general population. However, evidence in the patients with silicosis is limited. This study was aimed to investigate the relationship between RSP and the risk of death in a silicotic cohort.

METHOD

This retrospective cohort study used data from the Pneumoconiosis Clinic, Hong Kong Department of Health that containing 4315 patients aged 18-80 years and diagnosed with silicosis during 1981-2019, with a follow-up till 31 December 2019. Spirometry was carried out at the diagnostic examination of silicosis. Lung function categories were classified as normal spirometry (FEV1/FVC ≥ 0.7, FVC ≥ 80% predicted), RSP only (FEV1/FVC ≥ 0.7, FVC < 80% predicted), AFO only (FEV1/FVC < 0.7, FVC ≥ 80% predicted), and RSP&AFO mixed (FEV1/FVC < 0.7, FVC < 80% predicted). The hazard ratio (HR) and 95% confidence intervals (95% CI) were computed using a Cox proportional hazards model adjusting for age, body mass index, history of tuberculosis, smoking status, pack-years, and radiographic characteristics of silicotic nodules.

RESULTS

Among the 4315 patients enrolled in the study, the prevalence of RSP was 24.1% (n = 1038), including 11.0% (n = 473) with RSP only and 13.1% (n = 565) with mixed RSP and AFO. During the follow-up period, a total of 2399 (55.6%) deaths were observed. Compared with the silicotics with normal spirometry, those with RSP only had significantly increased risk of all-cause mortality (HR = 1.63, 95% CI 1.44-1.85) and respiratory-related mortality (HR = 1.56, 95% CI 1.31-1.85). Notably, a higher risk of mortality was observed in silicotics with mixed ventilatory defects of both RSP and AFO (all-cause mortality: HR = 2.22, 95% CI 1.95-2.52; respiratory-related mortality: HR = 2.59, 95% CI 2.18-3.07) than in those with RSP only.

CONCLUSION

RSP is significantly associated with increased risk of all-cause and respiratory-related mortality in the silicotics, and patients with mixed restrictive and obstructive ventilatory defect have higher risk of mortality than those with single RSP or AFO. These findings emphasize the importance of recognizing RSP in the occupational settings, especially for the silicotic patients with mixed ventilatory defect.

Smoking cessation sharply reduced lung cancer mortality in a historical cohort of 3185 Chinese silicotic workers from 1981 to 2014

BACKGROUND

Population-based studies showed an over 50% decrease in lung cancer risk after quitting smoking for 5-6 years, but the beneficial effect in silicotics remains unknown. We aimed to rectify this knowledge gap using a large historical cohort of 3185 Chinese silicotics since 1981 and followed-up till 2014.

METHODS

Baseline information on workers' socio-demographics, smoking habits, occupational history, and medical history was collected. Smoking status was reassessed during follow-up. Multiple Cox proportional hazards model was performed to evaluate the impact of smoking cessation on lung cancer mortality.

RESULTS

Overall, 1942 deaths occurred and 188 lung cancer deaths were identified. Compared with never quitters, silicotics who were new quitters had almost halved their lung cancer risk [hazard ratio (HR) = 0.51, 95%CI: 0.34-0.76], while persistent quitters had a 53% risk reduction (HR = 0.47, 95%CI: 0.33-0.66). Lung cancer mortality approximately halved after quitting smoking for 10 years. While the risk kept decreasing with years since cessation, it did not reverse back to that of never smokers. Persistent quitters with small opacities tended to have higher beneficial effects than those with large opacities.

CONCLUSIONS

Smoking cessation for 10 years halved lung cancer mortality among silicotics, while the beneficial effect was prominent for patients with small opacities.

Comparison of a 10-Year Cumulative Age-Standardized Incidence Rate of Lung Cancer among Metropolitan Cities in Korea (During the 2000⁻2009 Period): Review of Occupational and Environmental Hazards Associated with Lung Cancer

In Korea, lung cancer is a common cancer, and has the highest mortality rate in both males and females. Approximately 80% of lung cancer is due to smoking, and the remaining cases are known to be due to genetic factors, history of respiratory disease, infection, diet, and occupational and environmental factors. Since the occupational and environmental hazards may differ from region to region, the lung cancer risk may differ too. To identify this, we selected seven metropolitan cities, and compared occupational and environmental hazards. Furthermore, we calculated smoking rate adjusted standardized rate ratio (ratio of 10-year cumulative age-standardized incidence rate of lung cancer during the 2000⁻2009 period at target region versus reference region) to compare the regional lung cancer risk. The result showed that the emissions and concentrations of air pollutant were higher in high-risk regions, and the risk of lung cancer was significantly elevated in such area. In this study, we simultaneously consider the cumulative incidence, age-standardization and smoking rate adjustment. Therefore, we can conclude that the validity of the finding of this study is higher than that of past studies. In conclusion, the occupational and environmental hazards have an effect on lung cancer.

Occupational exposure to respirable crystalline silica and chronic non-malignant renal disease: systematic review and meta-analysis

Background

While occupational exposure to respirable silica is known to lead to lung disease, most notably silicosis, its association with chronic kidney disease is unclear.

Objectives

This review explores the association between occupational exposure to respirable silica and chronic non-malignant renal disease such as glomerulonephritis. The evidence has been collected and compiled. Possible sources of bias are thoroughly discussed.

Methods

Cohort studies with silica exposure and case–control studies of renal disease were searched in PubMed until January 2015. Two authors independently abstracted data; any disagreement was resolved by consulting a third reviewer. A meta-analysis was performed to evaluate the association to silica exposure.

Results

A total of 23 cohort and four case–control studies were included in the analysis. The meta-analysis of cohort studies yielded elevated overall SMRs for renal disease. Some studies, however, included dose–response analyses, most of which did not show a positive trend. The approaches and results of the case–control studies were very heterogeneous.

Conclusions

While the studies of cohorts exposed to silica found elevated SMRs for renal disease, no clear evidence of a dose–response relationship emerged. The elevated risk may be attributed to diagnostic and methodological issues. In order to permit a reliable estimation of a possible causal link, exposed cohorts should be monitored for renal disease, as the information from mortality studies is hardly reliable in this field.

Electronic supplementary material

The online version of this article (doi:10.1007/s00420-017-1219-x) contains supplementary material, which is available to authorized users.

Joint effects of smoking and silicosis on diseases to the lungs

Smokers are subject to being more susceptible to the long-term effects of silica dust, whilst it remains unclear whether the joint effect of smoking and silicosis differs amongst diseases to the lungs; this study aims to address this knowledge gap. This was a historical cohort study comprised of 3202 silicotics in Hong Kong during 1981-2005 who were followed up till 31/12/2006. We estimated the standardized mortality ratio (SMR) in the smoking and never smoking silicotics using the mortality rates of male general population indiscriminately by smoking status, but these SMRs were regarded as biased. We adjusted these biased SMRs using "smoking adjustment factors (SAF)". We assessed the multiplicative interaction between smoking and silicosis using 'relative silicosis effect (RSE)' that was the ratio of SAF-corrected SMR of smoking silicotics to the never smokers. A RSE differs significantly from one implies the presence of multiplicative interaction. A significant excess SMR was observed for respiratory diseases (lung cancer, chronic obstructive pulmonary diseases [COPD], silicosis) and other diseases to the lungs (pulmonary heart disease, tuberculosis). All the 'biased-SMRs' in smokers were higher than those in never smokers, but the SAF-corrected SMRs became higher in never smokers. The RSE was 0.95 (95%CI: 0.37-3.55), 0.94 (95%CI: 0.42-2.60), and 0.81 (95%CI: 0.60-1.19) for lung cancer, COPD, and silicosis; whilst it was 1.21 (95%CI: 0.32-10.26) for tuberculosis and 1.02 (95%CI: 0.16-42.90) for pulmonary heart disease. This study firstly demonstrated the joint effect of smoking and silicosis may differ amongst diseases to the lungs, but power is limited.

Laryngeal cancer and silica dust exposure: a systematic review and meta-analysis

BACKGROUND

Individual epidemiological studies generally lack the power to examine the association between silica exposure or silicosis and laryngeal cancer. We summarized pertinent evidence from published literature by using meta-analysis.

METHODS

A systematic literature search was performed to identify cohort and case-control studies, and the method of meta-analysis was used to combine standardized mortality ratios (SMRs) or standardized incidence ratios (SIRs) from cohort studies and odds ratios (ORs) from case-control studies.

RESULTS

A significantly increased risk of laryngeal cancer (pooled OR = 1.39, 95% confidence interval (95% CI): 1.17-1.67) among workers exposed to silica dust was observed by combining six case-control studies with adjustment for smoking and alcohol consumption. A similarly increased but statistically non-significant risk estimate was observed from cohort studies, with a pooled SMR of 1.38 (95% CI: 0.79-1.96) for silicosis cases; and a pooled SMR of 1.13 (95% CI: 0.82-1.45) and a pooled SIR of 1.50 (95% CI: 0.59-2.42) for workers with silica dust exposure.

CONCLUSION

This systematic review demonstrated a weak association between silica or silicosis and laryngeal cancer. Owing to the inherent limitations of the original studies, interpretation of the results of this meta-analysis should be cautious.

Occupational risks and lung cancer burden for Chinese men: a population-based case-referent study

OBJECTIVE

We aimed to fill in the gap of knowledge on the lung cancer burden resulting from occupational exposures among Chinese men through a population-based case-referent study.

METHODS

Detailed information on lifestyle and full occupational histories of 1,208 male lung cancer incident cases and 1,069 age-matched male community referents were obtained through interviews during 2004-2006. The associations between lung cancer risk and exposures to specific or group of agents that were confirmed or suspected occupational carcinogens were analyzed.

RESULTS

After adjustment of smoking and other potential confounding factors, significant odds ratio of lung cancer was observed for workers employed in major industrial divisions of "construction" (1.37, 95% CI: 1.00-1.89) and "financing, insurance, real estate, and business services" (0.48, 95% CI: 0.23-0.97), as well as in the occupational groups of "bricklayers, carpenters, and other construction workers" (1.49, 95% CI: 1.07-2.06). Significantly elevated odds ratios were found for occupational exposures to silica dust (1.75, 95% CI: 1.16-2.62), welding fumes (1.74, 95% CI: 1.13-2.68), diesel exhaust (2.18, 95% CI: 1.23-3.84), and man-made mineral fibers (7.45, 95% CI: 1.63-34.00), while a significantly reduced risk (OR = 0.67, 95% CI: 0.47-0.95) was linked to cotton dust. The population attributable fraction of lung cancer was 3.2% (95% CI: 0.1-7.3%) for construction workers and 9.5% (95% CI: 4.8-15.1%) for the four significant specific exposures.

CONCLUSIONS

Our study indicates that previous exposure to occupational carcinogens remains an important determinant of lung cancer burden for Hong Kong Chinese men. However, results obtained from this study should be confirmed by future analyses based on job exposure matrix.

An exposure-response threshold for lung diseases and lung cancer caused by crystalline silica

Whether crystalline silica (CS) exposure increases risk of lung cancer in humans without silicosis, and, if so, whether the exposure-response relation has a threshold, have been much debated. Epidemiological evidence is ambiguous and conflicting. Experimental data show that high levels of CS cause lung cancer in rats, although not in other species, including mice, guinea pigs, or hamsters; but the relevance of such animal data to humans has been uncertain. This article applies recent insights into the toxicology of lung diseases caused by poorly soluble particles (PSPs), and by CS in particular, to model the exposure-response relation between CS and risk of lung pathologies such as chronic inflammation, silicosis, fibrosis, and lung cancer. An inflammatory mode of action is described, having substantial empirical support, in which exposure increases alveolar macrophages and neutrophils in the alveolar epithelium, leading to increased reactive oxygen species (ROS) and nitrogen species (RNS), pro-inflammatory mediators such as TNF-alpha, and eventual damage to lung tissue and epithelial hyperplasia, resulting in fibrosis and increased lung cancer risk among silicotics. This mode of action involves several positive feedback loops. Exposures that increase the gain factors around such loops can create a disease state with elevated levels of ROS, TNF-alpha, TGF-beta, alveolar macrophages, and neutrophils. This mechanism implies a "tipping point" threshold for the exposure-response relation. Applying this new model to epidemiological data, we conclude that current permissible exposure levels, on the order of 0.1 mg/m³, are probably below the threshold for triggering lung diseases in humans.

Mortality in Vermont granite workers and its association with silica exposure

OBJECTIVES

To assess mortality in Vermont granite workers and examine relationships between silica exposure and mortality from lung cancer, kidney cancer, non-malignant kidney disease, silicosis and other non-malignant respiratory disease. Methods Workers employed between 1947 and 1998 were identified. Exposures were estimated using a job-exposure matrix. Mortality was assessed through 2004 and standardised mortality ratios (SMRs) were computed. Associations between mortality and exposure to silica were assessed by nested case-control analyses using conditional logistic regression. Results 7052 workers had sufficient data for statistical analysis. SMRs were significantly elevated for lung cancer (SMR 1.37, 95% CI 1.23 to 1.52), silicosis (SMR 59.13, 95% CI 44.55 to 76.97), tuberculosis (SMR 21.74, 95% CI 18.37 to 25.56) and other non-malignant respiratory disease (SMR 1.74, 95% CI 1.50 to 2.02) but not for kidney cancer or non-malignant kidney disease. In nested case-control analyses, significant associations with cumulative exposure to respirable free silica were observed for silicosis (OR 1.13, 95% CI 1.05 to 1.21 for each 1 mg/m(3)-year increase in cumulative exposure) and other non-malignant respiratory disease (OR 1.10, 95% CI 1.03 to 1.16) but not for lung cancer (OR 0.99, 95% CI 0.94 to 1.03), kidney cancer (OR 0.96, 95% CI 0.84 to 1.09) or non-malignant kidney disease (OR 0.95, 95% CI 0.84 to 1.08). Conclusions Exposure to crystalline silica in Vermont granite workers was associated with increased mortality from silicosis and other non-malignant respiratory disease, but there was no evidence that increased lung cancer mortality in the cohort was due to exposure. Mortality from malignant and non-malignant kidney disease was not significantly increased or associated with exposure.

Prior lung disease and lung cancer risk in an occupational-based cohort in Yunnan, China

We used the data from a prospective cohort study among tin miners in Yunnan, China to investigate whether prior lung disease is a risk factor for lung cancer. Information on prior lung disease was obtained from baseline questionnaires. The Cox proportional hazards model was used to examine the relationship between prior lung disease and lung cancer risk. From 1992 to 2001, a total of 502 lung cancer cases were confirmed among 9295 cohort participants. Prior chronic bronchitis was associated with an increase in lung cancer risk with an adjusted HR of 1.50 (95% CI: 1.24-1.81). There was an increased risk of developing squamous cell carcinoma in the setting of prior chronic bronchitis and small cell carcinoma in association with asthma with an adjusted HRs of 1.57 (95% CI: 1.19-2.09) and 2.56 (95% CI: 1.38-4.75), respectively. This prospective study provides further evidence that prior chronic bronchitis correlates with increased lung cancer risk, especially for squamous cell carcinoma. Asthma is associated with increased risk of small cell lung carcinoma.

Silica dust, diesel exhaust, and painting work are the significant occupational risk factors for lung cancer in nonsmoking Chinese men

Background:

Few epidemiological studies have explored the associations between occupational exposures and lung cancer in lifelong nonsmoking men.

Methods:

We obtained lifetime occupational history and other relevant information for 132 newly diagnosed lung cancer cases among nonsmoking Chinese men and 536 nonsmoking community referents. Unconditional multiple logistic regression analysis was performed to estimate the odds ratio (OR) of lung cancer for specific occupational exposures.

Results:

Significantly increased lung cancer risk was found for nonsmoking workers occupationally exposed to silica dust (OR=2.58, 95% confidence interval (CI): 1.11, 6.01), diesel exhaust (OR=3.47, 95% CI: 1.08, 11.14), spray painting (OR=2.81, 95% CI: 1.14, 6.93), and nonspray painting work (OR=2.36, 95% CI: 1.04, 5.37). Silica dust exposure was associated with a significantly increased risk of adenocarcinoma (OR=2.91, 95% CI: 1.10, 7.68). We observed a positive gradient of all lung cancers and of adenocarcinoma with duration of employment for workers exposed to silica dust and spray painting.

Conclusion:

This study found an increased risk of lung cancer among nonsmoking Chinese men occupationally exposed to silica dust, diesel exhaust, and painting work.

Dose-response meta-analysis of silica and lung cancer

OBJECTIVE

To examine the association between occupational exposure to silica and lung cancer from a systematic review (and meta-analysis) of the epidemiologic literature, with special reference to the methodological quality of observational studies.

METHODS

We searched Medline, Toxline, BIOSIS, and Embase (1966-December 2007) for original articles published in any language. Observational studies (cohort and case-control studies) were selected if they reported the result of dose-response analyses relating lung cancer to occupational exposure to silica after appropriate adjustment for smoking.

RESULTS

Ten studies (4 cohort studies and 6 case-control studies) met the inclusion criteria of the meta-analysis, nine of which contributing to the main analysis (dose-response analysis, no lag time). We found increasing risk of lung cancer with increasing cumulative exposure to silica, with heterogeneity across studies however. Posthoc analyses identified a set of seven more homogeneous studies. Their meta-analysis resulted in a dose-response curve that was not different from that obtained in the main analysis.

CONCLUSION

Silica is a lung carcinogen. This increased risk is particularly apparent when the cumulative exposure to silica is well beyond that resulting from exposure to the recommended limit concentration for a prolonged period of time.