Is silica carcinogenic?

Monitoring of DNA damage in foundry and pottery workers exposed to silica by the alkaline comet assay

BACKGROUND

Workers in foundry and pottery are exposed to a mixture of chemicals and silica, which is suspected to cause genetic alterations.

METHODS

To investigate the potential hazard associated with the occupational exposure to silica, DNA damage in the peripheral lymphocytes of 30 foundry and 22 pottery workers were examined by the alkaline single-cell gel electrophoresis or Comet Assay, and compared to 52 healthy subjects with no history of occupational silica or chemical exposure.

RESULTS

The DNA damage observed in the lymphocytes of both foundry and pottery workers was significantly higher than that in their controls. Cigarette smoking was also related to DNA damage since the DNA damage observed in smoking silica-exposed workers compared with the non-smoking workers was significantly higher.

CONCLUSIONS

Occupational exposure of silica from foundry and pottery workplaces has been associated with the increased DNA damage and smoking which represents an additional risk factor and must be avoided.

Mortality from lung cancer among silicotic patients in Sardinia: an update study with 10 more years of follow up

OBJECTIVES

To evaluate the association between silica, silicosis and lung cancer, the mortality of 724 patients with silicosis, first diagnosed by standard chest x ray film between 1964 and 1970, has been analysed by a cohort study extended to 31 December 1997.

METHODS

Smoking and detailed occupational histories were available for each member of the cohort as well as the estimated lifetime exposure to respirable silica dust and radon daughters. Two independent readers blindly classified standard radiographs according to the 12 point International Labour Organisation (ILO) scale. Lung function tests meeting the American Thoracic Society's criteria were available for 665 patients. Standardised mortality ratios (SMRs) for selected causes of death were based on the age specific Sardinian regional death rates.

RESULTS

The mortality for all causes was significantly higher than expected (SMR 1.35, 95% confidence interval (95% CI) 1.24 to 1.46) mainly due to tuberculosis (SMR 22.0) and to non-malignant chronic respiratory diseases (NMCRD) (SMR 6.03). All cancer deaths were within the expected numbers (SMR 0.93; 95% CI 0.76 to 1.14). The SMR for lung cancer was 1.37 (95% CI 0.98 to 1.91, 34 observed), increasing to 1.65 (95% CI 0.98 to 2.77) allowing for 20 years of latency since the first diagnosis of silicosis. Although mortality from NMCRD was strongly associated to the severity of radiological silicosis and to the extent of the cumulative exposure to silica, SMR for lung cancer was weakly related to the ILO categories and to the cumulative exposure to silica dust only after 20 years of lag interval. A significant excess of deaths from lung cancer (SMR 2.35) was found among silicotic patients previously employed in underground metal mines characterised by a relatively high airborne concentration of radon daughters and among ever smokers who showed an airflow obstruction at the time of the first diagnosis of silicosis (SMR 3.29). Mortality for lung cancer related to exposure was evaluated with both the Cox's proportional hazards modelling within the entire cohort and a nested case-control study (34 cases of lung cancer and 136 matched controls). Both multivariate analyses did not show any significant association with cumulative exposure to silica or severity of silicosis, but confirmed the association between mortality for lung cancer and relatively high exposure to radon, smoking, and airflow obstruction as significant covariates.

CONCLUSIONS

The findings indicate that the slightly increased mortality for lung cancer in this cohort of silicotic patients was significantly associated with other risk factors-such as cigarette smoking, airflow obstruction, and estimated exposure to radon daughters in underground mines-rather than to the severity of radiological silicosis or to the cumulative exposure to crystalline silica dust itself.

Lung cancer mortality and airways obstruction among metal miners exposed to silica and low levels of radon daughters

Starting from a cross-sectional survey in 1973, the mortality of two cohorts of Sardinian metal miners was followed through December 31, 1988. In mine A, the quartz concentration in respirable dust ranged between 0.2% and 2.0% and the exposure to radon daughters averaged 0.13 working level (WL), with the highest estimated cumulative exposure around 80-120 WLM. In mine B, the silica content was much higher (6.5-29%), but exposure to radon daughters was significantly lower than in mine A. More than 98% of the overall work force in 1973 (1,741 miners) entered the cohort, providing 25,842.5 person-years. Smoking, occupational history, chest radiographs, and lung function tests were available for the cohort members at admission. Mortality for all causes was slightly lower than expected. A significant excess for nonmalignant chronic respiratory diseases was noticed in both mines. Twenty-four subjects died of lung cancer, 17 from mine A (SMR: 128; 95% confidence interval [CI]: 75-205) and 7 from mine B (SMR: 85; 95% CI: 34-175). The SMR for lung cancer was highest among the underground workers from mine A (SMR: 148; 95% CI: 74-265), with a significant upward trend by duration of employment in underground jobs. Mine B underground miners showed lung cancer SMRs close to 100 without a significant trend by duration of employment. Among underground miners with spirometric airways obstruction in 1973, those from mine A showed the highest risk (SMR: 316; 95% CI: 116-687). The relationship did not change after adjusting for age and smoking. Based on the present findings, crystalline silica per se does not appear to affect lung cancer mortality. A slight association between lung cancer mortality and exposure to radon daughters, though within relatively low levels, may be considered for underground miners from mine A. Impaired pulmonary function may be an independent predictor of lung cancer and an important risk factor enhancing the residence time of inhaled carcinogens, i.e., alpha particles or PAHs, by impairing their bronchial and alveolar clearance.