Mortality from Non-Malignant Respiratory Diseases among Silicotics in Hong Kong: Exposure-Response Analyses with Silica Dust Exposure

Lap Ah TSE ¹, Ignatius T.S. Yu ^2*, Chi Chiu Leung ³, Wilson Tam ² and Tze Wai Wong ²

¹ The Chinese University of Hong Kong, Faculty of Medicine, Hong Kong
² The Chinese University of Hong Kong, Hong Kong
³ Department of Health, Hong Kong, Hong Kong

^* To whom correspondence should be addressed. E-mail: iyu{at}cuhk.edu.hk.

Accepted 25 August 2006

Abstract

Objectives: This study was undertaken to examine the exposure-response relationships between various indices of silica dust exposure and the mortality from non-malignant respiratory diseases (NMRD) or chronic obstructive pulmonary diseases (COPD) among a cohort of silicotic workers 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). Penalized smoothing spline models were used as a preliminary step to detect outliers and guide further analyses. Multiple Cox’s proportional hazard model was performed to estimate the dust effects on the risk of NMRD or COPD mortality after truncating the highest exposures. Results: 371 of the 853 deaths (43.49%) occurring among 2789 silicotic workers during 1981-1999 were from NMRD and 101 (27.22%) NMRD were from COPD. Multiple Cox’s proportional hazard models showed that CDE and MDC were only significantly associated with NMRD mortality. Subgroup analysis showed that deaths from NMRD and COPD were significantly associated with both CDE and MDC among underground caisson workers and those ever employed in other occupations with high silica dust exposure. No exposure-response relationship was observed for surface construction workers with low exposures. A clear upward trend for both NMRD and COPD mortality was found with increasing severity of radiological silicosis. Conclusion: Our study documented an exposure-response relationship between silica dust exposure and the risk of death from NMRD and/or COPD among silicotic workers except for the surface construction workers with low exposures. The risk of NMRD mortality increased significantly with the progression of ILO categories, independent of dust effects.

Keywords: chronic obstructive airway diseaase, crystalline silica, exposure-response relationship, non-malignant respiratory disease

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