Abstract
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.
Highlights
Occupational epidemiological studies conducted in workers with silicosis have consistently shown increased risks of lung cancer and chronic obstructive pulmonary diseases (COPD) [1,2,3]; these increased risks, could be linked to their smoking behaviors because silicotic workers were generally more prone to cigarette smoking than the referent population
We found that workers with silicosis were associated with an increased risk of mortality from all causes and the respiratory diseases, with a relatively stronger risk ratio effect of silicosis in never smokers given the relative silicosis effect (RSE) of lower than one
An excess risk of death was observed from pulmonary tuberculosis, but the silicosis effect was relatively weaker in never smokers, which indicates a positive multiplicative interaction may exist between smoking and silicosis on the mortality from pulmonary tuberculosis
Summary
Occupational epidemiological studies conducted in workers with silicosis have consistently shown increased risks of lung cancer and chronic obstructive pulmonary diseases (COPD) [1,2,3]; these increased risks, could be linked to their smoking behaviors because silicotic workers were generally more prone to cigarette smoking than the referent population. Long-term cigarette smoking may damage the respiratory system [4], leading the smokers to being more susceptible than the never smokers to the adverse health effects due to the prolonged exposure to hazardous silica dust or the associated silicosis. It remains unclear whether the joint effect of smoking and silicosis differs amongst a variety of diseases to the lungs; this study aims to address this knowledge gap by using data from a historical cohort of Chinese workers with silicosis
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