Secondhand and thirdhand smoke exposure and the risk of liver cancer: a prospective cohort study of 315,833 Chinese never smokers.

Whether secondhand smoke (SHS) exposure confers an increased risk of liver cancer remains elusive, and no epidemiological studies have examined the potential impact of thirdhand smoke (THS) exposure on the risk of this cancer. Therefore, we aimed to examine the potential associations of SHS and THS exposure with the risk of liver cancer. A population-based cohort of 315,833 Chinese never smokers was identified from the China Kadoorie Biobank. SHS and THS exposure were assessed by the retrospective self-reports. Multivariable Cox regression was used to compute hazard ratio (HR) for liver cancer incidence. Restricted cubic spline regression was used to test nonlinearity. Over an average follow-up of 11.89 years, 876 liver cancer cases were documented. Daily or almost daily SHS exposure [HR 1.36, 95% confidence interval (CI) 1.16-1.60] and THS exposure at enrollment (HR 1.23, 95% CI 1.02-1.47) were found to be associated with an elevated risk of liver cancer, which were more pronounced in participants aged <60 years than in those aged ≥60 years (both Pinteraction<0.05). Moreover, a nonlinear positive dose-response association with the risk of liver cancer was found for the usual duration of SHS exposure (Pnonlinearity = 0.001). No significant joint association of SHS and THS exposure with the risk of liver cancer was found (Padditive interaction = 0.343). Our findings suggest that reducing SHS or THS exposure may be beneficial in the primary prevention of liver cancer. Nevertheless, these findings should be interpreted with much caution, given that the retrospective self-reports cannot clearly distinguish SHS from THS exposure.