Long-term exposure to ambient PM_2.5, active commuting, and farming activity and cardiovascular disease risk in adults in China: a prospective cohort study

Background Increased physical activity is associated with a reduced risk of cardiovascular disease, but outdoor physical activity can be accompanied by increased inhalation of fine particulate matter (PM2.5). The extent to which long-term exposure to PM2.5 can offset the cardiovascular benefits of physical activity is unknown. We aimed to evaluate whether the associations between active commuting or farming activity and incident risks of cerebrovascular disease and ischaemic heart disease were consistent between populations with different ambient PM2.5 exposures. Methods We did a prospective cohort study using data from people aged 30-79 years without cardiovascular disease at baseline from the China Kadoorie Biobank (CKB). Active commuting and farming activity were assessed at baseline using questionnaires. A high-resolution (1 x 1 km) satellite-based model was used to estimate annual average PM2.5 exposure during the study period. Participants were stratified according to PM2.5 exposure (54 mu g/m(3) or greater vs less than 54 mu g/m(3)). Hazard ratios (HRs) and 95% CIs for incident cerebrovascular disease and ischaemic heart disease by active commuting and farming activity were estimated using Cox proportional hazard models. Effect modifications by PM2.5 exposure were tested by likelihood ratio tests. Analyses were restricted to the period from Jan 1, 2005, to Dec 31, 2017. Findings Between June 25, 2004, and July 15, 2008, 512 725 people were enrolled in the CKB cohort. 322 399 eligible participants completed the baseline survey and were included in the analysis of active commuting (118 274 non-farmers and 204 125 farmers). Among 204125 farmers, 2985 reported no farming time and 201140 were included in the farming activity analysis. During a median follow-up of 11 years, 39 514 cerebrovascular disease cases and 22 313 ischaemic heart disease cases were newly identified. Among non-farmers with exposure to annual average PM2.5 concentrations of less than 54 mu g/m(3), increased active commuting was associated with lower risks of cerebrovascular disease (highest active commuting vs lowest active commuting HR 0.70, 95% CI 0.65-0.76) and ischaemic heart disease (0.60, 0.54-0.66). However, among non-farmers with exposure to annual average PM2.5 concentrations of 54 mu g/m(3) or greater, there was no association between active commuting and cerebrovascular disease or ischaemic heart disease. Among farmers with exposure to annual average PM2.5 concentrations of less than 54 mu g/m(3), increased active commuting (highest active commuting vs lowest active commuting HR 0.77, 95% CI 0.63-0.93) and increased farming activity (highest activity vs lowest activity HR 0.85, 95% CI 0.79-0.92) were both associated with a lower cerebrovascular disease risk. However, among farmers with exposure to annual average PM2.5 concentrations of 54 mu g/m(3) or greater, increases in active commuting (highest active commuting vs lowest active commuting HR1.12, 95% CI 1.05-1.19) and farming activity (highest activity vs lowest activity HR 1.18, 95% CI 1.09-1.28) were associated with an elevated cerebrovascular disease risk. The above associations differed significantly between PM2.5 strata (all interaction p values <0.0001). Interpretation For participants with long-term exposure to higher ambient PM2.5 concentrations, the cardiovascular benefits of active commuting and farming activity were significantly attenuated. Higher levels of active commuting and farming activity even increased the cerebrovascular disease risk among farmers with exposure to annual average PM2.5 concentrations of 54 mu g/m(3) or greater.