Differentiating the effects of air pollution on daily mortality counts and years of life lost in six Chinese megacities

Ambient air pollution has been widely associated with increased mortality and years of life lost (YLL) from various diseases. However, no study has assessed that the effects of air pollution on overall YLL were due to increased number of mortalities or average YLL per death.We first conducted a time-series study from 2013 to 2016, covering six Chinese megacities. Generalized additive models with a Gaussian link were utilized to estimate the associations of fine particulate matter (PM2.5) and nitrogen dioxide (NO2) with daily overall YLL and average YLL per death from various causes, including non-accidental causes, cardiovascular diseases (CVD), respiratory diseases (RD), ischemic heart disease (IHD), chronic obstructive pulmonary diseases (COPD), stroke and acute myocardial infraction (AMI). The city-specific estimates were then pooled by random-effects meta-analysis.A total of 1,586,741 deaths from non-accidental causes and 21,916,857 YLLs were recorded in the six cities, providing an average of 13.81 daily YLLs per death. Significant effects of PM2.5 and NO2 on daily overall YLL and daily mortality count were observed, but there were no significant effects on average YLL per death. At the pooled level, each 10 μg/m3 increase in PM2.5 and NO2 was associated with increased YLL and higher mortality due to non-accidental causes [PM2.5: 5.45 years, 95% CI: 1.47, 9.42 and ERR (excess relative risk) = 0.25%, 95% CI: 0.14%, 0.35%; NO2: 20.46 years, 95% CI: 10.77, 30.15 and ERR = 1.13%, 95% CI: 0.63%, 1.63%]. Consistent results and patterns were observed for other cause-specific diseases, including IHD, COPD, stroke and AMI.Our study indicates observed associations between air pollution and YLL might be mainly induced by increasing mortality count, rather than increasing average life lost for each death. More relevant intervention should be performed to reduce the number of deaths due to air pollution.