Modifiers of the effect of short-term variation in PM_2.5 on mortality in Beijing, China

Epidemiologic studies have reported associations between short-term exposure to particulate matter <2.5 μm in aerodynamic diameter (PM_2.5) and mortality, but the role of modifiers remains unclear with studies reporting inconsistent results. We evaluated the impact of individual (age, gender and education) and township (geographic area, socioeconomic status, background air pollution and road density) level factors on the relationship between short-term variation in PM_2.5 with cause-specific mortality in Beijing (population: 21.7 million in 2016), China.

Daily PM_2.5 concentrations in each township (n = 327; township population: 2000-359,400; township area: 1-392 km²) within Beijing were estimated by kriging with external drift using measurements from 35 air quality monitoring stations and geographic variables. Time-stratified case-crossover analysis with township-level mortality data from Oct. 1st, 2012 to Dec. 31st, 2013 was then used to examine associations between PM_2.5 exposure estimates and cause-specific mortality, stratified by the potential effect modifiers.

A 10-μg/m³ increase in PM_2.5 concentration was associated with a 0.17% [95% confidence interval (CI): 0.05%-0.29%] and 0.27% (95%CI:0.01%-0.52%) increase in non-accidental and stroke mortality with no lag, a 0.81% (95%CI:0.39%-1.23%) and 0.96% (95%CI:0.35%-1.57%) increase in respiratory disease (RD) and chronic obstructive pulmonary disease (COPD) mortality at a lag of two-day moving average. For individual-level effect modifiers, the elderly showed higher effects for all the specific causes of mortality; those with lower education level showed higher effects for non-accidental, cardiovascular disease and stroke mortality; females showed higher effects for non-accidental and cause-specific cardiovascular diseases. For township-level effect modifiers, effect estimates tended to be larger for suburban areas, areas of lower road density, lower PM_2.5 and lower socioeconomic status.

Short-term exposure to township-level ambient PM_2.5 was associated with increased mortality in Beijing, with indications of effect modification by both individual and township-level factors.