Spatial variation in lag structure in the short-term effects of air pollution on mortality in seven major South Korean cities, 2006-2013.

Lag is one of the major uncertainties in the heterogeneity of short-term effects of particulate matter with aerodynamic diameter <10 μm (PM) on mortality. This study aimed to explore spatial variations in extended lag effects of PM on all-cause mortality, cardiovascular mortality and respiratory mortality in seven major South Korean cities over a period spanning 2006-2013. We did time-series analysis using generalized linear models and adjusted for temporal trend, day of the week, holiday, influenza epidemic, and weather. Single lag models and distributed lag models were extensively compared, specifically in terms of lag interval, and adjustment for temporal trend. We also conducted a time-stratified case-crossover analysis. Multivariate meta-regressions with city characteristic variables were conducted in order to assess spatial variation in the lag structure. When considering up to previous 45 days of exposure, we found longer lag associations between PM and mortality, particularly in all-cause mortality and respiratory mortality. SO, the ratio of SO to PM and gross regional domestic product were all found to positively contribute towards the associations between PM and all-cause mortality and cardiovascular mortality. Ulsan (Korea's largest industrial city) was found to have the strongest cumulative percentage increases in all-cause mortality and cardiovascular mortality per 10 μg/m increase of PM: 4.9% (95% CI: 2.5, 7.3) and 4.3% (95% CI: -0.9, 9.7) respectively. Busan (Korea's largest seaport city) was found to have the highest cumulative percentage increase in respiratory mortality with an 8.2% increase (95% CI: 2.8, 13.8). In summary, the short-term effects of PM on mortality may persist over a period of not just few weeks but longer than a month, and may differ according to regional economy. This study provides public health implication that, in order to minimize the health effects of PM, air quality interventions should focus on not only particulate pollution but also gaseous pollution, such as SO.