The difference in effect of ambient particles on mortality between days with and without yellow dust events: Using a larger dataset in Seoul, Korea from 1998 to 2015

Background: Yellow Dust (YD) is a natural source of particulate matter (PM) in Korea. It remarkably increases the concentration of PM. However, characteristics of PM in YD period are different from those of PM in non-YD period. Objectives: To investigate whether the association of PM with mortality is different between all days and non-YD days in Seoul, Korea, 1998–2015. Methods: We applied time-stratified case-crossover design to estimate effects of PM₁₀ and PM_2.5 on non-accidental cardiovascular and respiratory mortality. Effect estimates of PM were compared for all days in the study period and days without YD events. To identify whether different effect estimates between all days and non-YD days were not merely caused by the exclusion of high PM concentrations but rather by YD itself, we estimated effects of PM by randomly excluding the same number of days as days of YD. Results: A total of 4,509,392 deaths were observed during the study period. A 10 μg/m³ increase in PM₁₀ or PM_2.5 was associated with a 0.15% (95% CI: 0.06% to 0.24%) or 0.27% (95% CI: 0.07% to 0.47%) increase in risk of non-accidental mortality for all days, respectively. These associations were changed to 0.30% (95% CI: 0.18% to 0.42%) and 0.33% (95% CI: 0.10% to 0.55%) when YD days were excluded from analyses. We also found that effect estimates of PM were larger when YD days were excluded than those when high PM concentrations were randomly excluded. Conclusions: The effect estimates of PM differed between all days and non-YD days. Our study suggests that including YD days in the analyses is likely to attenuate the effect of PM in a usual urban environment.