The relationships between short-term exposure to particulate matter and mortality in Korea: Impact of particulate matter exposure metrics for sub-daily exposures.

Most studies of short-term particulate matter (PM) exposure use 24-hour averages. However, other pollutants have stronger effects at shorter timeframes, which has influenced policy (e.g., ozone 8-hour maximum). Selecting appropriate exposure timeframes is important for effective regulation. The U.S. EPA identified health effects for sub-daily PM exposures as a critical research need. Unlike most areas, Seoul, Korea has hourly measurements of PM10, although not PM2.5. We investigated PM10 and mortality (total, cardiovascular, respiratory) in Seoul (1999-2009) considering sub-daily exposures: 24-hour, daytime (7am-8pm), morning (7-10am), nighttime (8pm-7am), and 1-hour daily maximum. We applied Poisson generalized linear modeling adjusting for temporal trends and meteorology. All PM10 metrics were significantly associated with total mortality. Compared to other exposure timeframes, morning exposure had the most certain effect with total mortality (based on statistical significance). A 10µg/m3 increase in 24-hour, daytime, morning, nighttime, and 1-hour maximum PM10 was associated with a 0.15% (95% confidence interval 0.02-0.28%), 0.14% (0.01-0.27%), 0.10% (0.03-0.18%), 0.12% (0.03-0.22%), and 0.10% (0.00-0.21%) increase in total mortality, respectively. PM10 was significantly associated with cardiovascular mortality for 24-hour, morning, and nighttime exposures. We did not identify significant associations with respiratory mortality. Results support use of a 24-hour averaging time as an appropriate metric for health studies and regulation, particularly for PM10 and mortality.