title: Determining the threshold effect of ozone on daily mortality: an analysis of ozone and mortality in Seoul, Korea, 1995-1999.
abstract: Many studies have shown a positive association between ambient ozone levels and mortality. Typically, these findings are based on models that assume a linear relationship between log mortality and ozone level. In this study, we adapted generalized additive models in which ozone effects are presumed to occur in three different ways: as a simple linear term, as a cubic natural spline term, and as a combination of two linear terms (a threshold model). We applied these models to daily time-series data for Seoul, Korea for the years 1995-1999 and found that the threshold model always fits best among the three. A 2.6% (95% CI: 1.7-3.5) increase of estimated relative risk (RR) in the total mortality associated with a 21.5 ppb increase of daily 1-h maximum ozone lagged by 1 day was observed by linear Poisson's regression. However, a 3.4% (95% CI: 2.3-4.4) increase in the estimated RR was observed using the threshold model. Adjustments for other ambient pollutants caused little changes to these results; 2.4-2.5% in the linear models and 3.2-3.4% in the threshold models. In addition, the largest difference in the estimated RRs of the linear and threshold models was observed in the summer: 1.9% (95% CI: 0.5-3.3) by the linear model and 3.8% (95% CI: 2.0-5.7) by the threshold model. These findings indicate that the conventional time-series Poisson regression model, which dose not take threshold into consideration, could underestimate the true risk of the ozone effect on daily mortality.
authors.affiliation: Department of Epidemiology and Biostatistics, School of Public Health, and Institute of Health and Environment, Seoul National University, 28 Yongon Dong, Chongro-Gu, Seoul 110-799, South Korea.
authors.forename: Sun-Young
authors.initials: SY
authors.lastname: Kim
authors.forename: Jong-Tae
authors.initials: JT
authors.lastname: Lee
authors.forename: Yun-Chul
authors.initials: YC
authors.lastname: Hong
authors.forename: Kyu-Jeong
authors.initials: KJ
authors.lastname: Ahn
authors.forename: Ho
authors.initials: H
authors.lastname: Kim
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000393:Air Pollutants / Q000506:poisoning*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007723:Korea / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D009026:Mortality / Q000639:trends*; D010083:Oxidants, Photochemical / Q000506:poisoning*; D010126:Ozone / Q000506:poisoning*
chemical_list: D000393:Air Pollutants; D010083:Oxidants, Photochemical; D010126:Ozone