Determining the threshold effect of ozone on daily mortality: an analysis of ozone and mortality in Seoul, Korea, 1995-1999

Environ Res. 2004 Feb;94(2):113-9. doi: 10.1016/j.envres.2003.09.006.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Air Pollutants / poisoning*
  • Child
  • Child, Preschool
  • Female
  • Humans
  • Infant
  • Infant, Newborn
  • Korea / epidemiology
  • Male
  • Middle Aged
  • Models, Theoretical*
  • Mortality / trends*
  • Oxidants, Photochemical / poisoning*
  • Ozone / poisoning*


  • Air Pollutants
  • Oxidants, Photochemical
  • Ozone