Relationship between ozone air pollution and daily suicide mortality: a time-stratified case-crossover study in Taipei

Several studies suggested short-term exposure to air pollution might be associated with suicide mortality, although results have been inconsistent and vary depending upon the type of air contaminants. While seasonal variation associated with suicide was reported to occur and that in the spring and early summer there are peaks in ozone (O₃) distribution, the relationship between these two parameters is not known. The aim of this study was to examine the potential association between O₃ levels and daily mortality rate related to suicide in Taipei for the period 2004–2008 using a time-stratified case-crossover analysis. In our single pollutant model without adjustment for other pollutants, the risk of suicide increased by 11% on warm days and 15% on cool days for each interquartile range (IQR) rise in O₃ concentration, respectively. The relationship was positive but did not reach significance. In our two-pollutant models, O₃ remained non-significant on warm days after inclusion of one of any other ambient air contaminants into the model. However, on cool days, a significant association was found between O₃ levels and enhanced risk of mortality due to suicide after nitrogen dioxide (NO₂) or carbon monoxide (CO) were included. The overall positive but not significant findings of elevated risk of mortality frequently attributed to suicide on days with higher O₃ levels suggest that outdoor exposures to this gaseous contaminant may contribute to increases in daily mortality rate related to suicide.