Our analysis using two-pollutant models showed that the addition of SO2 and Ox but not NO2 slightly attenuated the risk of SPM compared to in the single pollutant model. With the available data however, it is not possible to determine whether this difference between the one and
two-pollutant models reflects a real difference in toxicity. For example, our results may have been influenced by statistical co-linear effects among the pollutants, while a previous study in Japan suggested that the effects of PM on mortality can be distorted after adjustment for co-pollutants (i.e.NO2, SO2 and Ox).