Synoptic weather patterns and modification of the association between air pollution and human mortality

Daily and seasonal fluctuations in mortality have been positively and significantly associated with ambient concentrations of atmospheric pollution and meteorological variables such as temperature and barometric pressure. However, much less is known about the health risk from atmospheric pollutants in the context of frequently occurring weather patterns. The objective of this study is to assess the relationship between short-term exposure to urban air pollution and all-cause, and cardio-respiratory mortality under typical meteorological conditions. Concentrations of air pollutants (including carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone and fine particulate matter) and human mortality from non-accidental causes were examined according to typical winter and summer synoptic climatologies in Toronto, Canada, between 1981 and 1999. Air masses were derived using a hybrid spatial synoptic classification procedure associating each day over the 19-year period with one of six different weather types, or a transition between two weather types typical to southern Ontario. Generalized linear models (GLMs) were used to assess the risk of mortality from air pollution within specific air mass type subsets. Patterns of mortality follow a distinct seasonal pattern with a maximum in winter and a minimum in summer. Average air pollution concentrations were similar in both seasons with the exception of elevated sulfur dioxide levels in winter and elevated ozone levels in summer. Mortality rates and air pollution concentrations vary significantly among air mass types and there is evidence that the effect of air pollution on mortality can be modified according to the presence or absence of specific weather conditions. Typical strategies to control for specific weather variables in the study of air pollution and health effects are effective if the objective is an assessment of overall risk. A synoptic weather modeling strategy has more utility when attempting to discern the modification of air pollution and health associations according to natural variation of weather. These results also provide evidence of a possible interaction between air pollution and warmer than average weather conditions.