Schwartz, J; Bind, MA; Koutrakis, P
BACKGROUND: While many time series studies have established associations of daily pollution variations with daily deaths, there are fewer at low concentrations, or focused on locally generated pollution, which is becoming more important as regulations reduce regional transport. Causal modeling approaches are also lacking.
OBJECTIVE: To use causal modeling to estimate the impact of local air pollution on mortality at low concentrations.
METHODS: Using an instrumental variable approach, we developed an instrument for variations in local pollution concentrations that is unlikely to be correlated with other causes of death, and examined its association with daily deaths in the Boston area. We combined height of the planetary boundary layer and wind speed, which impact concentrations of local emissions, to develop the instrument for PM2.5, BC, or NO2 variations that were independent of year, month, and temperature. We also used Granger causality to assess whether omitted variable confounding existed.
RESULTS: We estimated that an interquartile range increase in the instrument for local PM2.5 was associated with a 0.90% increase in daily deaths (95% CI 0.25, 1.56). A similar result was found for BC, and a weaker association with NO2. The Granger test found no evidence of omitted variable confounding for the instrument. A separate test confirmed the instrument was not associated with mortality independent of pollution. Furthermore, the association remained when all days with PM2.5 concentrations above 30 μg/m(3) were excluded from the analysis (0.84% increase in daily (95% CI 0.19, 1.50).
CONCLUSIONS: We conclude that there is a causal association of local air pollution with daily deaths at concentrations below EPA standards. The estimated attributable risk in Boston exceeded 1,800 deaths during the study period, indicating important public health benefits can follow from further control efforts.
