Can mortality displacement mask thresholds in the concentration-response relation between particulate matter air pollution and mortality?

Abstract

Two critical issues for public health assessment and regulation of exposure to particulate matter air pollution (PM) are whether there is a threshold in the concentration-response relation between PM and mortality and whether there is mortality displacement. In this vein, a number of time-series studies have concluded that a linear relation without a threshold is appropriate for describing the concentration-response relation, and a number of other studies have concluded that the mortality effects of PM exposure cannot be attributed to mortality displacement alone. Using three-state (healthy, frail, dead) population models that incorporate actual time series data from Cook County, Illinois for the period 1987-2000, the author investigates the shape of the concentration-response relation between PM and mortality, as observable from time-series data, in the presence of mortality displacement. It is found that thresholds in the concentration-response relation can be masked, or hidden, by linear concentration-response relations if some of the effect of PM exposure is attributable to mortality displacement. This is an important finding that has implications for studies that have found a linear concentration-response relation appropriate, particularly given the markedly different implications for public health assessment and regulation of a linear versus threshold concentration-response relation.