Harvesting-resistant estimates of air pollution effects on mortality

A number of studies have recently shown an association between particle con centrations in outdoor air and daily mortality counts in urban locations. I n the public health interpretation of this evidence, a key issue is whether the increased mortality associated with higher pollution levels is restric ted to very frail persons for whom life expectancy is short in the absence of pollution. This possibility has been termed the "harvesting hypothesis." We present an approach to estimating the association between pollution and mortality from times series data that is resistant to short term harvestin g. The method is based in the concept that harvesting alone creates associa tions only at shorter time scales. We use frequency domain log-linear regre ssion to decompose the information about the pollution-mortality associatio n into distinct time scales, and we then create harvesting resistant estima tes by excluding the short-term information that is affected by harvesting. We illustrate the methods with total suspended particles and mortality cou nts from Philadelphia for 1974-1988. The total suspended particles mortalit y association in Philadelphia is inconsistent with the harvesting-only hypo thesis, and the harvesting-resistant estimates of the total suspended parti cles relative risk are actually larger-not smaller-than the ordinary estima tes.