Interpretation of air pollution mortality: Number of deaths or years of life lost?

This paper examines the relation between the results of epidemiologic studies of air pollution mortality and impact indicators that can be informative for environmental policy decisions. Using models that are simple and transparent, yet contain the essential features, it is shown that (1) number of deaths is not meaningful for air pollution, whereas loss of life expectancy (LLE) is an appropriate impact indicator; (2) the usual short term (time series) studies yield a change in daily number of deaths attributable to acute effects of pollution, without any information on the associated LLE (although some information on this has recently become available by extending the observation window of time series); and (3) long-term studies yield a change in age-specific mortality, which makes it possible to calculate the total population averaged LLE (acute and chronic effects) but not the total number of premature deaths attributable to air pollution. The latter is unobservable because one cannot distinguish whether few individuals suffer a large or many suffer a small LLE. The paper calculates the LLE from exposure to PM10, as implied by the long-term mortality studies of adults and infants; population LLE for infants turns out to be an order of magnitude smaller than for adults. The LLE implied by short-term studies is a small fraction of the total loss implied by long-term studies, even if one assumes a very high loss per death. Applied to environmental policy, taking a permanent 50-70% reduction of PM10 as a reasonable goal, one finds a corresponding increase of average life expectancy in urban areas of the European Union (EU) and the United States of approximately four months.