Assessment of deaths attributable to air pollution: Should we use risk estimates based on time series or on cohort studies?

Epidemiologic studies are crucial to the estimation of numbers of deaths at tributable to air pollution. In this paper, the authors present a framework for distinguishing estimates of attributable cases based on time-series st udies from those based on cohort studies, the latter being 5-10 times large r. The authors distinguish four categories of death associated with air pol lution: A) air pollution increases both the risk of underlying diseases lea ding to frailty and the short term risk of death among the frail; B) air po llution increases the risk of chronic diseases leading to frailty but is un related to timing of death; C) air pollution is unrelated to risk of chroni c diseases but short term exposure increases mortality among persons who ar e frail; and D) neither underlying chronic disease nor the event of death i s related to air pollution exposure. Time-series approaches capture deaths from categories A and C, whereas cohort studies assess cases from categorie s A, B, and C. In addition, years of life lost can only be derived from coh ort studies, where time to death is the outcome, while in time-series studi es, death is a once-only event (no dimension in time). The authors conclude that time-series analyses underestimate cases of death attributable to air pollution and that assessment of the impact of air pollution on mortality should be based on cohort studies.