Inappropriate use of daily mortality analyses to estimate longer-term mortality effects of air pollution.
BACKGROUND
To avoid the usual problems of multi-population correlation studies of air pollution and mortality, and for reasons of convenience, daily time-series mortality studies within single populations have recently become popular in air pollution epidemiology. Such studies describe how the short-term distribution of deaths relates to short-term fluctuations in air pollution levels. The regression-based risk coefficients from these acute-effects studies have been widely used to estimate the excess annual mortality within a population with a specified average level of air pollution. Such calculations are inappropriate. Since daily time-series data provide no simple direct information about the degree of life-shortening associated with the excess daily deaths (many of which are thought to be due to exacerbation of well-advanced disease, especially cardiovascular disease), such data cannot contribute to the estimation of the effects of air pollution upon chronic disease incidence and long-term death rates. Yet it is that category of effect that is of most public health importance.


CONCLUSIONS
Such effects are best estimated from long-term cohort studies that incorporate good knowledge of local (or personal) exposure to air pollutants and of potential confounders. Time-series studies, properly evaluated, can identify the existence of acute toxic effects of transient peak levels of air pollution; they are thus useful for monitoring acute toxicity and for identifying the most noxious pollutants. However, to quantify the long-term health impacts of air pollution we cannot use acute-effects data.
McMichael
A J
AJ
Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, UK.
Anderson
H R
HR
Brunekreef
B
B
Cohen
A J
AJ
D000397:Air Pollution / Q000009:adverse effects* / Q000706:statistics & numerical data; D002423:Cause of Death; D003627:Data Interpretation, Statistical; D004739:England; D004784:Environmental Monitoring; D006801:Humans; D009026:Mortality; D012106:Research; D012306:Risk