AIR-POLLUTION AND MORTALITY - ISSUES AND UNCERTAINTIES

Results from 31 epidemiology studies linking air pollution with premat ure mortality are compared and synthesized. Consistent positive associ ations between mortality and various measures of air pollution have be en shown within each of two fundamentally different types of regressio n studies and in many variations within these basic types; this is ext remely unlikely to have occurred by chance. In this paper, the measure of risk used is the elasticity, which is a dimensionless regression c oefficient defined as the percentage change in the dependent variable associated with a 1% change in an independent variable, evaluated at t he means. This metric has the advantage of independence from measureme nt units and averaging times, and is thus suitable for comparisons wit hin and between studies involving different pollutants. Two basic type s of studies are considered: time-series studies involving daily pertu rbations, and cross-sectional studies involving longer-term spatial gr adients. The latter include prospective studies of differences in indi vidual survival rates in different locations and studies of the differ ences in annual mortality rates for various communities. For a given d ata set, time-series regression results will vary according to the sea sonal adjustment method used, the covariates included, and the lag str ucture assumed. The results from both types of cross-sectional regress ions are highly dependent on the methods used to control for socioecon omic and personal lifestyle factors and on data quality. A major issue for all of these studies is that of partitioning the response among c ollinear pollution and weather variables. Previous studies showed that the variable with the least exposure measurement error may be favored in multiple regressions; assigning precise numerical results to a sin gle pollutant is not possible under these circumstances. We found that the mean overall elasticity as obtained from time-series studies for mortality with respect to various air pollutants entered jointly was a bout 0.048, with a range from 0.01 to 0.12. This implies that about 5% of daily mortality is associated with air pollution, on average. The corresponding values from population-based cross-sectional studies wer e similar in magnitude, but the results from the three recent prospect ive studies varied from zero to about five times as much. Long-term re sponses in excess of short-term responses might be interpreted as show ing the existence of chronic effects, but the uncertainties inherent i n both types of studies make such an interpretation problematic.