Assessing the human health risk of atmospheric particles.

The health implications of atmospheric particulate matter have become a topic of much interest on both sides of the Atlantic, but especially in the USA, where they were at the centre of the debate over the Environmental Protection Agency's (EPA) revision of the National Ambient Air Quality Standard in 1997. Much of this debate revolves around claims that particulate matter (usually represented by PM10) in the atmosphere directly influences mortality, hospital admissions with respiratory diseases, and so on. In this paper, we take a critical look at one of these issues, the influence of PM10 on deaths in the elderly population. Two data sets are considered, one from Birmingham (Alabama) and the other from Chicago. In both cases we find a significant PM10-mortality relationship in some of the models fitted, but not in others. Other issues considered include the existence of a threshold below which PM10 has no discernible influence, the interaction with other pollutants and the mortality displacement or harvesting effect (the theory that the direct effect of PM10 is limited to a very small subset of the population who are already critically ill and whose death is only advanced by a few hours or days as a result of air pollution). Overall, our conclusion is that there are too many uncertain issues to allow us to make definitive statements about a causal relationship between PM10 and mortality.