Assessment of human exposure to ambient particulate matter

Recent epidemiological studies have consistently shown that the acute morta lity effects of high concentrations of ambient particulate matter (PM), doc umented in historic air pollution episodes, may also be occurring at the lo w to moderate concentrations of ambient PM found in modern urban areas. In London in December 1952, the unexpected deaths due to PM exposure could be identified and counted as integers by the coroners. In modern times, the PM -related deaths cannot be as readily identified, and they can only be infer red as fractional average daily increases in mortality rates using sophisti cated statistical filtering and analyses of the air quality and mortality d ata. The causality of the relationship between exposure to ambient PM and a cute mortality at these lower modern PM concentrations has been questioned because of a perception that there is little significant correlation in tim e between the ambient PM concentrations and measured personal exposure to P M from all sources (ambient PM plus indoor-generated PM). This article shows that the critical factor supporting the plausibility of a linear PM mortality relationship is the expected high correlation in time of people's exposure to PM of ambient origin with measured ambient PM conc entrations, as used in the epidemiological time series studies. The presenc e of indoor and personal sources of PM masks this underlying relationship, leading to confusion in the scientific literature about the strong underlyi ng temporal relationship between personal exposure to PM of ambient origin and ambient PM concentration. The authors show that the sources of PM of no n-ambient origin operate independently of the ambient PM concentrations, so that the mortality effect of non-ambient PM, if any, must be independent o f the effects of the ambient PM exposures.