ALTERNATIVE HYPOTHESES LINKING OUTDOOR PARTICULATE MATTER WITH DAILY MORBIDITY AND MORTALITY

Numerous epidemiologic studies have reported associations between fluc tuations in ambient particulate matter (PM) and small changes in daily mortality and morbidity. However, it has not been possible to causall y link outdoor PM with changes in daily mortality/morbidity because of , among other things, problems relating to the exposure-dose relations hip. Alternative explanations are possible, but remain untested. Varyi ng levels oi ambient PM may be correlated to factors that modify morta lity/morbidity by alternative pathways. We identify three such pathway s: (1) weather conditions directly affecting health, (2) human behavio r patterns directly affecting health, and (3) weather conditions and p atterns of human behavior affecting exposure to indoor air contaminant s, which directly affect health. These three pathways also are related to day-to-day fluctuations in ambient PM. Thus, PM levels may only be a surrogate for other, causal factors. For example, unpleasant weathe r or poor outdoor air quality causes people to spend more time indoors , and also promotes the use of climate-control systems, which can gene rate increased indoor levels of potentially toxic airborne particles. At the same time, weather extremes tend to elevate outdoor PM levels b ecause of increased use of vehicles (instead of walking or cycling) an d increased combustion emissions (for providing electric power for hea ting or air conditioning). Therefore, personal exposure to indoor cont aminants may be linked to outdoor PM concentrations. The major signifi cance of indoor air contaminants is that, unlike outdoor PM, toxicolog ical and epidemiological studies of indoor air support biologic plausi bility. Our analysis supports the importance of considering the factor s that drive outdoor PM fluctuations in the first place to determine i i the observed mortality/morbidity changes may be caused by pathways o ther than the hypothetical frank toxicity from inhalation of ambient P M.