The short- and long-term relationships between air pollution and health effects, in terms of mortality and morbidity, have been extensively demonstrated by numerous epidemiological studies. Short-term effects are acute functional or biological clinical manifestations that occur within a short period of time (a few days or weeks) following changes in ambient air pollution levels. Long-term effects may occur after chronic exposure (several months or years) to air pollution and ultimately lead to illness or even death. (R.A.)