from air pollution. Both causes of death are associated with exposure to PM, ozone, and sulfates. Mortality attributable to exposure to air pollution occurs mainly in individuals who already suffer from cardiac and/or respiratory diseases. Increased mortality in these groups occurs within one to five days following the hazardous exposure (Schwartz 1994a, Wilson and Spengler 1996).

Short-term exposure to high levels of air pollutants is also associated with diseases of the respiratory tract, both upper and lower: bronchitis, pneumonia, chronic obstructive pulmonary disease, and cough with phlegm. Symptoms aggravated by exposure to certain pollutants such as ozone and PM include asthmatic attacks, cough without phlegm, and wheezing (Wilson and Spengler 1996, Ghio and others 1999).

Episodes of extremely high pollution documented in cities around the world have demonstrated the consequence of human exposure to high concentrations of air pollution. These episodes, however, occur sporadically, whereas exposure to low concentrations of pollutants over long periods of time is a daily phenomenon. Recent studies have focussed on establishing the effects of long-term exposure to low levels of air pollutants.

Health effects due to long-term, low-level exposure to air pollution (hereafter "chronic effects and chronic exposure") are similar to those reported for short-term exposure to high levels of air pollution. A synthesis of the available information concerning chronic exposure is an extremely complex task because many different factors can cause the same symptoms. There are several reports of increased mortality related to chronic exposure, however, most cases involve mainly elderly individuals for whom respiratory and cardiovascular problems are already the principal cause of death (Pope and Dockery 1999). Increased respiratory diseases (such as bronchitis) have also been reported as associated with chronic exposure.

In both acute and chronic exposure to air pollutants, populations are exposed to a complex mixture of compounds whose combined toxic effects could differ from that of each compound alone. A study performed on volunteers exposed to ozone with and without preexposure to H2SO4 showed that the preexposed group suffered more severe toxic effects than the group that was not preexposed (Wilson and Spengler 1996).

Particulate matter and ozone are often correlated spatially and over time, making it difficult to separate the effects of the individual pollutants. The mixture of PM1O and ozone, however, has proven more toxic than the individual compounds alone (Katsouyanni 1995). Unfortunately models and protocols to analyze the different interactions among environmental pollutants are not yet available (Samet and Speizer 1993). Thus, it is not clear how much each pollutant individually influences elevated mortality and morbidity rates. As a result some cost-benefit studies have chosen to use one index air pollutant rather than estimating effects for multiple air pollutants individually and then adding their effects to get a total air pollution effect. Focusing on a single pollutant provides a conservative approach to estimating air pollution effects. In fact, recent analyses (for example Thurston and Ito 1999) suggest that ozone and PM air pollution effects are relatively independent, since controlling for one pollutant has only modest effects on the concentration-response of the other. Thus, use of a single index pollutant underestimates the overall public health effects and monetary valuations _19_