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Studies of urban air pollution episodes during the 1950s documented that large, short-term increases in air pollution could cause increased mortality. More recently, a growing number of epidemiological studies, often employing sophisticated statistical methods, have observed that much smaller short-term increases in particulate air pollution also are associated with increased mortality from respiratory and cardiovascular disease. A number of studies are currently being conducted to re-examine these original findings, to analyze the implications of different methodological approaches and underlying assumptions, to identify the population groups at increased risk of mortality, to understand the pathophysiological mechanisms involved, and to characterize the properties of particulate air pollution most strongly associated with mortality.
Without prejudging the outcomes of these studies, the TAP Project is developing a framework to connect the findings of such health impact studies with available information on emission sources, dispersion characteristics, air quality monitoring results and population characteristics. Ultimately, this work at IIASA aims
The analysis addresses a number of important aspects. Some of the findings about acute mortality impacts of fine particles are presently being strongly debated. Although further studies focusing on the most critical aspects are being carried out, it is anticipated that uncertainties will not be entirely eliminated in the near future. The assessment framework envisaged as the outcome of this work will account for these uncertainties. Further, there is relatively little information about the health impacts of chronic exposure to fine particles. The lack of conclusive information does not, however, necessarily mean a lack of impacts, and a strategic assessment should not ignore this aspect. It is crucial, therefore, to consider the properties (size, chemical composition, shape) of the fine particles that are most strongly related to observed health impacts. Only a solid understanding of these key properties will enable a meaningful connection with cost-effective emission control options. Further, certain emission control measures may change the size distribution profiles of particles, i.e., they may remove larger fractions of the coarse particles, but less of fine particulate matter.
Particulate air pollution is caused by diverse sources. One major source of primary particle emissions is fuel combustion, including mobile sources such as diesel and gasoline-powered vehicles and power plants. Secondary particles (including sulfate and nitrate aerosols) are formed from gaseous pollutants (SO2, NOx). The relative contributions of these different sources to the particle concentration at a given site may vary significantly. At least for secondary particles their long-range transport in the atmosphere is an important aspect to be taken into account when designing cost-effective control strategies.
In designing cost-effective strategies for reducing health impacts of air pollution, the interactions of particles with other pollutants such as ozone and SO2 will be another important subject. Critical interactions exist not only for actual impacts (i.e., simultaneous exposure of a population to several pollutants), but also in relation to atmospheric processes (e.g., non-linear relations between particle concentrations and ozone levels). Also, some control technologies affect more than one pollutant and have different impacts on environmental effects (e.g., flue gas desulfurization simultaneously removes SO2 and particulate matter, which has impacts on acidification, on ambient levels of SO2 and primary particles and on secondary sulfate aerosols). Therefore, a further aspect for deriving priorities on future emission controls is the fact that ambient levels of primary and secondary particles are expected to decline in the future due to already agreed measures.
December 6, 2000