dioxins and furans are more important on a regional than a local scale, whereas the emissions of some metals, such as mercury, are important close to the stack and on a regional scale. Third, it appears that incinerators with similar waste streams, operating conditions, and emission controls to those studied are unlikely to now be a major contributor to local ambient concentrations of the chemicals of concern noted in this report.
Although releases to the environment from incineration facilities occur mainly by air emissions, multiple potential pathways in the environment to humans exist. Variations in the physical properties of substances of concern and the extent to which they persist in the environment result in differences in the types of pathways. Results of environmental monitoring studies around incineration facilities indicate that the specific facilities studied are unlikely to be major contributors to local ambient concentrations of the substances of concern noted in this report. However, methodological limitations of the studies do not allow for general conclusions to be made about waste incineration's contributions to environmental concentrations of those contaminants. They also do not allow for characterization of total human exposure.
The air concentration of gases (including carbon monoxide and acid gases) and fine particles can be estimated by dispersion models to data on stack emissions, stack height, and local meteorological conditions. For the case of heavy metals carried by the particles, the deposition fluxes of the particles and the transfer from the soils into the food chain through vegetable produce and animal fodder needs to be taken into account. Models for the dispersion and the uptake into the food chain are available for use in risk assessments.
Mercury and dioxins and furans are classes of pollutants where historically incinerators are estimated to have contributed a significant portion of the total national emissions. These classes of pollutants are characterized by their long-range air transport, persistence, and relative uniformity of deposition fluxes on regional bases. Whereas one incinerator might only contribute a small fraction of the total environmental concentration of these compounds, the sum of the emissions of all the incineration facilities in a region could be considerable. Because a number of older incinerators had been closed down and replaced by modern low-emitting units, it is now uncertain how much incineration contributes to the environmental concentrations of these compounds.
Computational models for the environmental fate and transport of mercury and the dioxins and furans provide useful information for assessing the major exposure pathways for humans, but are unable to provide overall