Evaluating the health impacts of incinerator emissions

Before a municipal solid waste incinerator (MSWI) can be operated, it is generally required that a health risk assessment be performed and that human health risks predicted fall below permissible levels. There are several components to the risk assessment paradigm, including: (1) determination of stack emissions for potentially toxic chemicals, (2) calculation of atmospheric dispersion and exposure point concentrations, (3) development of scenarios by which humans become exposed to airborne chemicals, (4) identification of dose-response functions for carcinogenic and noncarcinogenic effects, and (5) prediction of the probability of health impacts. Typical MSWI air contaminants of concern are metals (e.g., Ag, As, Be, Cd, Cr, Hg, Ni, Pb, Sb) and organic compounds (e.g., benzene, PCBs, B(a)P, polychlorinated dioxins/furans). MSWI risk assessments include both direct exposure pathways (air inhalation, incidental ingestion of soil), and indirect pathways (food-chain exposures such as human consumption of produce, beef, fish, and milk). To perform a risk assessment for direct and indirect routes of exposure, both atmospheric concentration and deposition rate are required; assumptions need to be made about toxicity as a function of route of exposure. Interpretation of risk-assessment results requires understanding how some of the conservative assumptions made in the risk-assessment process play out relative to real-world health hazards. Some attempts have been made to verify that predicted concentrations of airborne contaminants are reflected by measured levels, but in most cases the predicted air and soil concentrations fall below limits of detection and always within background variability. In summary, health risk assessments are useful for regulatory guidance, but it has not been possible to verify that health risks of MSWI emissions contribute measurably to population health risks.