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Dose Additivity <br /> "Synergistic" effects are those in which the net effect of the simultaneous action of two <br /> or more toxic agents is greater than the sum of their individual effects. As an example, <br /> synergistic effects have been documented for the effects of asbestos exposure and <br /> smoking. "Antagonistic" effects are those in which the net effect of exposure to two or <br /> more agents is less than the sum of their individual effects. Data concerning combined <br /> effects of complex mixtures of chemicals are generally lacking. In the absence of such <br /> data, the approach recommended by EPA for evaluating the health impact of chemical <br /> mixtures assumes dose additivity. This approach does not account for potential <br /> synergism, antagonism, or differences in target organ specificity and mechanism of <br /> action. This approach may over- or underestimate the potential human health impact. <br /> 7.5 RISK CHARACTERIZATION <br /> Because there are uncertainties in each step of the risk assessment process, these <br /> uncertainties are often magnified in the final risk characterization. The final quantitative <br /> estimates of risk may be one or several orders of magnitude different from the potential <br /> risk associated with a given exposure. In an attempt to minimize the consequences of <br /> uncertainty, EPA guidance typically relies on use of conservative estimates of hazards <br /> in the absence of strong scientific data. The overall result is that risk estimates <br /> presented in this report are much more likely to overestimate the potential risk rather <br /> than to underestimate it. <br /> In this risk assessment average and reasonable maximum exposures (RME) were used <br /> to estimate risk. Average risks are considered to be best estimates, whereas RME risk <br /> represents an upperbound estimate. Because RME risk estimates are based on a <br /> combination of conservative assumptions, the RME estimates are highly unlikely to r <br /> reflect risks generally encountered at the site, whereas the average risk estimates are still <br /> very conservative but are more likely to reflect potential risk. <br /> The assessment has been prepared in a manner consistent with that generally used in the <br /> consulting community and agency guidance at the time it was prepared. It is likely that <br /> risk assessment methods and data identifying and quantifying the toxicity of chemicals -- <br /> SALDC\YELLO.RYf May 4, 1995 7-7 <br />