In some cases, the exposure assumptions are the default assumptions, as in the VRP Soil Levels. Others may deviate from the default values provided by guidance. For example, worker exposure for a specific scenario may be derived from residential exposures. In these cases, the deviation will require review and evaluation to determine if the appropriate adjustments have been made. This task will be approached on a case-by-case basis and applies mainly to simple and medium risk assessments.
Many risk assessments require evaluation of a central tendency estimate (CTE) in addition to reasonable maximum exposure (RME). Although RME is generally used for remedial decisions, CTE provides insight into the reasonableness of the RME estimate relative to the variance in the data. This approach will be evaluated with WDEQ on a case-by-case basis when developing action levels.
If it is not always possible to obtain site-specific information regarding behavior patterns (e.g., trespassing), which must be characterized using guidance. EPA's Exposure Factor Handbook is a good source for certain specific behavior patterns associated with a wide range of human activities. However, professional judgment can be a significant and essential part of a risk assessment for site specific or unusual situation.
Terra Mentis staff has reviewed or prepared hundreds of risk assessments and in each of these risks have been estimated for both non-cancer and cancer effects. The most typical calculation is a point estimate of risk, but Terra Mentis has also calculated risk distributions using ranges of assumptions and chemical concentrations. Evaluating the correctness of the data interpretations for risk assessment and the reasonableness of the risk characterization requires linking the interpretations to the DQOs and the factors identified above. Issues such as the probability of exposure, the types of exposure upon which the toxicity values are based compared to anticipated exposure at the site, controversial toxicity values, appropriate consideration of additivity and bioaccumulation, and other uncertainties in the risk characterization are all important considerations. Non-cancer risk estimates determine an HI, which, if it exceeds one, then risks are estimated by target organ. Risk characterization evaluates the critical toxicological effects of a compound, both independently and when some risk characterizations are performed. For example, an evaluation of lead would evaluate blood lead as a critical parameter, especially for children, either by modeling or by direct measurement. Summed non-carcinogenic risk estimates typically sum by adverse health effect. Summed non-carcinogenic risk estimates are typically added by target organ. Cancer risk estimates are summed, not by target organ but by each pathway and then for all pathways together.
Evaluating the uncertainty in the risk estimates is important in some risk assessments. The uncertainty analysis should convey an overall sense in the confidence the authors have in the risk assessment and should identify areas that may impact the results of the risk assessment. The uncertainty analysis, if one is necessary, should be comprehensive and discuss the site data, fate and transport analyses, exposure assumptions, and toxicity criteria. Whenever possible, the degree and direction of uncertainty associated with each item should be identified. It is important to distinguish between variability and uncertainty. An example of variability is the difference in results typically obtained when analyzing splits of a soil sample. Uncertainty relates to estimates that may not accurately reflect reality. Examples of items associated with uncertainty include modeling result (e.g., Johnson and Ettinger Model) and generic exposure assumptions when actual activity patterns are not known.