Conceptual risk-based assessment model

Contaminated sites may pose risks to both the environment and human health. The impacts of contaminated sites in the UK and internationally are managed using a conceptual risk-based assessment model ... 

U.S. Army. 2000e. Conceptual Site Model and Assessment Methodology for the Human Health Risk-Based RCRA Permit Closure of the JACADS Facility, Johnston Island, Johnston Atoll, North Pacific (Ocean). Prepared by United States Army Center for Health Promotion and Preventive Medicine for the Project Manager for Chemical Stockpile Disposal, June 27. Report No. 39-EJ-8929-99. Aberdeen Proving Ground, Md. U.S. Army Program Managerfor Chemical Demilitarization. 

The basic concept is that estimated results for pesticide movements and exposure levels vary greatly with the model types and modeling philosophy. Before con-dncting a model exercise, a conceptual check of the model is needed to ascertain if the model contains aU relevant routes of exposure. A simple model, such as SCIES, is based on worst-case assumptions, and may be sufficient for inhalation risk assessment. More complicated simulation models, such as CONSEXPO and InPest, provide information on the amounts of pesticides on the room materials, as well as the airborne concentration, and they are appropriate for risk assessment via aU routes. Even in complicated models, each mechanistic model contains assumptions to simplify the process description of the pesticide movement in the real world . The underlying assumptions for each of the models, and the relevant processes they implicate, are criteria to consider when selecting an appropriate model. Therefore, the validity of the assumptions used for the assessment should be considered before using the model, and they should be well documented. A simple phrase such as, we used model xx to estimate an exposure level of yy, is inadequate for documentation purposes. 

The site assessment must provide sufficient information to identify relevant health and ecological concerns and assess the need for remedial action. To support a risk-based site evaluation, these data must be sufficient to construct a site conceptual exposure model characterizing affected environmental media (i.e., soil, groundwater, surface water, and sediment), applicable exposure pathways and receptors, and potential contaminant transport mechanisms. General guidelines regarding key data requirements and data acquisition procedures are summarized below. 

PBPK models improve the pharmacokinetic extrapolations used in risk assessments that identify the maximal (i.e., the safe) levels for human exposure to chemical substances (Andersen and Krishnan 1994). PBPK models provide a scientifically sound means to predict the target tissue dose of chemicals in humans who are exposed to environmental levels (for example, levels that might occur at hazardous waste sites) based on the results of studies where doses were higher or were administered in different species. Figure 3-4 shows a conceptualized representation of a PBPK model. 

Bayesian statistics are applicable to analyzing uncertainty in all phases of a risk assessment. Bayesian or probabilistic induction provides a quantitative way to estimate the plausibility of a proposed causality model (Howson and Urbach 1989), including the causal (conceptual) models central to chemical risk assessment (Newman and Evans 2002). Bayesian inductive methods quantify the plausibility of a conceptual model based on existing data and can accommodate a process of data augmentation (or pooling) until sufficient belief (or disbelief) has been accumulated about the proposed cause-effect model. Once a plausible conceptual model is defined, Bayesian methods can quantify uncertainties in parameter estimation or model predictions (predictive inferences). Relevant methods can be found in numerous textbooks, e.g., Carlin and Louis (2000) and Gelman et al. (1997). 

The risk of exposure to individual chemicals as calculated using the SSD method is based on the same mathematical principles used in the derivation of concentration-response curves in single-species toxicity evaluations. As for individual species, both the concentration addition and response addition models can conceptually be applied in ecological risk assessment for species assemblages exposed to mixtures of toxicants, which are now being formulated probabilistically (Traas et al. 2002 Posthuma et al. 2002a De Zwart and Posthuma 2005). 

As a result of inadequate resources and the belief that applied engineering held the key, and seeking to establish a rational and consistent basis for making decisions, EPA personnel soon adopted a judgment-based technique that often relied heavily on mathematical models for calculating risks quantitative risk assessment. Somewhat later the assessment and management of risks were conceptually separated, though the two activities have always remained interdependent. 

First, risk assessments must construct a conceptual model that embeds a series of potential cause-and-effect relationships. In many cases these relationships, such as the concentration-response, are based upon laboratory... 

Number of monitoring points based on conceptual model and impact/risk assessment... 

---