7.4 Risk assessment methodology for conventional and alternative  (Page 6/17)

There are a number of other valuable sources for toxicity values (RfDs/RfCs for non-carcinogens and URFs/CSFs for carcinogens), which can be compiled via the following sources:

• EPA Region 9 tabulated " Preliminary Remediation Goals (PRGs)" or "Regional Screening Levels (RSL)" for Chemical Contaminants at Superfund Sites," which also lists toxicity values (oral/inhalation RfD and oral/inhalation CSF) used in the medium-specific PRG/RSL calculation for each chemical. This table can be accessed via (External Link)
• The Hot Spot Guidelines published by California EPA for Air Toxics Program includes technical background documentation for toxicity criteria/values for chemicals (i.e. Cancer Potency Factors (CPFs) , which is equivalent to EPA's CSFs and Chronic Recommended Exposure Limits (RELs) , which are similar to USEPA's RfCs). The most recent version of REL Table is located at: (External Link) . The Technical Support Document for CPFs that contains cancer unit risks and potency factors for 121 of the 201 carcinogenic substances or groups of substances can be accessed via (External Link) .
• The Department of Energy's Oak Ridge National Laboratory (ORNL) maintains Risk Assessment Information System (RAIS) website, which contains useful information for risk assessment, including chemical-specific toxicity values. The RAIS information can be accessed via (External Link) .
• Toxicology Excellence in Risk Assessment (TERA) , a non-profit organization, manages and distributes a free Internet database of human health risk values and cancer classifications for over 600 chemicals of environmental concern from multiple organizations worldwide. This database, Integrated Toxicity Estimates for Risk (ITER) , can be accessed via: (External Link) or via NLM's TOXNET database at: (External Link) .

The dermal RfDs and CSFs can be derived from oral RfDs and CSFs, adjusted for chemical-specific gastrointestinal absorption efficiency, based on the recommended methodology in EPA's Guidance for Dermal Risk Assessment ( EPA, 2004a ).

Exposure assessment

In the third step of risk assessment, the magnitude of exposure is determined by measuring or estimating the amount of an agent to which humans are exposed (i.e. exposure concentration) and the magnitude of dose (or intake) is estimated by taking the magnitude, frequency, duration, and route of exposure into account. Exposure assessments may consider past, present, and future exposures.

While estimates of past or current exposure concentration/dose can be based on measurements or models of existing conditions, estimates of future exposure concentration/dose can be based on models of future conditions. In the case of inhalation exposures, personal or area monitoring to sample for contaminants in the air can be employed. The sampling data can be augmented with modeling efforts using default and/or site-specific input parameters. The model application can begin with simple screening level dispersion models and/or can utilize higher-level 2-D or 3-D models depending on the complexity of the environmental pollution problem in hand.