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3. 4 Potential Problems As with any risk-based tool, the potential exists for misapplication. In most cases the root cause will be a lack of understanding of the intended use of Region 9 PRGs. In order to prevent misuse of PRGs, the following should be avoided: Applying PRGs to a site without adequately developing a conceptual site model that identifies relevant exposure pathways and exposure scenarios, • Not considering background concentrations when choosing PRGs as cleanup goals, • Use of PRGs as cleanup levels without the nine-criteria analysis specified in the National Contingency Plan (or, comparable analysis for programs outside of Superfund), = Use of PRGs as cleanup levels without verifying numbers with a toxicologist, • Use of antiquated PRG tables that have been superseded by more recent publications, • Not considering the effects of additivity when screening multiple chemicals, and • Adjusting PRGs upward by factors of 10 or 100 without consulting a toxicologist. 4.0 TECHNICAL SUPPORT DOCUMENTATION Region 9 PRGs consider human exposure hazards to chemicals from contact with contaminated soils, air, and water. The emphasis of the PRG equations and technical discussion are aimed at developing initial goals for soils, since this is an area where few standards exist. For air and water, additional reference concentrations or standards are available for many chemicals (e.g. non-zero MCLGs, AWQC, and NAAQS) and consequently the discussion of these media are brief. 4.1 Inhalation of Volatiles and Fugitive Dusts Agency toxicity criteria indicate that risks from exposure to some chemicals via inhalation far outweigh the risk via ingestion; therefore soil PRGs have been designed to address this pathway as well. The models used to calculate PRGs for inhalation of volatiles / particulates are updates of risk assessment methods presented in RAGS Part B (USEPA 1991a) and are consistent with the Soil Screening Guidance: User's Guide and Technical Background 10