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consulting a staff toxicologist at state and / or federal regulatory agencies.. Where anthropogenic background levels exceed PRGs and EPA has determined that a response action is necessary and feasible, EPA's goal will be to develop a comprehensive response to the widespread contamination. This will often require coordination with different authorities that have jurisdiction over the sources of contamination in the area. 33 Risk Screening A suggested stepwise approach for screening sites with PRGs is as follows: • Perform an extensive records search and compile existing data. • Identify site contaminants in the PRG Table. Record the PAG concentrations for various media and note whether PRG is based on cancer risk (indicated by "ca") or noncancer hazard (indicated by "nc"). Segregate cancer PRGs from non-cancer PRGs and exclude (but don't eliminate) non-risk based PRGs ("sat" or "max"). • For cancer risk estimates, take the site-specific concentration (maximum or 95 UCL) and divide by the PRG concentrations that are designated for cancer evaluation ("ca"). Multiply this ratio by 10' to estimate chemical-specific risk. For multiple pollutants, simply add the risk for each chemical cont cont, cont Risk = ( ( PRGPRG 1 PRG rl l x 10i s Y • • For non-cancer hazard estimates. Divide concentration term by its respective non-cancer PRG designated as "nc" and sum the ratios for multiple contaminants. [Note that carcinogens may also have an associated non-cancer PRG that is not listed in the printed copy of the table and these will also need to be obtained in order to complete the non-cancer evaluation.] The non-cancer ratio represents a hazard index (HI). A hazard index of 1 or less is generally considered safe . A ratio greater than 1 suggests further evaluation: cont cont cont Kazard index I ( PRGsI ` ( PRGr 1 t PRG')] For more information on screening site risks, the reader should contact EPA Region 9's Technical Support Team. a