duration. Dourson and Stara (1983) and U.S. EPA (1987a) discuss the 

 methods for deriving RfD values and the criteria for selecting uncer- 

 tainty factors. In brief, an uncertainty factor of 1000 is based on 

 combining a factor of 10 to account for animal-tb-human extrapolation, 

 a factor of 10 to protect sensitive individuals, and a factor of 10 to 

 account for use of a LOAEL in place of a NOAEL. 



In many cases, EPA regions and other agencies will be able to rely on 

 dose-response assessments generated previously. Current values for 

 Carcinogenic Potency Factors and RfDs are given in IRIS (U.S. EPA 

 1987a; e.g., see Appendix B). Before using these values, investigators 

 should consult the IRIS database and current EPA health assessment 

 documents for information on their derivation and associated uncert- 

 ainties. Contacts for information on specific chemicals are listed in 

 IRIS Chemical Files. 



The Carcinogenic Potency Factors calculated by the EPA Carcinogen 

 Assessment Group are pubUshed in IRIS and in each health assess- 

 ment document produced by the Office of Health and Environmental 

 Assessment (e.g., U.S. EPA 1985a). The EPA Carcinogen Assessment 

 Group develops these carcinogenic potency values and updates them 

 periodically. Before being entered into IRIS, Carcinogenic Potency 

 Factors and supporting documentation are reviewed by the Car- 

 cinogen Risk Assessment Verification Endeavor (CRAVE) work 

 group. The list of Carcinogenic Potency Factors published in each 

 health assessment document is intended only to provide comparative 

 information for various chemicals. IRIS should be used as the primary 

 source of Carcinogenic Potency Factors for risk assessment. 



IRIS is the primary source of RfD values. An example of an IRIS data 

 sheet for the pesticide hndane is shown in Appendix B. The data sheet 

 provides information on the RfD, the endpoints (biological effects) of 

 concern, experimental data sets, doses, uncertainty factors, additional 

 modifying factors, confidence in the RfD, reference documentation, 

 and dates of agency RfD reviews. 



Individual program offices within EPA may need to be consulted for 

 information on chemicals not yet incorporated into IRIS. For example, 

 the Office of Drinking Water is a source of RfDs for selected chemi- 

 cals. In May 1987, the Office of Drinking Water released draft Health 

 Advisories containing RfDs and guidelines for short-term effects for 

 16 pesticides: alachlor, chiordane, l,2-dibromo-3-chloropropane 

 (DBCP), 2,4-dichlorophenoxyacetic acid (2,4-D), 1,2-dichloro- 

 propane, endrin, ethylene dibromide (EDB), heptachlor and hep- 

 tachlor epoxide, lindane, methoxychlor, oxymyl, pentachlorophenol, 

 toxaphene, and 2,4,5-trichlorophenoxypropionic acid (2,4,5-TP). Of- 

 fice of Drinking Water Health Advisories will eventually be incor- 

 porated into IRIS. 



Sources of Information 



Carcinogenic Potency Factors 



Reference Doses 



27 



