Why were ERPG-2 values selected as toxic endpoints instead of ERPG-3?

An endpoint is needed for analysis of offsite consequences of potential accidental releases of regulated substances. The endpoint to be used for each regulated toxic substance is provided in Part 68, Appendix A, and is the Emergency Response Planning Guideline level 2 (ERPG-2) developed for the substance by the American Industrial Hygiene Association, unless an ERPG-2 has not been developed for the substance. Why were ERPG-2 values selected instead of ERPG-3?

EPA based its decision to utilize ERPG-2 values as the toxic endpoints for offsite consequence analyses on comments received from the public and the regulated community. The Agency's focus was on exposure resulting in serious, irreversible health effects, which is best represented by the ERPG-2 values. ERPG-3 values, which represent lethal exposure levels, were rejected on the basis that they are not protective enough of the public in emergency situations. For toxic substances which have no ERPG-2 values, the endpoint to be used is the level of concern (LOC), as identified in the Technical Guidance for Hazards Analysis (December 1987), updated where necessary to reflect new toxicity data (61 FR 31672; June 20, 1996). LOCs are intended to be protective of the general public for exposure periods of up to an hour. These levels have been peer reviewed by EPA's Science Advisory Board, and are widely accepted by the emergency response planning community.