When the air toxic rules were first promulgated, an initial backlog of requests for screening levels developed, so interim procedures were implemented to reduce this backlog. These interim procedures are outlined in the staff activity report, "Air Toxic Rules - Implementation Procedures", dated January 20, 1993. Part of the interim procedures included doing a shortened review of the data for a chemical and establishing an "interim" screening level. Interim screening levels were maintained on a separate list from the screening levels undergoing a more complete review.

Under the interim procedures a chemical was placed on the interim list when there was both an EPA RfD and an OEL. In this case, no determination was made regarding the appropriateness of using the RfD to establish the ITSL, and an ITSL determined from both the RfD and OEL was added to the list. If an applicant met the most restrictive value, then they had demonstrated compliance with Rule 230. If an applicant could not meet the most restrictive value, the Permit Section submitted a request to the Toxics Unit to determine a final screening level. Chemicals were also placed on the interim list if there were no EPA established RfCs or RfDs, and the OEL was not health based or based only on acute data.

The backlog of requests for screening levels has been eliminated and the interim procedures are no longer being implemented. As a result, no new chemicals are being added to the interim screening level list. As time permits, the Toxics Unit intends to review those chemicals on the interim screening level list and make a final determination regarding the screening level. Eventually, the interim list will be eliminated. Until that time, the interim list of screening levels is also available to anyone upon request.