US EPA

EPA has conducted a peer review of the scientific basis supporting the human health hazard and dose-response assessment of 2,2,4-trimethylpentane, also called TMP, that will appear on the Integrated Risk Information System (IRIS) database. Peer review is meant to ensure that science is used credibly and appropriately in derivation of the dose-response assessments and toxicological characterization.

The draft Toxicological Review of 2,2,4-Trimethylpentane provides scientific information on the effects pertaining to exposure to 2,2,4-trimethylpentane. 2,2,4-Trimethylpentane (TMP) was nominated by OTAQ and OAQPS for evaluation by the IRIS Program, based on its classification as a Title I Clean Air Act Hazardous Air Pollutant and because it is a major component of motor vehicle exhaust. TMP is a hydrocarbon and is also known as isooctane or isobutyltrimethylmethane. TMP is released into the environment through the manufacture, use, and disposal of products associated with the gasoline and petroleum industry. TMP comprises about 10% of unleaded gasoline. Automotive exhaust and automotive evaporative emissions are important sources of atmospheric TMP. Thus, the most probable route of exposure to the general population is by inhalation.

The current TMP assessment posted on IRIS (1991) does not include an evaluation of the oral noncancer database or the cancer database, and contains a message for the inhalation RfC that the health effects data for TMP were reviewed by the U.S. EPA RfD/RfC Work Group and were determined to be inadequate for the derivation of an inhalation RfC. An evaluation of the TMP database is not available from other agencies. As a part of the IRIS nomination process for selecting chemicals for assessment, a quick literature review of the database was conducted in 2003. While no new critical studies via the inhalation route were identified, newer mechanistic studies, gavage studies, and toxicokinetic studies were found. This information has been used to update the assessment.

The draft Toxicological Review of 2,2,4-Trimethylpentane provides scientific information on the effects pertaining to exposure to 2,2,4-trimethylpentane.

Following the External Peer Review, the final assessment will be posted in 2007 in the IRIS database.

U.S. EPA. IRIS Toxicological Review and Summary Documents For 2,2,4-Trimethylpentane (External Review Draft).

Comments on the assessment may be submitted and reviewed using the e-Government Regulations.gov Web site. From the site, select Environmental Protection Agency and the keyword EPA-HQ-ORD-2006-1009 (for the docket ID).