The evolution of EPA's Exposure Factors Handbook and its future as an exposure assessment resource.

The need to compile and summarize exposure factors data into a resource document was first established in 1983 after the publication of the National Academy of Sciences (NAS) report on Risk Assessment in the Federal Government: Managing the Process and subsequent publication of the Environmental Protection Agency's (EPA's) exposure guidelines in 1986 (NAS, 1983; US EPA, 1986). During the same time frame, the EPA published a report entitled Development of Statistical Distributions or Ranges of Standard Factors Used in Exposure Assessment to promote consistency among various exposure assessment activities in which EPA was involved and to serve as a support document to the 1986 exposure guidelines (US EPA, 1985). As the exposure assessment field continued to advance during the 1980s and 1990s, so did the need for more comprehensive data on exposure factors. The Exposure Factors Handbook was first published in 1989 in response to this need (US EPA, 1989). It became an important reference document and has been revised and updated since its original publication (US EPA, 1989; US EPA, 1997a; US EPA, 2011a). This paper reviews the evolution of the Exposure Factors Handbook, and explores anticipated needs and some of the potential options for future updates of the handbook.