In response to increasing concerns over the potential impacts of emissions from combustion devices through indirect exposure routes, the U.S. EPA formed a multidisciplinary working group to review and update Agency guidance on conducting indirect exposure assessments. The approach used by the working group was to review the 1990 document, Methodology for Assessing Health Risks Associated with Indirect Exposure to Combustor Emissions and to identify and resolve issues. The working group concluded that the 1990 document was fundamentally sound, although specific issues were identified. The working group was divided into three subgroups to deal with specific technical issues, as follows: (1) modeling emissions, air dispersion, and deposition; (2) food chain modeling, including fate and transport modeling methodology and input parameters; and (3) human exposure and contact rates.