The purpose of this report is to evaluate new scientific developments and advancements in gas dosimetry focusing on tracheobronchial (TB), pulmonary (PU), and systemic/extrarespiratory (ER) inhalation dosimetry related to the U.S. EPA's 1994 Methods for Derivation of Inhalation Reference Concentrations and Applications of Inhalation Dosimetry (U.S. EPA, 1994), hereafter RfC Methods. Particular emphasis is given on animal to human extrapolation and inhalation dosimetry in children. The goal of this project is to provide information necessary for ensuring that methods and guidance used and implemented by EPA in inhalation risk assessment reflects the state-of-the-science. Overall, the scientific advances support and, in some cases, actually build further upon the approaches of the current methodology for gas dosimetry in the tracheobronchial (TB), pulmonary (PU) and extrarespiratory (ER) regions. With regards to gas dosimetry, there appears to be insufficient quantitative evidence to modify the RfC Methods specifically for children; however, in some cases, chemical-specific information may warrant consideration of alternative approaches or adjustments to account for this lifestage. It is anticipated that information will continue to become available to further inform this issue.