The purpose of this report is to evaluate and summarize the pertinent scientific developments and advancements in gas dosimetry focusing on extrathoracic (ET) or upper respiratory tract (URT), tracheobronchial (TB), pulmonary (PU), and extrarespiratory (systemic, SYS) inhalation dosimetry related to the current methodology used by EPA. Particular emphasis is placed on animal to human dosimetric extrapolation performed in derivation of a chronic reference concentration (RfC). An RfC is defined as an estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure for a chronic duration (up to a lifetime) to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. In addition, this report summarizes available data pertaining to inhalation dosimetry throughout the respiratory tract of children as it relates to derivation of an RfC. This report provides the scientific foundation necessary for ensuring that methods and guidance used and implemented by EPA in inhalation risk assessment of gases reflects the state of the science.