This document presents background information and justification for the Integrated Risk Information System (IRIS) Summary of the hazard and dose-response assessment of phosgene. IRIS Summaries may include an oral reference dose (RfD), an inhalation reference concentration (RfC), and a carcinogenicity assessment. The RfD and RfC provide quantitative information for use in risk assessments for health effects known or assumed to be produced through a nonlinear (possibly threshold) mode of action. The RfD (expressed in units of mg/kg-day) is defined as an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. The inhalation RfC (expressed in units of mg/m3) is analogous to the oral RfD, but provides a continuous inhalation exposure estimate. The inhalation RfC considers toxic effects for both the respiratory system (portal-of-entry) and for effects peripheral to the respiratory system (extrarespiratory or systemic effects). This document does not attempt to develop concentration values protective of acute toxicity. For reference purposes, Appendix A presents a summary of the phosgene Acute Exposure Guideline Levels (AEGLs) that was prepared by the National Academy of Sciences.