IRIS provides information on how chemicals affect human health and is a primary source of EPA risk assessment information on chemicals of environmental concern. The database serves as a guide for the hazard identification and dose-response assessment steps of EPA risk assessments. IRIS makes chemical-specific risk information readily available to those who must perform risk assessments and also increases consistency in risk management decisions. The principal section of IRIS is the chemical files. The chemical files contain: - oral reference doses (RfD) and inhalation reference concentrations (RfC) for noncarcinogens - oral and inhalation carcinogen assessments - summarized Drinking Water Health Advisories - summaries of selected EPA regulations - supplementary data (for example, acute toxicity information and physical-chemical properties) and - full bibliographic citations The primary types of health assessment information in IRIS are oral RfDs, inhalation RfCs and carcinogen assessments. Reference doses and concentrations are estimated human chemical exposures over a lifetime which are just below the expected threshold for adverse health effects. The carcinogen assessments include: a weight-of-evidence classification, oral and inhalation quantitative risk information, including slope factors, along with unit risks calculated from those slope factors. A slope factor is the estimated lifetime cancer risk per unit of the chemical absorbed, assuming lifetime exposure. The health assessment information contained in IRIS, except as noted, has been reviewed and agreed upon by two interdisciplinary review groups of EPA scientists who have extensive experience in risk assessment. Thus, the information in IRIS represents and expert Agency consensus. This Agency-wide agreement on risk information is one of the most valuable aspects of IRIS. Chemicals are added to IRIS on a regular basis. Chemical file sections in the system will be updated as new information is made available to the two review groups. Because exposure assessment pertains to exposure at a particular place, IRIS cannot provide situational information on exposure. IRIS can be used with an exposure assessment to characterize the risk of chemical exposure. This risk characterization can be used to decide what must be done to protect human health. There are currently over 450 chemicals in IRIS. For these chemicals, oral RfDs are provided for 340, inhalation RfC summaries for 25 and carcinogen slope factors are provided for 180.