Occupational exposures to chemicals have long been recognized as having the potential to adversely affect the lives and health of workers. Acute or short-term exposures to high concentrations of some airborne chemicals have the ability to quickly overwhelm workers, resulting in a wide spectrum of undesirable health outcomes that may include irritation of the eyes and respiratory tract, severe irreversible health effects, impairment of the ability to escape from the exposure environment, and, in extreme cases, death. Airborne concentrations of chemicals capable of causing such adverse health effects or impeding escape from "high risk" situations or conditions may arise from a variety of situations affecting workers, including special work procedures (e.g., in confined spaces), industrial accidents (e.g., chemical spills or explosions), or chemical releases into the community (e.g., during transportation incidents or other uncontrolled release scenarios). Many organizations develop acute inhalation limits or guidelines. These are typically presented as airborne concentrations. However, airborne chemicals can also contribute to toxicity through other exposure routes, such as the skin and eyes. The "immediately dangerous to life or health air concentration values (IDLH values)" developed by the National Institute for Occupational Safety and Health (NIOSH) characterize these high-risk exposure concentrations and conditions and are used as a component of the respirator selection criteria first developed in the mid-1970s [NIOSH 1994]. Since the development of the original IDLH values in the 1970s and their subsequent revision in 1994, NIOSH has continued to review relevant scientific data and conduct research on methods for developing acute exposure guidelines. This document reflects continuing enhancements in risk assessment approaches and provides a detailed description of the methodology used to derive IDLH values. The documentation for specific IDLH values is available as separate NIOSH publications and on the NIOSH website (https://www.cdc.gov/niosh/idlh/default.html
). The primary objectives of this Current Intelligence Bulletin (CIB) are: 1. To provide a brief history of the development of IDLH values, 2. To update the scientific bases and risk assessment methodology used to derive IDLH values from quality toxicity and human health effects data, 3. To provide transparency behind the rationale and derivation process for IDLH values, and 4. To demonstrate how scientifically credible IDLH values can be derived from available data resources.