Immediately dangerous to life and health, the maximum amount of a toxic material that a healthy adult can be exposed to for up to 5 min (National Institute for occupational Safety and Health (NIOSH)) and escape without irreversible health effects. There are also IDLH values established for oxygen-deficient atmospheres and explosive or near-explosive atmospheres (above, at, or near the LELs). At the scene of a hazardous materials incident, it should be assumed that the concentrations present are above the TLV-ceiling, and only responders wearing SCBA and proper chemical protective clothing should be allowed near the incident scene.