IDLH stands for Immediately Dangerous to Life or Health, a term used by the Occupational Safety and Health Administration (OSHA) to identify hazardous environments that can cause irreversible harm or death. Synonyms for IDLH include extremely hazardous, life-threatening, and critical. Other terms that may describe an IDLH environment are deadly, fatal, lethal, perilous, and precarious. It is important to recognize these synonyms and their potential implications in order to quickly identify and respond to any hazardous situations that may put employees at risk. Employers and workers must always be vigilant and take appropriate safety precautions to avoid the deadly consequences of exposure to IDLH conditions.