Before a worker enters a dangerous or permitted confined space, all hazards should be thoroughly assessed. As part of the assessment, it should be determined whether a hazardous atmosphere is present. The National Institute for Occupational Safety and Health (NIOSH) has created values known as Immediately Dangerous to Life or Health (IDLH) for a group of chemicals that can have an immediate and dangerous effect. NIOSH defines them as exposure to airborne contaminants that is “likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment”. Values should be used to compare the results of pre-entry testing for chemicals that may be present and select the necessary respiratory and personal protective equipment.