"The guidance recommended in this fact sheet will help respiratory protection program administrators, managers, and air-purifying respirator (APR) wearers understand the special features of a NIOSH-approved Chemical, Biological, Radiological, and Nuclear (CBRN) APR. These types of respirators have unique performance, use limitations, and storage requirements compared to NIOSH-approved industrial APR. The respiratory protection program administrator should assure that CBRN APR manufacturer recommendations are addressed. This information may also be used by managers and APR wearers to optimize personal protection. When using, purchasing, or storing a CRBN APR, the requirements set by NIOSH, Occupational Safety and Health Administration (OSHA), and the manufacturer of the specific unit should all be taken into consideration. Remember, approved CBRN APR may only be used for escape from an atmosphere that has become immediately dangerous to life or health. These respirators must not be used in oxygen-deficient atmospheres or to enter an atmosphere immediately dangerous to life or health. OSHA defines an atmosphere immediately dangerous to life or health as follows: An atmospheric concentration of any toxic, corrosive or asphyxiant substance that poses an immediate threat to life or would interfere with an individual's ability to escape from a dangerous atmosphere (29 CFR 1910.120(a)(3))."--NIOSHTIC-2