It is a Federal OSHA regulation
... request by the International Association of Fire Fighters (IAFF) for a clarification of OSHA respiratory protection regulation 29 CFR 1910.134. This section of the Code of Federal Regulations covered industrial employees operating in confined spaces, toxic environments, or oxygen-deficient atmospheres. These work areas are classified as immediately dangerous to life and health (IDLH).
OSHA ruled in 1996 that fire fighters working within a structure fire were operating in an IDLH atmosphere. Fire departments must comply with 29 CFR 1910.134 while self-contained breathing apparatus is being used. A minimum of two fire fighters enter the IDLH area together and remain in visual or voice contact with one another at all times.
In addition, at least two properly equipped and trained fire fighters must:
• Be positioned outside the IDLH atmosphere
• Account for the interior teams
• Remain capable of rescue of the interior team or teams
This interpretation became known as the two-in-two-out rule and evolved into the Rapid Intervention Team concept that was incorporated into NFPA 1500.
taken from Fire Officer: Principles and Practice (2006) NFPA/Jones and Bartlett