Before revision: An interior structural fire (an advanced fire that has spread inside of the building where high temperatures, "heat" and dense smoke are normally occurring) would present an IDLH atmosphere and therefore, require the use of respirators.
After revision: An interior structural fire (an advanced fire that has spread inside of the building where high temperatures, "heat" and dense smoke are normally occurring) would present an "Immediately Dangerous to Life or Health" (IDLH) atmosphere and therefore, require the use of respirators.