What event would signal the need for an emergency withdrawal from an IDLH atmosphere?

An emergency withdrawal from an Immediately Dangerous to Life or Health (IDLH) atmosphere is warranted primarily due to the failure of a Self-Contained Breathing Apparatus (SCBA). SCBA is vital for a firefighter's safety, especially in IDLH environments, as it provides breathable air and protects against toxic smoke and gases. If there is a failure, including issues like equipment malfunction or depletion of air supply, the firefighter is immediately at risk and must evacuate to a safe area as quickly as possible.

While decreased visibility, high heat levels, and the end of a shift may indicate unsafe conditions in their own right, they do not represent an immediate and critical life-threatening failure of personal protective equipment. The other events could warrant caution or strategic planning for withdrawal but would typically allow for a more measured response rather than the immediate need that a malfunction or failure of the SCBA represents. Therefore, a SCBA failure is the most clear-cut trigger for an emergency withdrawal in an IDLH atmosphere.