At what percentage of air in the cylinder must a firefighter exit the structure?

A firefighter must exit a structure when there is 50% air remaining in the cylinder to ensure safety and maintain sufficient oxygen levels for continued operation and effective decision-making. When the air supply drops below this level, the risk of running out of breathable air increases significantly, which could lead to dire consequences in an emergency situation where every second counts.

Maintaining a reserve allows firefighters to safely navigate back to the exit without rushing or compromising their safety due to a lack of oxygen. Moreover, a 50% threshold also takes into account the unexpected circumstances that may arise within a structure, such as increased air demand due to exertion or environmental factors that may compromise a firefighter's air supply faster than anticipated.