Why do fire fighters use oxygen gas tank?

SCBA tanks use regular air, not pure oxygen. Modern SCBA cylinders are carbon fiber wrapped aluminum. They do get hot under high temperature conditions. But, the most common ones today are tested to 6000 PSI, and filled to 4500. This gives them some buffer for increase in pressure due to heating. Furthermore, any increase in pressure from heat is usually offset by the rapid release of air as we breathe the tank down. Firefighting is heavy, hot, difficult work, and a tank of air usually lasts about 2/3 of what it states on the label.

When firefighters are spending a long time in high heat conditions, they will experience an increase in the temperature of the air coming from the regulator.Pure oxygen is the last thing they want to be carrying around when they are fighting a fire.