Abstract: At approximately 3:45 p.m. on January 1, 2017, Employee #1, coworker #1 and coworker #2, all firefighters, responded to a structure fire. They donned self-contained breathing apparatus, SCBA, to make entry to the second floor for an initial fire attack. Employee #1 was on the nozzle, Coworker #1 was on the hose line and had the thermal imaging camera and Coworker #2 was at the door advancing the attack line. They finished the initial attack on the second floor and thought that the fire was above them in the attic space and radioed for a ladder. Coworker #1 asked Employee #1 to shut the hose line down so he could radio command and ask for the ladder. Employee #1 stated he closed the bail and turned towards Coworker #1 and took a breath and the mask sucked to his face as if there was no air to breathe. Employee #1 checked the donning /air saver switch thinking it may have been hit with debris and had been depressed but it was ok. Employee #1 felt he needed to take a breath so he unhooked the regulator and started breathing in the immediate danger to life or health atmosphere, IDLH, while checking the regulator. He reconnected the regulator and took another breath and the mask again sealed to his face. Employee #1 started to panic a little bit, and started to disconnect the regulator again to take another breath. Employee #1 dropped the hose and told Coworker #1 he was out of air and needed to get out now. When Employee #1 reached the outside he collapsed to the ground and started coughing at which time fire department emergency medical services provided medical treatment. Employee #1 was hospitalized.