Before I retired, I had to wear one in certain departments, sometimes all day, because of dangerous chemicals. We were trained on wearing and fitting them and had to remove them every two hours. Depending on the department, sometimes we had to use a new one every two hours. Some departments required air packs with tanks worn on the back and an enclosed helmet. I didn’t work in those departments. With my asthma, I wasn’t allowed near them.
Before they relocated to Ft. Leonard Wood in MO, the Army's Military Police and Chemical schools were at Ft. McClellan, AL. When I was a student at the MP Officer Advanced Course, we got (had?) to go through the Chemical Defense Training Facility (CDTF) where live chemical weapons were used in order to train with detection and decontamination equipment.
Needless to say there was a lot of prep one must go through in order enter, including giving a blood sample to be used as a baseline for evaluation in case one was accidentally exposed. We were all thoroughly indoctrinated with the list of symptoms to be watchful for after attending the training and advised to get to the post hospital at the first sign of onset. One of these symptoms was headaches.
The only problem with listing that as a symptom was that every student was going to develop headaches because it was a natural impulse to cinch your protective mask on as tight as you possibly could.
I kind of liked SCBA’s.
Clean, cool air. No smell, and you got to pretend you are Darth Vadar.
Of course, if you lost track of time that dang bell would go off and then it was head to the exits to get a new tank. But lung capacity was not something I lacked.