I've noticed that even with good hearing protection (foamies + earmuffs), being next to loud guns (like .50BMG outdoors or most rifle calibers indoors) disorients me and causes me to feel a little strange for hours afterwards. I think that anyone who spends a lot of time near such explosions risks brain injury.
I'm pretty sure the effect comes from the sound and not something like toxic gasses because I never get the feeling shooting smaller calibers, nor do I get it when shooting with a silencer. It's too bad silencers are so restricted in the US. I think a lot of shooters would be in better health if they were more common.
In the army I had the opportunity to fire the Carl Gustaf 84mm recoilless rifle, which is normally a two person job (one to hold, aim and fire, the other to load and check the backblast area).
We're instructed to exhale before firing because the concussion of the round leaving the front while its propellent leaves the back of the tube creates a brief vacuum. If you don't exhale, the air is forced out of your lungs so violently you feel like you got punched in the chest.
The noise is undoubtedly part of it, but the atmospheric effect is not insignificant, I think.
I'm pretty sure the effect comes from the sound and not something like toxic gasses because I never get the feeling shooting smaller calibers, nor do I get it when shooting with a silencer. It's too bad silencers are so restricted in the US. I think a lot of shooters would be in better health if they were more common.