I took a hit of 100% nitrogen by mistake in the lab, things got awfully dark in seconds. it has the the same effect as a hypobaric chamber, very pleasnt way uo go.
Yeah. I always put eyes on the O2 sensor before opening the door.
I once did this with helium, yeah, thinking I would enhance the voice effects by taking more than one breath..
Boom, I was down in seconds. No pain, panic, nothing, just out...