@GothicSoul posted the link below in a thread on the rookie forum recently. This study contains some very unique and very
thorough data on workers (and would-be rescuers which about a third to a half of all of these victims in these studies were would-be rescuers, btw) that have died in confined spaces. NIOSH has pulled data from a large cross-section of industries, both rural and urban. It's further broken down into a large amount of subsets (what kind of confined space, specific cause of death, like oxygen deprivation [asphyxiation] vs poisoning from various other toxic gases, "drowning," - like in grain, for instance). Anyways, I highly recommend checking this out if you partake the underground adventure varieties, or even if confined spaces are in your job description: https://www.cdc.go...03/pdfs/94-103.pdf
I'm not by any means a PhD, but I had to scour countless studies in college and for what it's worth, I'd say the validity/reliability factors in the way this study was conducted is very good.