Comment by e12e

I suppose there are other hydrocarbons to worry about (both naturally occurring from compost, and from leaking gas pipes/old tanks). Not so much because of suffocation (that's why you have the O2 gauge) but from the not blow up/be burned alive part. I suppose it depends a bit where you go exploring - AFAIK old coal mines can be dangerous wrt fires/explosions. It's a good reason to avoid open flame/non ex certified electronics, if you have reason to suspect an explosive atmosphere. Fires can be dangerous just by binding oxygen - in addition to the more obvious danger.

I think the main thing is to realize that enclosed spaces can be really dangerous. Farmers die in grain/grass silos every year. And people die from CO poisoning from using coal as heat-sources indoors, or just from theoretically sound, but flawed fire based heating solutions (eg: paraffin/diesel/oil heaters).