I'm beginning to get the impression, though I need to read a lot more on this, that emplaced sediment may hold (and emit) a lot of radon, especially close to the shale boundary, as much as deep-seated gas rising up through fissures. Aren't there higher-than-usual levels of uranium within the Edale Shales? I seem to remember reading somewhere (probably TDF) that investigations were made around the Mam Tor/Odin fault area in the 1950s as to the viability of mining for it, but the generally unstable ground (or perhaps uneconomic levels, or both) made it impractical. Maybe I imagined all that, but it rings a bell.
Anyway, if prolonged digging in low-air sediment-filled tubes, which occupied a lot of diggers in the 1960s-90s, is a possible heightened risk, then it does add complexity (or extra time at least) for current digging, smoking or not. I was at the inner end of a very long passage such as this only today, digging out layered sediments with my hands, as there literally wasn't room to swing a micro-pick. My first visit for two years though, and it was amazing to see how far it had got since I was last there. The main diggers definitely aren't smokers.