Wow, someone seems to have got their knickers in a knot for no good reason!? But I'll play along:
1) I think everyone here except you got the point that I was making. That is, that most people prefer to be inhaling dust to cigarette smoke, ceteris paribus;
2) Sure, when referring to particulate matter in the air, mg/m^3 should be used (and if my undergrads made a similar error I wouldn't award marks), but this is frequent flyer forum, not the BIPM!;
3) The evidence to which you refer in your more important (?) point is actually about as close as you can get to irrefutable proof.
I don't have any knickers in a knot.
I think I understood clearly that your point was to dismiss the suggestion of reduced air quality with a throw away comment about a
few more ppm of dust. I just applied the first test I was taught in undergrad physics - does the answer make sense. A few ppm when converting into a mass concentration is considered medium to high pollution. It is about 1/3 of the dust limit in NSW coal mines :!: I'm sorry but IME very few people would prefer to inhale a
few more ppm of dust over passive cigarette smoke.
My more important point is based on my time as a postgrad in a air quality research center. I left them some time ago and I can only refer to the state of knowledge when I was there. However, I'm sure that their knowledge has vastly expanded since then and a google search would find the most up to date references.
Of course, since this is only the
frequent flyer forum and it is acceptable to erroronously refer to ppm of dust, it must also be perfectly acceptable to not provide the most up to date references to peer reviewed papers.
Maybe my language was not clear, but the point is that fine particles of a non-carcinogenic substances are likely to still have a carcinogenic effect depending on a number of factors.
Edit: perhaps, you missed my point that inhaling dust is not necessarily harmless compared to passive smoking, ceteris paribus.