danny burstein submitted this idea :
In FromTheRafters
writes:

[snip]

CO is heavier than 'air'. Smoke is generally in hot air and rises.--
follow their instructions for placement.

Carbon Monoxide has an atomic, well molecular,
weight of Carbon [12] + Oxygeb [16] == 28

Air is a mix of lots of stuff, but if we
just look at nitrogen, which is 78 or
so percent of it, N2 = [14 times 2] = 28

plus, of course, these gases don't remain as
discrete entities but mix in with each other.

So CO is going to be pretty much diffused
throught the air.

Yes, thanks to you and Frank for making this clear.