There was a thread on this topic quite some time back. I can't find that now, or I may be searching for it wrong. My understanding of it all is that since it was coming up from under you also, that you didn't have a "welcome mat" placed under you. With a tarp over and a ground cover under, and if your not huffing and puffing in there creating a bunch of your own vapor, then you should be fine.
There something about the moisture also rising from the grass and greenery under you. Just don't know how to explain the science. I'll let one of our many brain children elaborate on that