Although I've never been fire-fighting I have been amazed at just how far down the downwash will travel in an almost perfect column if you stay in the hover for any length of time in light winds.
At 400 feet in the hover (A109 for a film job - probably not a very clever idea in the first place!) I remember seeing a tight pool of downwash in the corn off to one side. It was so clearly defined that it was almost as if there was an invisible helicopter hovering there at ten feet.
I'd assumed up until then that the effect would be largely disipated over that distance but, man, those are powerful hair dryers (and fire fanners) we drive!
Cheers
JerryG