That is GREAT QUESTION!!
I started using thermacells last spring in the bear stand..I'm always leary about scent, as you know bears got wicked noses....but I must say, the Thermacell gives off a little scent...What I started doing is leaving old thermacell pads in the stand, when I wasn't there, and then used the Thermacell when I was sitting.
Let me tell you, it changed my hunting...I could sit buck naked and and not have a bug within 5 feet. It was aweasome, and I saw lots of bears last year...Including the GIANT when I had the thermacell running...
click.
http://www.menoutdoors.com/forums/showthread.php?t=4097