I will add some input to the subject............. Being from the "great white north" we use these things all the time.
I have at least 10 of them(picked up a whole box full at a garage sale once) I use them pretty much the entire hunting season, usually I place one in my shirt pocket and one in my "hand muff tube" that hangs around my waist. I have had them still putting out heat after 8 hrs in the woods.
I will say there is a small amount of odor, but it has never stopped me from seeing/shooting dead. I have even placed a few drops of cover scent on the outer cover bag, worked similar to a thermocell.
I would highly recommend the product to someone suffering with cold hands/body in the stand.
FDXX75