Matt,<br><br>Those are quite common around here and seem to have largely suplanted the fuel-type catalytic hand warmers. It seems to matter greatly whose fuel sticks one uses - some of them are quite stinky. They do put off CO, of course - nothing to worry about in most circumstances.<br><br>My wife likes to toss one in each boot (toe area) and stuff with her last-worn socks when she sacks out for the night in the winter. I think they go out for lack of O2 about 1/2 the time, depending on which boots...