David, I have used the chemical ones and like them. They are cheap, portable, and you can take extras, as a precaution, if you get lost, or hurt. Some you mix the dry chemicals, some you just open the package, and let the air get to it, and it starts heating. They range in time from six to eight hours I think. My hands, and feet have always gotten cold, and stayed cold,
so these were great. Or you could do like Eddie does, and get a thermal cell, and save the tabs for when the skeeters get bad. You can get therma cells, and refills, at Wally World, and Walgreens, and you can order them on line from Sportsmans guide, and other sites on line. If you have a mosquito problem, they are a Godsend!
Wayne