I've used several different brands that have worked very well, I sure don't like to venture out on a cold day without them. Keep them in an Insulated pocket though! I went out a couple of weeks ago, I had on my reversible snow camo coveralls. My hands wouldn't warm up. I had the hand warmers in my pocket of the coveralls, but the pocket wasn't insulated, warmers didn't work while hunting. When I got done and was in my truck, I wrapped the warmers in my hat. When I completed the 7 mile drive home, those hand warmers were as hot as they could be, so I guess that if you overexpose them to oxygen they won't work as well.