I'm a military flightline worker. it was 29 degrees out so i threw one warmer in each of my gloves, one in each of my boots and one on my shirt. the most heat i got was for half an hour, and that was only one the boot&hands. i had them all on for about 2hours and the back warmer never warmed up, the rest were gone within the 2 hours and i just threw them out. dont buy this brand ever, unless you feel like wasting your money on 2 hours of hear, and then wasting it on another pack. I will never by this ever again, I'm going to go buy a new pack of another brand ASAP