i am extremely sensitive to cold and use two types of warmers.

1. Therma-Pak (medical grade, not sure mfg) and EZHeat (by Pristech) are rechargeable warmth packs made of sodium acetate and water (nontoxic). Each pack provides an hour of heat and can be recharged by placing in boiling water for 10 minutes. EZHeat packs are found at REI and outdoor stores for about $4. Therma-Pak comes in various sizes from hand size to lower back size and is more expensive. I purchased through my physical therapist.

2. Survival outdoor stores like GI Joes sell a small rechargeable metal heatpack where the liquid fuel like lighter fluid saturates a sponge inside which is set afire briefly and then blown out. The heater smolders without smoke and goes in a pouch in your pocket to keep warm for 8 hours. i found it puts off a slight smell of liquid fuel and so i couldn't keep using it, but some folks might not be sensitive.

cz