Someone gave me a few of these. They are so awesome, I bought some when I was nearly out.
You can get a single set for around $2 or pay less per set if you buy a ten pack or whatever. They are available at places like Target, Walgreen's and sporting goods stores.
You rip open the plastic casing, shake to activate. They are air activated. They do not get hot enough to burn you. They do not seem to emit any odors or chemicals or anything that bothers me (I am extremely chemically sensitive). They do not give off light. They can be stuck in your pockets or held in your hands to keep you warm. They have made a big differernce on cold nights here recently for us.