When your hands are cold and you’ve still got 200 miles to go, pull two of these out of your tank bag. The chemical pack is activated for a one-time use, producing heat of 150°F. Safe and odorless. They have a shelf life of 3 years. The expiration date is stamped on the package. They pack smaller than a handkerchief. Handwarmer (2.25"×4"; 10 hrs), Footwarmer (4"×4"; 7 hrs), Bodywarmer (4"×5"; 24 hrs). Note: footwarmers are formulated to work in a low oxygen environment.