I first started using these when I hung iron in December in Fairbanks, Alaska. These saved the day by keeping my hands warm in the coldest of weather. Once the outer package is opened, an oxidation process takes place which causes a safe, natural reaction to release heat. Biodegradable, won't harm the environment. Sold by the pair, or 5 pack. Time released to last all day. Hand warmers lasts 7 to 11 hours, toe warmers up to 5 hours.