Ideal for cold weather work & recreation, N-Ferno Warming Packs heat up in seconds & provide continuous warmth. The self-contained packs add heat anywhere when exposed to air. To activate, simply open the pack. All-natural ingredients heat up when they contact oxygen. Seal from oxygen in Zip-Loc bag & reuse the next day. Environmentally friendly hand warmers keep your hands warm up to 8 hours. They are great for construction, freight / baggage, drilling / mining, oil / gas refining, iron / steel work, delivery / service, landscaping / grounds, cold storage / warehousing & material handling