Ergodyne - EGO16990 Ideal for cold weather work and recreation, N-Ferno Warming Packs heat up in seconds and 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 and 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 and material handling