Heat Factory Footwarmer Insoles are designed to heat in the restricted air environment of a shoe or boot for up to 8 hours. When placed under sock, these thin and comfortable insoles provide soothing warmth underneath your feet. Ideal for daily use in shoes, ski boots, or whenever feet are cold. Not intended for use in open air. One size fits all. The Heat Factory Footwarmer Insole is a soft, light-weight pouch containing an inert mixture of iron powder, charcoal, salt, water and vermiculite. When exposed to air, a natural exothermic reaction generates consistent and prolonged heat. The insole pouch is made of soft cloth lined with a very thin layer of porous plastic for maximum durability. Peel back the adhesive backing and press warmer to outside of sock underneath foot. Great for fishing, hunting or outdoor activities. The Heat Factory Foot Warmer Insoles last approximately 8 hours and measure: 9" x 2.75". Sold as pair.