From same brand
The Thermopad series of heaters are air activated. They bring a natural heat thanks to the unique composition of the contents. They heat up to 45 °C (113 °F) and keep an average temperature of 37 °C (98.6 °F) for up to 12 hours.
The body warmer is intended for placement on the lower back, where it distributes the heat to that area. Simply stick the adhesive side of the warmer to your base layer, and put on the rest of your winter clothing.
Notice: Thermopad products are single-use, meaning that once activated by air, they cannot be re-heated for any purpose.