Air-circulating blankets and mattresses are not in common use in the United States, although variations on this technology have been used in the past. This technology relies on warmed-air flowing over the body to transfer heat to the patient. The advantages of warmed-air technology are that it is safe, lightweight, and theoretically more effective than warmed hospital blankets or water-circulating blankets. Products using this technology are not widely found in the U.S. market, however.