Fig. 5. The linear second phase of the hypothermia curve. The second phase of the hypothermia curve results from heat loss exceeding metabolic heat production. Typical heat production is approximately 1 kcal · kg⁻¹· h⁻¹. At steady state, heat loss must equal this amount. During anesthesia and surgery, however, heat loss is often far greater. The major cause of loss is radiation, accounting for roughly 60% of the total. Radiative loss is proportional to the difference of the fourth powers of room wall (i.e. , ambient) and skin temperatures. The remaining loss is largely convective, which is proportional to the difference between skin and air temperatures multiplied by the square root of air speed. Respiratory evaporative loss contributes only approximately 10% to the total and cutaneous evaporative loss remains relatively small except during sweating. However, evaporative loss from within surgical incisions can be substantial. Conductive loss is unimportant during anesthesia.