The precise indicator of the amount of absorption in heat therapy is the in vivo temperature rise per unit volume. Chemicals that are mild skin irritants may make a patient feel warm, but they will not produce any in vivo temperature rises or any of the physiological effects of heat. Many clinicians tend to think about heat therapy in terms of superficial heat modalities, such as hydro-therapy, and "deep" (penetrating) heat modalities, such as ultrasound and diathermy. Although, in the-ory, ultrasound and diathermy can be used as heat modalities, modern Food and Drug Administration