Abstract
Most ultrasound sources do not transmit acoustic energy into air nearly as effectively as into liquids or solids. Because of this, only low level ultrasound reaches humans through the air in most circumstances. Instead, most human exposure to this agent derives from its use in medicine, either for therapeutic or diagnostic purposes. Sonographers and other medical personnel receive occupational exposure during training when they learn diagnostic techniques by using themselves as subjects. Also, sales personnel and engineers employed by manufacturers of diagnostic equipment often find their own bodies to be convenient objects for demonstrations or tests. There have been no confirmed reports of harm resulting from exposure to diagnostic ultrasound as practiced heretofore, whether to patients, sonographers, sales representatives, or engineers. However, users are advised to minimize exposure, since biological damage is theoretically possible if some of the newer equipment is not used wisely. Laboratory experiments have shown that ultrasound can produce a wide variety of physical, chemical, and biological effects. These are attributed to sonically produced heat, or to any of a variety of nonthermal mechanisms, some of which are unique to sound. Criteria for avoidance of thermal risk have been developed which make use of computations based on principles of thermoacoustics.