Ultrasound is sound (a mechanical vibration phenomenon) having a frequency above the range of human hearing (typically above 16 kHz) which, unlike electromagnetic radiation, requires a medium through which to propagate. Exposure to ultrasound can be divided into two distinct categories: airborne and liquid-borne. Exposure to airborne ultrasound occurs in many industrial applications such as cleaning, emulsifying, welding and flaw detection; through the use of consumer devices such as dog whistles, bird and rodent controllers, and camera rangefinders; and commercial devices such as intrusion alarms. Liquid-borne exposure occurs predominantly through medical exposure in diagnosis, therapy, and surgery. As with any other physical agent, ultrasound has the potential to produce adverse effects at sufficiently high doses. In addition, biological effects of unknown significance have been reported at low exposure levels under laboratory conditions.
Exposure of human beings to low frequency ultrasound (16 - 100 kHz) can be divided into two distinct categories; one is via direct contact with a vibrating solid or through a liquid coupling medium, and the other is through airborne conduction. For airborne ultrasound exposure, at least one of the critical organs is the ear. Effects reported in human subjects exposed to airborne ultrasound include: temporary threshold shifts in sound perception, altered blood sugar levels, electrolyte imbalance, fatigue, headaches, nausea, tinnitus, and irritability. Studies on skeletal tissue indicate that bone growth may be retarded following exposure to ultrasound at high therapeutic intensities, even if the transducer is kept in motion during treatment. If the transducer is held stationary, bone and other tissue damage occurs at lower intensities. Both in vitro and in vivo exposures of muscle tissue have been reported to trigger contractions. Therapeutic intensities of ultrasound have also been reported to alter thyroid function in man. A number of reports indicate that lower foetal weight and increased foetal abnormalities occur following exposure to ultrasound in the low therapeutic intensity range. One study suggests that lower birth weights may result from exposure to diagnostic ultrasound in utero. As the practice of ultrasound diagnosis becomes more widespread, it will be difficult to find adequate control populations and opportunities for satisfactory epidemiological studies may become increasingly rare.