Many concerns have been raised in the medical community about ultrasound's safety to human beings under observation, particularly when used in prenatal care. Ultrasound waves are energy (heat) waves, and as a result, they potentially can be harmful to human tissues and organs if the heat levels used are high or sustained for long periods of time.
Tissues absorb heat, which increases the temperature locally. Cavitation, or the formation of bubbles when dissolved gases come out of a solution due to local ultrasound heating, is another concern. The larger concern is with the use of CW ultrasound waves, which have higher energy levels over a specific period of time than pulsed sonography.
Heat is only one way ultrasound can potentially affect human tissue negatively. Ultrasound itself does not produce audible noise. But secondary vibrations can produce audible noises as loud as 100 dB, causing fetuses to move. Other still poorly understood effects include shearing forces within tissues, induced flows within fluids, and the creation of minute amounts of toxic chemicals.
One trend is clear. Since the mid-1990s, ultrasound energy dosage levels have been rising (but still within federal guidelines) as the medical profession increases its use of sonography. Medical professionals believe the benefits of ultrasound far outweigh any perceived or potential risks.
However, there have been no known documented ill effects associated with ultrasound in either humans or animals. Granted, some medical providers are understandably concerned that expectant mothers may have their fetuses exposed to too much ultrasound imaging. But the U.S. Food and Drug Administration (FDA) has established safety levels for ultrasound imaging power levels and frequencies when used for prenatal applications.