of cataracts depends on frequency, power density, duration of exposure, and intervals between exposures. There is some evidence that microwaves also affect the central nervous system in various ways. They also affect the performance of some types of cardiac pacemakers. For a time, a beneficial use of microwaves on humans was diathermy. Diathermy is a therapeutic means of generating heat in body tissues. Localized application of levels up to 100mW/cm2 helped promote healing in joints, tendons, muscle, and other tissue. Diathermy devices operate in shortwave radio frequencies and ultrasonic frequencies in addition to microwave frequencies. If not applied carefully for limited times, the treatment can burn skin and injure deeper tissues. In addition, diathermy can cause severe injury or even death when used on patients with implanted leads or other objects or devices.