Microwave Diathermy (MWD) units and domestic microwave ovens usually operate at the internationally agreed frequency of 2450 MHz (2.45 GHz) Some clinical devices, particularly in the United States, emit radiation with a frequency of 915MHz The power output of therapeutic microwave units can approach 250W The depths to which microwave energy penetrates (expressed as the distance in which the field strength has fallen to 37% of the incident value) varies according to tissue and frequency. Microwaves with a frequency of 2450MHz penetrate 1.7cm in muscle and skin, whilst at 915MHz these values increase to 3.04 cm