The use of microwave transmission of power has been the most controversial issue in considering any SPS design. At the Earth's surface, a suggested microwave beam would have a maximum intensity at its center, of 23 mW/cm2 (less than 1/4 the solar irradiation constant), and an intensity of less than 1 mW/cm2 outside the rectenna fenceline (the receiver's perimeter). These compare with current United States Occupational Safety and Health Act (OSHA) workplace exposure limits for microwaves, which are 10 mW/cm2,[original research?] - the limit itself being expressed in voluntary terms and ruled unenforceable for Federal OSHA enforcement purposes.[citation needed] A beam of this intensity is therefore at its center, of a similar magnitude to current safe workplace levels, even for long term or indefinite exposure.