"The plan which is, recommended by the United States provides for certain discrete frequencies, with appropriate guard bands, for those industrial scientific and medical devices which cannot be shielded effectively. This assures a minimum disruption of vital communication services. The allocation of frequencies for this purpose is justified because of the tremendous investments in equipment and the public demand for the various services performed, together with the harmful interference to radio communications which would result if such devices were scattered indiscriminately throughout the spectrum. The frequencies proposed are:

13660 kHz

27320 kHz

40980 kHz

2450 MHz"

http://handle.itu.int/11.1004/020.1000/4.62.51.en.108, page 628.