Energy deposition processes in biological tissue: Nonthermal biohazards seem unlikely in the ultra-high frequency range

The prospects of ultra high frequency (UHF, 300-3000 MHz) irradiation produ cing a nonthermal bioeffect are considered theoretically and found to be sm all. First, a general formula is derived within the framework of macroscopi c electrodynamics for the specific absorption rate of microwaves in a biolo gical tissue; this involves the complex Poynting vector, the mass density o f the medium, the angular frequency of the electromagnetic field, and the t hree complex electromagnetic constitutive parameters of the medium. In the frequency ranges used for cellular telephony and personal communication sys tems, this model predicts that the chief physical loss mechanism will be io nic conduction, with increasingly important contributions from dielectric r elaxation as the frequency rises. However, even in a magnetite unit cell wi thin a magnetosome the deposition rate should not exceed 1/10 k(B)T per sec ond. This supports previous arguments for the improbability of biological e ffects at UHF frequencies unless a mechanism can be found for accumulating energy over time and space and focussing it. Second, three possible nonther mal accumulation mechanisms are then considered and shown to he unlikely: ( i) multiphoton absorption processes; (ii) direct electric field effects on ions; (iii) cooperative effects and/or coherent excitations. Finally, it is concluded that the rate of energy deposition from a typical field and with in a typical tissue is so small as to make unlikely any significant nonther mal biological effect. Bioelectromagnetics 22:97-105, 2001. (C) 2001 Wiley- Liss, Inc.