Thermal models for microwave hazards and their role in standards development

We consider the thermal response of the body to radiofrequency (RF) energy, with emphasis on partial-body exposure, to assess potential thermal hazard s. The thermal analysis is based on Pennes' bioheat equation. In this model , the thermal response is governed by two time constants. One (tau(1)) pert ains to heat convection by blood flow and is (for physiologically normal pe rfusion rates) on the order of 3 min. The second (tau(2)) characterizes hea r conduction, and varies as the square of a distance that characterizes the spatial extent of the heating. We examine three, idealized cases. The firs t is a region of tissue with an insulated surface, subject to irradiation w ith an exponentially decreasing SAR, which models a large surface area of t issue exposed to microwaves. The second is a region of tissue in contact wi th a hemispherical electrode that passes current into it, which models expo sure from contact with a conductor. The third is a region of tissue with an insulated surface, subject to heating from a dipole located close to it. I n all three cases, we estimate the maximum steady-state temperature increas e as a function of the relevant electrical and thermal parameters and the t hresholds for thermal hazard. We conclude that thermal models are a potenti ally fruitful but underutilized means of analyzing thermal hazards from RF fields. A quantitative analysis of such hazards enables the development of data-based uncertainty factors, which can replace arbitrary "safety factors " in developing exposure limits. Finally, we comment on the need to many qu antitative modeling of data and risk assessment, and to incorporate contemp orary approaches to risk assessment into RF standards development. Bioelect romagnetics 20:52-63, 1999. (C) 1999 Wiley-Liss. Inc.