An analysis of existing radiofrequency and microwave radiation absorption data has been performed to examine the frequency dependent phenomenon of biological tissue heating. This analysis restricts itself to thermal considerations and examines the exposure field intensities associated with various levels of RF and MW induced thermal loading on both the body as a whole and specific, selectively absorbing tissues in adult humans and infants. An underlying absorption factor of 1W/kg, is used for comparative purposes in the analysis. A method of specifying safety standard limits based on the electromagnetic field energy density rather than the plane wave, free-space equivalent power density is presented. The analysis reveals a particularly important resonance frequency range, 10 MHz < or = f < or = 1000 MHz, in which RF and MW absorption may lead to whole body thermal loads several times the whole body basal metabolic rate for exposures equal to the present safety standard in use in the United States. A discussion is developed for applications of this analysis to occupational environments and short duration exposure conditions.