Frequency and power windowing in tissue interactions with weak electromagnetic fields

Effects of nonionizing electromagnetic (EM) fields that raise tissue temperature in general differ very little from effects of hyperthermia induced by other means. However, fields raising tissue temperature orders of magnitude less than 0.1°C may result in major physiological changes not attributable to raised temperature per se. These weak fields have been observed to produce chemical, physiological, and behavioral changes only within windows in frequency and incident energy. For brain tissue, a maximum frequency sensitivity occurs between 6 and 20 Hz. Two different intensity windows have been seen, one for ELF tissue gradients around 10^-7V/cm, and one for amplitude modulated RF and microwave gradients around 10^-1V/cm. The former is at the level associated with navigation and prey detection in marine vertebrates and with control of human biological rhythms; the latter is at the level of the electroencephalogram (EEG) in brain tissue. Coupling to living cells appears to require amplifying mechanisms that may be based on nonequilibrium processes, with long-range resonant molecular interactions. These cooperative processes are now recognized as important in immune and hormonal responses, as well as in nerve cell excitation. Polyanionic proteinaceous material forming a sheet on cell membrane surfaces appears to be the site of detection of these weak molecular and neuroelectric stimuli.