7.3.2 Electrical Stimulation

At the lower end of the RF range, induced electric fields may stimulate electrically excitable tissues before heating becomes the only relevant mechanism. Nerve and muscle membranes require time to accumulate the charge needed to initiate an action potential. As frequency increases, the field reverses too rapidly for this process to occur efficiently, so stimulation becomes progressively less important.

Possible responses at sufficiently high induced fields include tingling, painful peripheral-nerve stimulation, and involuntary muscle contraction. Loss of muscular control or a startle response can also create a secondary safety hazard during work at height, near moving machinery, or around energized equipment.

The transition is gradual rather than a boundary at one frequency. ICNIRP specifies induced electric-field restrictions from 100 kHz to 10 MHz, while heating-based restrictions overlap this range. Both mechanisms may therefore need to be considered in the lower-frequency portion of the RF spectrum.

Lower-frequency induction systems can produce strong magnetic fields and induced currents, whereas dielectric heaters operating at frequencies such as 13.56 or 27.12 MHz are generally assessed principally for thermal exposure and leakage fields. The interaction mechanism should be identified from the frequency and exposure geometry rather than inferred from the broad description of the equipment.