7.2.2 Indirect Coupling
Indirect coupling occurs when an RF field first induces voltage or current in a conductive object and that object subsequently interacts with a person. Towers, antennas, guy wires, fences, pipelines, vehicles, handrails, and similar structures can acquire RF potentials whose magnitude depends on frequency, field strength, dimensions, orientation, and grounding.
Touching an energized object while providing a path to ground can produce contact current, shock, a spark discharge, or a localized burn. These hazards are especially relevant near high-power transmitters and large metallic structures and are controlled through isolation, grounding, bonding, shielding, access control, and safe work procedures.
RF fields may also interfere with implanted or body-worn medical devices or couple to conductive leads. The effect depends on the particular device and exposure conditions. Compliance with general human-exposure restrictions does not guarantee medical-device electromagnetic compatibility; manufacturer instructions and advice from an appropriate health professional must be followed.
Direct human exposure, contact-current hazards, and medical-device compatibility are related but distinct assessment questions. A complete RF safety assessment identifies which pathways are credible in the situation being considered.
7.2.3 Frequency Dependence And Penetration Depth
The distribution of RF absorption changes continuously with frequency. At lower frequencies, induced fields may extend through a substantial part of the body. As frequency increases, the depth over which most energy is absorbed generally decreases and absorption becomes progressively concentrated toward the surface.
Penetration depth is not a single property of the human body. It depends on the definition used, tissue permittivity and conductivity, frequency, polarization, angle of incidence, anatomical structure, and the presence of boundaries between tissues. A numerical depth quoted for one tissue model should therefore not be applied to the entire body or used as a sharp boundary between deep and superficial effects.
The 6 GHz transition used in current RF guidelines is a practical dosimetric boundary rather than a biological discontinuity. Below it, localized specific absorption rate is used to characterize absorption over a tissue mass. Above it, absorbed power density is used because the energy is increasingly confined near the surface. Whole-body thermal loading and local hot regions can still require consideration on either side of the boundary.
Frequency also affects coupling efficiency. Whole-body absorption can be high when body dimensions and the incident wavelength form a resonant relationship, but the frequency of greatest absorption depends on body size, posture, grounding, polarization, and field geometry.