Besides electric field strength in V/m, what other metrics may be used in RF exposure guidelines?

• A. Voltage across the coil only.
• B. Specific Absorption Rate (SAR) and power density.
• C. Inductance.
• D. Capacitance.

Answer: (B)

In RF exposure guidelines, the focus is on how much energy actually interacts with and heats body tissue, not just how strong the electric field is. That’s why the common metrics used besides field strength are Specific Absorption Rate (SAR) and power density.

SAR measures how much energy is absorbed per unit mass of tissue, expressed in watts per kilogram. It directly relates to heating effects and is especially important for devices that couple energy into the body at close distances, such as handheld or wearable devices. Because different tissues absorb energy differently and the amount absorbed depends on mass, SAR is often averaged over small regions (like 1 g or 10 g) to reflect localized heating risk.

Power density, on the other hand, describes how much power passes through an area (watts per square meter) and is typically used for exposures in the far field, where the wave can be treated as a plane wave spreading out from the source. It helps assess whole-body or regional exposure when the incident wave’s distribution is more about surface power flow than localized tissue heating.

Voltage across a coil, inductance, and capacitance are circuit parameters. They do not directly indicate how much RF energy is absorbed by the body, so they aren’t used as exposure metrics in safety guidelines.

So, SAR and power density are the metrics that most directly relate to the potential biological effects of RF exposure, which is why they are the correct choice.