Which EMF exposure range would you expect to cause skin heating effects at the skin surface?

• A. 3 kHz-5 MHz
• B. 100 kHz-3 GHz
• C. 3 GHz-300 GHz
• D. 30-300 MHz

Answer: (C)

Energy deposition depth in tissue depends strongly on frequency: as frequency goes up, the depth at which most energy is absorbed becomes smaller. The skin depth δ, which roughly scales as δ ≈ sqrt(2/(ω μ σ)), decreases with increasing angular frequency ω, so higher-frequency fields heat a much thinner layer right at the surface. In the 3 GHz to 300 GHz range, which covers microwaves to millimeter waves, the skin depth is tiny—often fractions of a millimeter to a few millimeters—so the energy is absorbed mainly in the outer skin layers, producing heating at the surface rather than deeper tissues. At lower frequencies (kHz to low GHz), the waves penetrate more deeply because δ is larger, leading to less surface-confined heating. So the exposure range that would cause skin heating at the surface is the 3 GHz to 300 GHz band.