Why do exposure limits vary with frequency?

The human body acts as a receiving antenna for radio waves. Just like any antenna, the body has a resonant frequency where it most efficiently absorbs electromagnetic energy. This resonance typically occurs in the VHF range (between 30 MHz and 300 MHz), depending on a person's height and size. Because the body absorbs much more energy at these resonant frequencies, the Maximum Permissible Exposure (MPE) limits must be set lower (stricter) in this range to prevent localized tissue heating. At lower (HF) or higher (UHF/Microwave) frequencies, the body is a less efficient antenna, so it absorbs less energy and the allowable limits can safely be higher.