Why do exposure limits vary with frequency?

The correct choice highlights a fundamental concept in radiofrequency (RF) exposure and biological interaction. The human body interacts with RF energy in complex ways that depend on frequency, leading to varying absorption rates at different frequencies. This is largely due to the differing electrical properties of tissues, which can affect how energy is absorbed.

At certain frequencies, particularly in the range of a few megahertz to several gigahertz, the resonance effects and the dielectric properties of body tissues can enhance the absorption of RF energy, resulting in greater potential for heating or other biological effects. For example, as frequency increases, the energy tends to be absorbed more by the skin, while lower frequencies might penetrate deeper into body tissues. Thus, this frequency-dependent absorption necessitates adjusted exposure limits to protect individuals from potential harm.

The other options do not accurately capture the relationship between frequency and RF energy absorption. Lower frequency fields, for example, do not necessarily have more energy; instead, it's the energy absorption characteristics at various frequencies that are critical. Lower frequency RF fields can penetrate human body tissues but do so differently than higher frequency fields. Additionally, higher frequency RF fields are not inherently transient; they simply interact differently with biological tissues compared to lower frequencies. This distinction is essential for understanding RF exposure