Unlike light, however, the wavelength of RF energy is usually larger than, or about the same size as, many of the objects that it interacts with. The wavelength of red light is 0.00007 cm (0.00003 inches). In comparison, the wavelengths of RF frequencies most commonly used by hams, 1.8 MHz to 460 MHz, vary from 171 meters (562 feet) to 65 cm (26 inches). This introduces resonance effects between the energy and human tissue. If an object is equal in size to one wavelength, or certain fractions of a wavelength (like one half or one quarter of a wavelength) it is likely to be resonant to that energy. When a resonance exists, the object absorbs more energy and reflects or passes less.
Likewise, when there is no resonance, much less energy is absorbed; most of it either passes through the tissue or is reflected. Thus, incident power density does not necessarily indicate how much of the energy gets absorbed in tissue. A different measure, that indicates
absorption, has been introduced: Specific Absorption Rate, or SAR, is measured in watts/kg or milliwatts/g of matter (1 W/kg = 1 mW/g). For a given volume of tissue, the SAR indicates the average rate at which energy is absorbed for each kilogram, or gram, of tissue weight.
The various RF safety standards, such as ANSI/IEEE C95.1-1992 and NCRP Report 86, base acceptable exposure limits on SAR values that have been determined to be safe. The limits are based on analysis of several decades of scientific study. Due to resonance effects, the acceptable exposure, expressed in power-density, differs with frequency in order to realize a consistent limit of SAR (the Maximum Permissible Exposure, or MPE, limits that follow are for controlled exposure. Similar relationships exist for general population limits). It is easy to see how the body absorbs very little energy in the 160 meter band and, as such, the MPE is relatively high (100 mW/cm2). As frequency increases, wavelength approaches dimensions of the human body and its parts. This is reflected in the MPE limits by a decreasing function with increasing frequency (900/f2 mW/cm2) from 3 to 30 MHz. At VHF frequencies, the wavelengths are very close to body dimensions, resonance is
high, absorption increases, and the MPE limits are at their minimum value (1 mW/cm2). As frequencies increase into the UHF region, wavelengths become smaller than the body and MPE limits correspondingly increase as frequency increases (f/300 mW/cm2). In the microwave region, the wavelengths are very small and most absorption occurs near the surface of the body. The decrease of resonance effects at these frequencies causes the MPE to level off (5 mW/cm2). This complicated set of relationships is designed to keep SAR below the accepted safe level of 0.8 W/kg whole body absorption.