SAR is measured in different ways that have applicability to the mechanisms that excessive RF absorption can harm tissue. Whole-Body SAR deals with the thermal load of the body. Like a fever, whole body heating deals with the addition of heat to the body that must be removed.
Often, the choice of SAR characterization has been based on limitations of the ability to calculate or measure detailed SAR. In the early days of electromagnetic research, laboratories were capable only of determining the total amount of energy that was absorbed by the tissue. After dividing by body weight, whole-body SAR was the result. Many studies were performed to look for any deleterious effects and relate them to a whole-body SAR dose. Later, as technology improved, making it feasible to both measure and model SAR with higher spatial resolution, localized SAR was used in many exposure situations. The smallest region of tissue for which localized SAR is defined is currently one gram (approximately a cube measuring one cm (0.4 inches) on each side). This resolution was chosen in the early 1990s, in part due to the constraints of the available technology at the time. As computers continue to become more powerful, the minimum resolution can continue to
drop if deemed necessary.
When RF exposure is localized to a certain area in the body, whole-body SAR can be misleading. A very high local SAR can appear to be much lower when averaged over the entire body. For example, consider a focused beam of RF energy that is absorbed in 100 g (3.5 oz), of brain tissue (a cube measuring approximately 5 cm (2 inches) on each side) with an SAR of 280 W/kg. If this were the only place that RF is absorbed in the body, the equivalent whole-body SAR for a 70-kg (150-lb), person would be 0.4 W/kg. Even though 280 W/kg is a very high level of absorption, one that approaches levels attained in a microwave oven, the whole-body equivalent value is deceptively small, and within guidelines for safety. Clearly, it is not correct to apply whole-body SAR calculations in every exposure situation. In developing the ANSI/IEEE C95.1-1992 safety standard, this was taken into account by specifying a maximum whole-body SAR as 0.4 W/kg, or a maximum localized
(averaged over any 1 gram (0.04 oz) cube of tissue) peak SAR of 8 W/kg. Again, these values are for controlled exposure. General population limits are one fifth of these. The safety standard also takes into account that some tissues are more sensitive than others, and peak SAR can be as high as 20 W/kg (4 W/kg for general population) averaged over any 10 gram cube of tissue in the hands and feet.