Exposure levels are evaluated either by measuring the energy absorbed by tissue (through SAR) in the case of mobile phones, or the power density of an equivalent plane wave in the case of base stations. Determining the maximum SAR value for a mobile phone is a relatively difficult operation. It’s done using models of the head, filled with a liquid whose properties are similar to the human brain in terms of wave propagation. The results obtained in this way are complemented by computer simulations using various models. In the case of base stations, the procedure is simpler, because they aren’t located in the immediate vicinity of the human body and it’s possible to directly measure certain physical properties which serve as reference. Usually, these are the power density of the equivalent plane wave or the intensity of electrical current. The SAR values recommended by the International Commission on Non-Ionising Radiation Protection are accompanied by a set of reference levels for these