The Specific Absorption Rate (SAR) is a number that measures the rate at which energy is absorbed by the human body when exposed to radio waves.
In mobile telephony, it is the specific absorption rate to measure the energy absorbed by a particular mass of human tissue, within a certain amount of time. SAR is measured in units of power per mass (W/kg). Generally this is reported for Head and Body.
 Safe limits
In the United States, phones should have an SAR value below or at 1.6 W/Kg, taken over the volume containing a mass of 1 gram of tissue. In Europe, the limit is at 2 W/kg, but in a sample of 10 grams of tissue. Because of the different sample used to measure it is not possible to compare the two numbers.