Each GSM handset has a radio transmitter and receiver in order to operate in the wireless GSM network. That transceiver is manufactured so that when used next to the ear and when worn on the belt, it won't exceed the limits for exposure to radio frequency energy set by the authorities.
The authorities in question here are the Federal Communications Commission (FCC) of the U.S. Government, Industry Canada of the Canadian Government (IC), and the Council of the European Union.
These limits are part of comprehensive guidelines that establish permitted levels of RF energy for the general population. The guidelines are based on standards that have been developed by independent scientific organizations through periodic and thorough evaluation of scientific studies.
The exposure standard for wireless devices employs a unit of measurement known as the Specific Absorption Rate, or SAR. Unfortunately the limits set by the FCC/IC and the Concil of the European Union are measured over different amount of tissue so they are not directly comparable.
The SAR limit set by the FCC/IC is 1.6W/kg averaged over 1 gram of actual tissue.
The SAR limit recommended by the Council of the European Union is 2.0W/kg averaged over 10 g of actual tissue.
Although the SAR is determined at the highest certified power level, the actual SAR level of the device while operating can be well below the maximum value. This is because GSM phones are designed to operate at multiple power levels so as to use only the power required to reach the network. In general, the closer you are to a wireless base station antenna, the lower the power output of the device and vice versa.
As of 2010, we at GSMArena.com have added the officially set SAR values for most models in our database.
You will find we list several SAR values. The reason for that is the FCC/IC have stipulated that SAR should be measured at both hip level (making calls while carrying the phone at the waist) and at head level (making calls with the phone put next to ear). So you are almost certain to see two SAR values for the FCC/IC standard.
The Council of the European Union only requires the measurements at ear level so most manufacturers don't measure SAR at hip level for European models. So in this case you are almost certain to see a single SAR value only for the EU standard.