SAR is a comparative value that supposes to reflect how much radiation is being absorbed into a flesh tissue when radiated by RF radiation source like a cellphone. The units are Watt per kilogram.
The measurement is based on heating of a water-sugar-salt based fluid inside a dummy which is bigger than 90% of the world adult population. The radiation source is located about 1" (2.54 cm) away from the dummy (This is way in most mobile phone user manuals it say "keep the phone at least 1" away from your body in all time") and is configured to transmit in max power. The measurement last for 6 minutes (in the time SAR was invented it was seem to be impossible that a person will use the phone for more than 6 minutes). Both temperature and RF Electric field levels are measured by a probe that is inserted to the dummy.
The SAR is addressed as the world wide official standard for cell-phones' radiation emission.
SAR measurement is not simple and there are only a small number of laboratories around the world that can actually measure it. The room of the measurement is RF and ELF shielded, and it should have no RF reflection from the walls.
Please see the video below about SAR measurement in Australia.
Why was the SAR invented?
Officially SAR was invented to create a protocol for safety measurement of cell-phoned. Since cell phones are held next to a person head and brain there was a need to assess the safety of the device before putting it on the market. There is a technical difficulty to measure RF radiation very close to the source (in the "near field") and there was a need to find a way to measure the levels without using RF radiation flux density. One way of doing this is to measure at least 3 field lengths away from the source and calculate backwards the RF radiation levels in the head and brain. The result of this calculation showed higher levels than what is allowed by ICNIRP so called safety standard which is based only on heat effect. So the need for a new standard that will give KOSHER stamp to all cellphone (even if they emit very high levels of RF radiation) appeared.
Because SAR is so difficult to understand people takes it as granted and don't question the logic behind it.
What is wrong with the SAR?
1. Temperature, heat effect based standard (heat concept, don't protect from biological influences, changes and damage at none thermal levels of RF radiation).
2. Measured on a dummy filled with sugar-salt water.
3. The sugar salt water is homogeneous. The brain is not, and it has more and less condensed parts and different tissues. Test has shown that there are hot spots in the brain where the SAR levels are about 10 times more than the measured SAR on a dummy.
4. The dummy size and shape is of a big grownup. Childrens' heads are much smaller and the radiation penetration to the head is higher. The SAR does not cover children and small adults.
5. The Dummy is made out of plastic and does not block or reflect RF radiation as the skull does. In a real head, hot spots are created due to the reflection of the RF radiation from the skull.
6. The basic assumption in the time that the SAR was invented was that the longest possible cellphone use will less tha 6 minutes (funny funny ha ha).
7. In the SAR measurement there is no expression to the ELF magnetic and electric fields that are created around a cellphone while it transmits. Exposure to this fields from levels of 2mG is considered to be a health risk. In GSM phones the ELF magnetic field can go up to several hundreds of mG, but it will not influence the SAR level.