SAR (Specific Absorption Rate)
In general, exposure standards are based on a measurement called the "specific absorption rate" (SAR).
The specific absorption rate is defined by the Institute of Electrical and Electronics Engineers (IEEE) as:
The specific absorption rate is defined by the ANSI standard reads:
In practice, the SAR is measured directly as a temperature increase in a localized area of tissue. To do this it is necessary to insert calorimetric probes into a live cell phone user's head in order to map SARs directly.
As a result model heads and mathematical simulations of exposed heads
seem the only viable options for estimating SARs. However building a model
head inherently involves approximations in tissue simulation and model
complexity. Similar problems exist for computer models. Thus, there is
considerable variation across different SAR studies.
Maximum Permissible Exposure
There is considerable debate over cellular telephone Maximum Permissible Exposure (MPE) standards - the maximum level of electromagnetic radiation to which humans should be exposed. There are therefore a wide range of MPE levels which are used in different countries. I am using an FCC definition here.
Normally, Maximum Permissible Exposures are based on power density measurements (milliwatts per square centimeter) to determine compliance. Unlike SARs, power densities are directly measurable using field probes and meters.
The FCC sets a maximum power density of 4mW/cm² for devices
that operate in the frequency band as cellular phones. But the standard
says that power density measurements can only be used for devices that
operate at a distance greater than 20 centimeters from the human body.
Devices operating within 20 centimeters of the human body, such as cellular
phones, must use the SAR of 1.6 W/kg.
Cellular Phones and Human Health
Cellular Communication May Damage your Health
Dosimetry of Radio Frequency Electromagnetic Fields