Radiation Standards and Measures

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:

When referring to human tissue, this means that the SAR is a measurement of the heat absorbed by the tissue.
Measuring SAR
Measuring specific absorption rates relies upon a simple process in theory, but a nearly impossible procedure in practice because of the intrusive nature of the ideal methodology.

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.
Interesting Links
Cellular Phones and Human Health
Cellular Communication May Damage your Health
Dosimetry of Radio Frequency Electromagnetic Fields

Last Updated: September 13th, 1998 by linda.doyle@tcd.ie