Cellular telephone makers soon will offer consumer information on Specific Absorption Rate or SAR. What is the SAR, and how can people use it? The cellular telephone industry has announced that it will present consumers with the Specific Absorption Rates, or SARs, measured or calculated from its phones. SAR is also the basis of the Maximum Permissible Exposure–or MPE–limits with which hams must comply. What is the SAR and how can people use it?
Why is it needed? Radio waves and light waves are both made up of electromagnetic energy, with different frequencies. Microwaves are radio waves with smaller wavelengths, typically less than 30 cm (one foot). In many ways, all electromagnetic waves have the same behavior when they interact with matter. Many aspects of the behavior of radio waves can be demonstrated with light, which may be easier to understand since we can visualize it. The amount of energy that impinges on an object depends on the size of the object and how much the energy has spread since leaving the antenna.
To take this additional factor into account, a new unit of measurement is introduced, power density, or power per unit of area, measured in watts/m2 or millwatts/cm2 (10 W/m2 = 1 mW/cm2). The ways that RF energy affects human tissue are not simply related to the power density of the signal in the air. The key is how much energy is absorbed in tissue, and particularly in sensitive tissues (internal organs tend to be sensitive to external energy while skin tends to be very resilient). As with light, RF energy can do three things when it interacts with an object: it can pass through, it can be reflected, or it can be absorbed.
Usually, interaction with tissue is a combination of all three of these things. Consider the sun shining on the water in a swimming pool. Some of the light reflects off the water surface and some of it passes through, illuminating the bottom of the pool. During the course of the day, the water in the pool gets warmer, indicating that some of the sunlight was absorbed and converted to heat.