What is the Specific Absorption Rate (SAR) , and how can people use it to buy cell phones? Part 3

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SAR is measured in different ways that have applicability to the mechanisms that excessive RF absorption can harm tissue. Whole-Body SAR deals with the thermal load of the body. Like a fever, whole body heating deals with the addition of heat to the body that must be removed.

Often, the choice of SAR characterization has been based on limitations of the ability to calculate or measure detailed SAR. In the early days of electromagnetic research, laboratories were capable only of determining the total amount of energy that was absorbed by the tissue. After dividing by body weight, whole-body SAR was the result. Many studies were performed to look for any deleterious effects and relate them to a whole-body SAR dose. Later, as technology improved, making it feasible to both measure and model SAR with higher spatial resolution, localized SAR was used in many exposure situations. The smallest region of tissue for which localized SAR is defined is currently one gram (approximately a cube measuring one cm (0.4 inches) on each side). This resolution was chosen in the early 1990s, in part due to the constraints of the available technology at the time. As computers continue to become more powerful, the minimum resolution can continue to
drop if deemed necessary.

When RF exposure is localized to a certain area in the body, whole-body SAR can be misleading. A very high local SAR can appear to be much lower when averaged over the entire body. For example, consider a focused beam of RF energy that is absorbed in 100 g (3.5 oz), of brain tissue (a cube measuring approximately 5 cm (2 inches) on each side) with an SAR of 280 W/kg. If this were the only place that RF is absorbed in the body, the equivalent whole-body SAR for a 70-kg (150-lb), person would be 0.4 W/kg. Even though 280 W/kg is a very high level of absorption, one that approaches levels attained in a microwave oven, the whole-body equivalent value is deceptively small, and within guidelines for safety. Clearly, it is not correct to apply whole-body SAR calculations in every exposure situation. In developing the ANSI/IEEE C95.1-1992 safety standard, this was taken into account by specifying a maximum whole-body SAR as 0.4 W/kg, or a maximum localized
(averaged over any 1 gram (0.04 oz) cube of tissue) peak SAR of 8 W/kg. Again, these values are for controlled exposure. General population limits are one fifth of these. The safety standard also takes into account that some tissues are more sensitive than others, and peak SAR can be as high as 20 W/kg (4 W/kg for general population) averaged over any 10 gram cube of tissue in the hands and feet.

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What is the Specific Absorption Rate (SAR) , and how can people use it to buy cell phones? Part 2

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Unlike light, however, the wavelength of RF energy is usually larger than, or about the same size as, many of the objects that it interacts with. The wavelength of red light is 0.00007 cm (0.00003 inches). In comparison, the wavelengths of RF frequencies most commonly used by hams, 1.8 MHz to 460 MHz, vary from 171 meters (562 feet) to 65 cm (26 inches). This introduces resonance effects between the energy and human tissue. If an object is equal in size to one wavelength, or certain fractions of a wavelength (like one half or one quarter of a wavelength) it is likely to be resonant to that energy. When a resonance exists, the object absorbs more energy and reflects or passes less.

Likewise, when there is no resonance, much less energy is absorbed; most of it either passes through the tissue or is reflected. Thus, incident power density does not necessarily indicate how much of the energy gets absorbed in tissue. A different measure, that indicates
absorption, has been introduced: Specific Absorption Rate, or SAR, is measured in watts/kg or milliwatts/g of matter (1 W/kg = 1 mW/g). For a given volume of tissue, the SAR indicates the average rate at which energy is absorbed for each kilogram, or gram, of tissue weight.

The various RF safety standards, such as ANSI/IEEE C95.1-1992 and NCRP Report 86, base acceptable exposure limits on SAR values that have been determined to be safe. The limits are based on analysis of several decades of scientific study. Due to resonance effects, the acceptable exposure, expressed in power-density, differs with frequency in order to realize a consistent limit of SAR (the Maximum Permissible Exposure, or MPE, limits that follow are for controlled exposure. Similar relationships exist for general population limits). It is easy to see how the body absorbs very little energy in the 160 meter band and, as such, the MPE is relatively high (100 mW/cm2). As frequency increases, wavelength approaches dimensions of the human body and its parts. This is reflected in the MPE limits by a decreasing function with increasing frequency (900/f2 mW/cm2) from 3 to 30 MHz. At VHF frequencies, the wavelengths are very close to body dimensions, resonance is
high, absorption increases, and the MPE limits are at their minimum value (1 mW/cm2). As frequencies increase into the UHF region, wavelengths become smaller than the body and MPE limits correspondingly increase as frequency increases (f/300 mW/cm2). In the microwave region, the wavelengths are very small and most absorption occurs near the surface of the body. The decrease of resonance effects at these frequencies causes the MPE to level off (5 mW/cm2). This complicated set of relationships is designed to keep SAR below the accepted safe level of 0.8 W/kg whole body absorption.

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