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.

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.

How SAR is Determined Makes a Difference

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.

FCC Regulations

The FCC defines a portable device as one that is normally used with its antenna within 20 cm of the body of the user. A handheld radio transmitter, such as a cellular telephone, is an example of such a device, and local SAR determinations are required, since its energy is absorbed in a small area of the body, usually the head. It is not meaningful to determine the power density at the head, as would be applicable for transmissions from an antenna that is farther away from the body. The FCC requires that many handheld radios meet the safety requirement that the SAR must be less than 1.6 W/kg in any one gram of tissue. It is assumed that not all cellular telephone users will be informed about RF safety, so the General Population limit is applied. It is acceptable to either measure absorption in a phantom head, or to perform a computer model of absorption, using one-gram regions of tissue in both cases.

The FCC has exempted amateur handheld transmitters from testing for compliance with SAR limits. There are several reasons for this. One is that hams are assumed to be in the controlled population, and the acceptable peak localized SAR of 8 W/kg is higher than a handheld transmitter would normally be expected to cause. Also, unlike cellular telephones, which transmit continuously when in use, an HT is a "Push-to-Talk" device, that only transmits when the ham is speaking.

The Cell Phone Debate

As the question of cell phone safety continues to be argued in the general society, we, as hams, cannot sit back and be spectators. Many of the same issues that affect cell phone users also affect us. With regard to exposure limits, the FCC considers us to be part of the informed population. We need to understand as much as possible about the issues affecting safe exposure. When using our H-Ts, we need to know why they were exempted from testing and operate accordingly. Since the exemption was based in part on the assumption of a relatively low duty cycle, we should avoid holding the PTT button and chatting nonstop for six minutes at a time. Above all, let's strive to operate safely and to understand how and why we are doing so.

Editor's note: Greg Lapin, N9GL, started working in the RF safety world after spending many years first studying cardiac function imaging and then brain tumor kinetics. He serves as chairman of the ARRL RF safety Committee and as a member of the IEEE Committee on Man and Radiation. A former professor of Biomedical Engineering and Neurology at Northwestern University, Lapin now works as a consulting professional engineer in the electronics industry. He was first licensed while a teenager in 1969 and continues to be fascinated by virtually all aspects of Amateur Radio. One of his many interests is electronic design, and he is the author of Chapter 8, "Analog Signal Theory and Components" in The ARRL Handbook for Radio Amateurs. His non-ham interests include making things grow in his garden and serving as commissioner of the local children's softball league. At other times--when he is not working or helping his kids with their homework--you might find him with the local emergency services agency, climbing his tower, building a new QRP rig, playing with his APRS setup, responding to QSL cards, going off on a DXpedition, or trying to get that "new one." You can reach him by email at g.lapin@ieee.org.