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With wi-fi access points projected to double by 2005 and a pro- jected 9 million users before the end of this year, the electromagnetic environment is becoming much more energetic. These Gartner Inc. projections beg the question, however, of popular acceptance of radio- frequency energy in the workplace and other venues.
Cellular telephone energy, with its possible effect on the brain for users of handheld transceivers with built-in antennas, has been a subject of controversy for years.
Added to that now are questions of safety for other fast-growing radio services, such as point-to-point microwave bridges and the 802.11x family of wireless transceivers.
Users, maintenance personnel and other affected persons will have to decide whether theyre relieved, or insulted, to know that radio-exposure measurements will now be made by modeling their heads as bags of poorly conductive fluid. The IEEE standard defines the methods by which electrical field probes will measure what is formally called the Peak Spatial-Average Specific Absorption Rate of radio energy in this brain-equivalent material.
Its important to note, however, that the standard merely describes a procedure for estimating the energy levels that are caused, for example, by handheld wireless transceivers operating next to the head at frequencies between 300MHz and 3GHz. The standard does not set exposure limits, which are left to the regulatory agencies of individual countries. This leaves plenty of room for argument.
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Radio waves combine electric and magnetic fields. Radio receivers work because the electric component of the electromagnetic wave induces a voltage difference across an antenna, creating a current that an amplifier can boost and filter to yield the desired signal. The question is whether the human body also functions, too well, as an antenna and whether the resulting currents in the body have harmful effects due to gross mechanisms, such as the heating of tissue, or to more subtle effects that result from particular frequencies of even low-powered signals.
Study of these effects depends, critically, on both the geometry and the composition of body parts, since antenna response to radio waves depends on the equivalent electrical length of an object and on its orientation with respect to the electrical field. The equivalent length of an object is greater than its physical length because electrical fields in physical objects (such as transmission lines or human brains) propagate at less than the speed of light in a vacuum. Ideally, a 10-centimeter antenna, placed at right angles to a radio beam and in the same plane as its electric field component, would be a perfect trap for energy at a frequency of about 1.5GHz, but in practice, the optimal antenna size will be smaller to offset the slower speed of the signal within the antenna material.
Whether radio waves essentially pass through an object, or resonate within it, is an important question, but live human heads are difficult to obtain for testing purposes. Its therefore been a challenge for the IEEE to develop a model of the brains response to radio waves that can be used to make valid measurements.
The newly recommended practice, IEEE 1528, has been in development for several years; in draft form, it has already influenced such key regulatory documents as the Federal Communications Commission Office of Engineering and Technology Bulletin 56, “Questions and Answers about Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields,” most recently updated in the fourth edition published in August 1999 (see www.fcc.gov/oet/info/ documents/bulletins/#56).
Building on measurement standards, the FCC then promulgates guidelines for radio-frequency exposure, such as those described at www.fcc.gov/cgb/ consumerfacts/rfexposure.html. For cellular telephone towers, for example, the FCC recommends that nearby persons should not be exposed to field strengths of more than 580 microwatts per square centimeter.
In free space, a transmitter radiating equally in all directions can be imagined as spreading its energy across the surface of a sphere with the transmitter antenna at its center, meaning that energy density falls with the square of the distance. Using this simple model, a person would have to be less than 46 inches from a 100-watt transmitters antenna to be exposed to more than the recommended field strength and would have to remain in that full-power beam for several minutes to be harmed.
Mobile phones are more of a concern: They operate at small fractions of a watt but at distances of only a few centimeters, with a 0.1-watt transmitter exceeding the FCC-recommended field strength—although only within a small volume of the body—at any distance of less than about 4 centimeters.
To a great extent, however, the existence of a standard for modeling radio-frequency absorption by the brain merely begins the debate. The argument is eloquently made by physicians such as Dr. Robert Becker, who is the author of two books (“Cross Currents” and “The Body Electric”) concerning the biological effects of electromagnetic fields.
In simple terms, the view of Becker and others is that the gross heating effects of simple energy absorption might be relevant in rating the performance of microwave ovens but that they fail to reflect the interactions that take place in living organisms.
In an interview with The EMR Network (emrnetwork.org/faq/faq.htm), Becker asserted that engineers and physicists see living cells as “little plastic bags filled with minestrone soup”—which is, in fact, not much different from what the IEEE suggests—and their question is How much does it take to heat this up?”
“Any biologist can tell you that the body is much more complicated than that,” Becker said. He described his work with “the bodys actual use of electric currents … that regulated certain things like healing. Wound healing is associated with a rather specific electrical current and voltage. So, the premise that was applied by the physicists and the engineers was erroneous from the start.”
Even those who dont fully share Beckers concerns will generally agree that there are significant differences between short-term, high-level exposures (which are easy to test in a laboratory) and long-term, low-level exposures (more costly to measure) at varying frequencies (with different resonance effects on different types and sizes of body structure).
And as radio links become more common in voice and data networks, the possible side effects of radio-frequency exposure may become an issue for more people and in more places.
Technology Editor Peter Coffee can be contacted at [email protected].