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A free electronic newsletter covering news and other topics for those interested in RF safety issues.
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Overview This section discusses the basics of thermal effects of RF radiation on the body and the factors that impact the amount of energy absorbed by the body. Thermal Effect of RF Radiation Early on we knew that the primary concern was thermal—quite simply the body heats up in the presence of significant RF energy. The first ANSI standard was a best guess and suggested limiting exposure to the same 10 mW/cm2 field level at all frequencies. But, as research continued, it became apparent that many factors impact how much the body heats up. The concept of Specific Absorption Rate, or SAR, evolved. SAR defines heat absorbed into the body in units of Watts per kilogram. Ultimately, it was determined that much of this follows basic antenna theory. In an ungrounded situation, the body represents a fat, lossy dipole. When well grounded, the body represents a grounded quarter-wave antenna. Researchers consider the “standard man” to be 1.75 meters tall, about 5 foot 9 inches. That makes him resonant at about 86 MHz. So the average adult makes a perfect antenna for channel six television! The biology is certainly more complicated than that but height, grounding, and polarization are the most important factors in determining SAR level. How much heat can the body tolerate? It was determined that the most heat the human body can deal with is approximately 4 W/kg. Much of this research was based on exercise levels rather than on actual exposure experiments. And these levels are averaged over the body since our circulatory system functions much like a radiator. For this reason, an arm exposed to a strong RF field from a satellite uplink dish can tolerate about twenty times as much energy as the whole body. The eyes and a male’s testes are particularly vulnerable, however, since the limited blood flow of these organs limits the benefits of the circulatory system. Time is also a factor—most standards average exposure over time, which only makes sense since we are dealing with heat. Six minutes is the averaging period for most occupational exposure limits. To put things in perspective, the normal metabolic rate when someone is sleeping is about 1.0 W/kg. It increases to about 2.4 W/kg during moderate exercise. How can RF energy hurt me? Moderate level exposures cause heat stress and behavioral changes. The effects are often mistaken for the flu because the symptoms are often similar. And as the level of exposure increases, the potential for harm increases. Human cells die at 107 degrees Fahrenheit. This is the reason that doctors get concerned if anyone’s temperature goes above 105 degrees. The body is constantly replacing cells so the amount of damage that is done depends on how many cells are killed and what kind of cells are killed. Kill off some cells and the effects may pass in minutes or hours. Destroy a lot of liver cells for instance and you will have liver damage. If the damage is not too severe, the body will be able to repair itself. However, if the damage is extensive, the effects may be permanent! |
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