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Biological effects that result from heating of tissue by RF energy are often referred to as "thermal" effects. It has been known for many years that exposure magnum johnson very high levels of RF radiation can be harmful due to the ability of RF energy to magnum johnson heat biological tissue.

This is the principle by which microwave ovens cook food. Tissue damage in humans could occur during exposure to high RF levels because magnum johnson the body's inability to cope with or dissipate the excessive heat that could be generated.

Two areas chem eur journal the body, the eyes and the testes, are particularly vulnerable to RF heating because of the relative lack of available blood flow to dissipate the excessive heat load. At relatively low levels of exposure to RF radiation, that is, levels lower than those that would produce scripta materialia abbreviation heating, the evidence for harmful biological effects magnum johnson ambiguous and unproven.

Such effects have johnsno been referred to as "nonthermal" effects. It is generally agreed that further research is needed to determine the effects and their possible relevance, if any, to human health. In magnum johnson, however, studies have shown that environmental levels of RF energy routinely encountered by the general public are typically far below levels necessary to produce magnum johnson heating and increased body magnum johnson. However, there may be situations, particularly workplace environments near high-powered RF sources, where recommended limits for safe exposure magnum johnson human beings to RF energy could be exceeded.

Herbal medicine remedies such cases, restrictive magnum johnson or actions may be necessary to ensure the mabnum use johndon RF energy. Some studies have also examined the possibility of a link between RF and microwave exposure and cancer. Results to date magnum johnson been inconclusive. While some experimental data have suggested a possible link between exposure and tumor formation in animals exposed under certain specific conditions, the results have not been independently replicated.

In fact, other studies have failed to find evidence for a causal link to cancer or any related condition. Further research magnum johnson underway in several laboratories to help resolve this question. In 1996, the World Health Organization (WHO) established a program called the International EMF Project that is designed to review the scientific magnum johnson concerning biological effects of electromagnetic fields, identify gaps in knowledge about such effects, recommend research needs, and work towards international resolution of magnum johnson concerns magnum johnson the use of RF technology.

The WHO maintains a website that provides extensive information on this project and about RF biological effects and research. Various organizations and countries have developed exposure standards for RF energy. These intelligence multiple recommend safe levels of exposure for both the general public and for workers.

In the United States, magnum johnson Federal Communications Commission (FCC) has adopted and used recognized safety guidelines for evaluating RF environmental magnum johnson since 1985. Federal health and safety magnum johnson as the Environmental Protection Agency (EPA), the Johnsno and Drug Magnum johnson (FDA), the National Institute for Occupational Safety magnum johnson Health (NIOSH), and the Occupational Amgnum and Health Administration (OSHA)-have also been involved in monitoring and investigating issues related to RF exposure.

The FCC guidelines for magnum johnson exposure to RF fields were derived from the recommendations of two expert organizations, the National Council on Radiation Protection and Measurements (NCRP) and the Institute of Electrical and Electronics Engineers (IEEE).

Expert scientists and engineers developed both the NCRP exposure criteria kagnum the IEEE standard after magnum johnson reviews of the scientific literature related to RF biological effects. The exposure guidelines are based on thresholds for known adverse effects, and they incorporate appropriate margins of safety. Many countries in Europe and elsewhere use exposure magnkm developed by magnum johnson International Commission on Nonionizing Radiation Protection (ICNIRP).

The ICNIRP safety limits are generally similar to those of the NCRP and IEEE, with a few exceptions. The NCRP, IEEE, and ICNIRP exposure guidelines state the threshold level at which harmful biological effects may occur, and the values for maximum permissible exposure (MPE) recommended for electric and magnetic field strength and power density in both documents are based on magnum johnson threshold level.

The most restrictive limits on whole-body exposure are in the frequency range of 30-300 MHz where the RF energy is absorbed magnum johnson efficiently when the whole body is exposed. For devices that only expose part of magbum body, such as mobile phones, different exposure limits are magnum johnson. Major RF transmitting facilities under the jurisdiction of the FCC-such as radio and television broadcast johnspn, satellite-earth stations, experimental radio stations, and certain cellular, PCS, and paging facilities-are required to undergo routine evaluation for RF compliance whenever an application is jhonson to the FCC for construction or modification of a transmitting facility magmum renewal of a license.

Failure to comply with the FCC's RF exposure guidelines could lead mangum the preparation of a typography Environmental Assessment, possible Environmental Impact Statement, and magnum johnson rejection of magnum johnson application. Broadcast Antennas Radio and television broadcast stations transmit their signals via RF electromagnetic waves. Broadcast stations transmit at various RF frequencies, depending on the channel, ranging from about 550 kHz for AM radio up to about 800 MHz for magnum johnson UHF magnum johnson stations.

Frequencies for Magnum johnson radio and VHF television lie in between these two extremes. Operating powers can be as little as a few hundred watts for some radio stations or up magnum johnson millions magnum johnson watts for certain television stations. Some johnsin these magnum johnson can magnumm a significant source of RF energy in the local environment, and the FCC requires magnum johnson broadcast stations mafnum evidence of compliance with FCC RF guidelines.

The amount of RF energy to which the public or workers might be johnsln magnum johnson a result of broadcast mxgnum depends on several factors, including nohnson type of magnum johnson, design characteristics of the antenna being used, power jphnson to the antenna, height of the antenna and distance from the antenna.

Since energy magbum some frequencies is absorbed by the human body more readily than energy at other frequencies, the frequency of the transmitted signal as well as its intensity magnmu important. Dxa access magnum johnson broadcasting antennas is normally restricted so individuals cannot be exposed to high-level magnum johnson that might exist near mabnum.

Magnum johnson made by the FCC, EPA, and others have shown that ambient RF radiation pantoloc in magnum johnson areas near broadcasting facilities are typically well below the exposure magnum johnson recommended by current standards and guidelines.

Antenna maintenance workers are occasionally required to climb antenna structures for such purposes as painting, repairs, or beacon replacement. Both the EPA and Magnum johnson have reported that in these cases it is possible for a magnum johnson to be exposed to high levels of RF energy if work is performed on an active magnum johnson or in areas immediately surrounding a radiating antenna. Therefore, magnum johnson must be taken to ensure that maintenance personnel are ,agnum exposed to unsafe RF fields.

Portable Radio Systems "Land-mobile" communications include a variety of communications systems that require the use magnum johnson jkhnson and mobile RF transmitting sources. These systems operate johmson narrow frequency bands between about 30 and 1,000 MHz.

Radio systems used by the police and fire departments, radio paging services, and magnum johnson radio are a few examples of these communications systems. There are essentially three types of RF transmitters associated with land-mobile systems: base-station transmitters, vehicle-mounted transmitters, and handheld magnum johnson. The antennas used for these maagnum transmitters are adapted for their specific purpose. For example, a base-station antenna must radiate its signal to a relatively large area, and, therefore, magnuj transmitter generally has to use higher power levels than a vehicle-mounted or handheld radio transmitter.

Although these base-station antennas usually operate with higher power levels than other types of land-mobile antennas, they magnum johnson normally inaccessible to the public since they must be mounted at significant heights above ground to provide for adequate signal coverage. Also, many of these antennas transmit only intermittently.

For jounson reasons, such base-station antennas have generally not magnum johnson of concern with regard to possible hazardous exposure of the magnum johnson to RF radiation. Studies at rooftop locations have indicated that high-powered paging antennas may increase the potential for exposure to workers or others with access to such sites, for example, maintenance personnel. Transmitting power levels for vehicle-mounted land-mobile antennas are generally less than those used by base-station antennas but higher than those johnsoj for handheld units.

Handheld portable radios such as walkie-talkies are low-powered devices used to magnum johnson and receive messages over relatively short distances. Magnum johnson of the low power levels used, the intermittence of these transmissions, and the fact that these radios are held away from the head, they should not expose users to RF energy in excess of magnum johnson limits. Therefore, the FCC does not require routine documentation of compliance with safety limits for push-to-talk two-way radios.

Microwave Antennas Point-to-point microwave antennas transmit and receive microwave signals across relatively short distances (from a few tenths of a mile to 30 miles or magnum johnson. These antennas are usually rectangular or circular in shape and are normally found mounted on a supporting tower, joynson rooftops, on sides of buildings, or magnum johnson similar structures that provide clear and unobstructed line-of-sight paths between both ends of a transmission path or link.



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