Radiofrequency (RF) Radiation
(Includes RF from broadcast antennas, portable radio systems, microwave antennas, satellite, and radar)Kelly Classic, Certified Medical Physicist
Electromagnetic radiation consists of waves of electric and magnetic energy moving together (that is, radiating) through space at the speed of light. Taken together, all forms of electromagnetic energy are referred to as the electromagnetic spectrum. Radio waves and microwaves emitted by transmitting antennas are one form of electromagnetic energy. Often the term electromagnetic field or radiofrequency (RF) field may be used to indicate the presence of electromagnetic or RF energy.
An RF field has both an electric and a magnetic component (electric field and magnetic field), and it is often convenient to express the intensity of the RF environment at a given location in terms of units specific for each component. For example, the unit "volts per meter" (V/m) is used to measure the strength of the electric field and the unit "amperes per meter" (A/m) is used to express the strength of the magnetic field.
RF waves can be characterized by a wavelength and a frequency. The wavelength is the distance covered by one complete cycle of the electromagnetic wave, while the frequency is the number of electromagnetic waves passing a given point in one second. The frequency of an RF signal is usually expressed in terms of a unit called the hertz (Hz). One Hz equals one cycle per second. One megahertz (MHz) equals one million cycles per second. Different forms of electromagnetic energy are categorized by their wavelengths and frequencies. The RF part of the electromagnetic spectrum is generally defined as that part of the spectrum where electromagnetic waves have frequencies in the range of about 3 kilohertz (3 kHz) to 300 gigahertz (300 GHz).
Probably the most important use for RF energy is in providing telecommunications services. Radio and television broadcasting, cellular telephones, radio communications for police and fire departments, amateur radio, microwave point-to-point links, and satellite communications are just a few of the many telecommunications applications. Microwave ovens are a good example of a noncommunication use of RF energy. Other important noncommunication uses of RF energy are radar and for industrial heating and sealing. Radar is a valuable tool used in many applications from traffic enforcement to air traffic control and military applications. Industrial heaters and sealers generate RF radiation that rapidly heats the material being processed in the same way that a microwave oven cooks food. These devices have many uses in industry, including molding plastic materials, gluing wood products, sealing items such as shoes and pocketbooks, and processing food products.
The quantity used to measure how much RF energy is actually absorbed in a body is called the specific absorption rate (SAR). It is usually expressed in units of watts per kilogram (W/kg) or milliwatts per gram (mW/g). In the case of whole-body exposure, a standing human adult can absorb RF energy at a maximum rate when the frequency of the RF radiation is in the range of about 80 and 100 MHz, meaning that the whole-body SAR is at a maximum under these conditions (resonance). Because of this resonance phenomenon, RF safety standards are generally most restrictive for these frequencies.
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 to very high levels of RF radiation can be harmful due to the ability of RF energy to rapidly 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 of the body's inability to cope with or dissipate the excessive heat that could be generated. Two areas of 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 significant heating, the evidence for harmful biological effects is ambiguous and unproven. Such effects have sometimes 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 general, however, studies have shown that environmental levels of RF energy routinely encountered by the general public are typically far below levels necessary to produce significant heating and increased body temperature. However, there may be situations, particularly workplace environments near high-powered RF sources, where recommended limits for safe exposure of human beings to RF energy could be exceeded. In such cases, restrictive measures or actions may be necessary to ensure the safe use of RF energy.
Some studies have also examined the possibility of a link between RF and microwave exposure and cancer. Results to date have 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 is 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 literature concerning biological effects of electromagnetic fields, identify gaps in knowledge about such effects, recommend research needs, and work towards international resolution of health concerns over the use of RF technology. The WHO maintains a Web site 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 standards recommend safe levels of exposure for both the general public and for workers. In the United States, the Federal Communications Commission (FCC) has adopted and used recognized safety guidelines for evaluating RF environmental exposure since 1985. Federal health and safety agencies-such as the Environmental Protection Agency (EPA), the Food and Drug Administration (FDA), the National Institute for Occupational Safety and Health (NIOSH), and the Occupational Safety and Health Administration (OSHA)-have also been involved in monitoring and investigating issues related to RF exposure.
The FCC guidelines for human 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 and the IEEE standard after extensive 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 guidelines developed by the International Commission on Non-Ionizing 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 this threshold level. The threshold level is a SAR value for the whole body of 4 watts per kilogram (4 W/kg). The most restrictive limits on whole-body exposure are in the frequency range of 30-300 MHz where the RF energy is absorbed most efficiently when the whole body is exposed. For devices that only expose part of the body, such as mobile phones, different exposure limits are specified.
Major RF transmitting facilities under the jurisdiction of the FCC-such as radio and television broadcast stations, 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 submitted to the FCC for construction or modification of a transmitting facility or renewal of a license. Failure to comply with the FCC's RF exposure guidelines could lead to the preparation of a formal Environmental Assessment, possible Environmental Impact Statement, and eventual rejection of an 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 some UHF television stations. Frequencies for FM 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 to millions of watts for certain television stations. Some of these signals can be a significant source of RF energy in the local environment, and the FCC requires that broadcast stations submit evidence of compliance with FCC RF guidelines.
The amount of RF energy to which the public or workers might be exposed as a result of broadcast antennas depends on several factors, including the type of station, design characteristics of the antenna being used, power transmitted to the antenna, height of the antenna and distance from the antenna. Since energy at 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 is important.
Public access to broadcasting antennas is normally restricted so individuals cannot be exposed to high-level fields that might exist near antennas. Measurements made by the FCC, EPA, and others have shown that ambient RF radiation levels in inhabited areas near broadcasting facilities are typically well below the exposure levels 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 OSHA have reported that in these cases it is possible for a worker to be exposed to high levels of RF energy if work is performed on an active tower or in areas immediately surrounding a radiating antenna. Therefore, precautions must be taken to ensure that maintenance personnel are not exposed to unsafe RF fields.
Portable Radio Systems
"Land-mobile" communications include a variety of communications systems that require the use of portable and mobile RF transmitting sources. These systems operate in narrow frequency bands between about 30 and 1,000 MHz. Radio systems used by the police and fire departments, radio paging services, and business 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 transmitters. The antennas used for these various transmitters are adapted for their specific purpose. For example, a base-station antenna must radiate its signal to a relatively large area, and, therefore, its 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 are 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 these reasons, such base-station antennas have generally not been of concern with regard to possible hazardous exposure of the public 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 used for handheld units.
Handheld portable radios such as walkie-talkies are low-powered devices used to transmit and receive messages over relatively short distances. Because 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 safe 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 more). These antennas are usually rectangular or circular in shape and are normally found mounted on a supporting tower, on rooftops, on sides of buildings, or on similar structures that provide clear and unobstructed line-of-sight paths between both ends of a transmission path or link. These antennas have a variety of uses, such as transmitting voice and data messages and serving as links between broadcast or cable TV studios and transmitting antennas. The RF signals from these antennas travel in a directed beam from a transmitting antenna to a receiving antenna, and dispersion of microwave energy outside of the relatively narrow beam is minimal or insignificant. In addition, these antennas transmit using very low power levels, usually on the order of a few watts or less. Measurements have shown that ground-level power densities due to microwave directional antennas are normally a thousand times or more below recommended safety limits. Moreover, as an added margin of safety, microwave tower sites are normally inaccessible to the general public. Significant exposures from these antennas could only occur in the unlikely event that an individual was to stand directly in front of and very close to an antenna for a period of time.
Satellite Systems
Ground-based antennas used for satellite-earth communications typically are parabolic "dish" antennas, some as large as 10 to 30 meters in diameter, that are used to transmit (uplinks) or receive (downlinks) microwave signals to or from satellites in orbit around the earth. The satellites receive the signals beamed up to them and, in turn, retransmit the signals back down to an earthbound receiving station. These signals allow delivery of a variety of communications services, including long-distance telephone service. Some satellite-earth station antennas are used only to receive RF signals (that is, just like a rooftop television antenna used at a residence) and, since they do not transmit, RF exposure is not an issue. Because of the longer distances involved, power levels used to transmit these signals are relatively large when compared, for example, to those used by the microwave point-to-point antennas discussed above. However, as with microwave antennas, the beams used for transmitting earth-to-satellite signals are concentrated and highly directional, similar to the beam from a flashlight. In addition, public access would normally be restricted at station sites where exposure levels could approach or exceed safe limits.
Radar Systems
Radar systems detect the presence, direction, or range of aircraft, ships, or other moving objects. This is achieved by sending pulses of high-frequency electromagnetic fields (EMF). Radar systems usually operate at radiofrequencies between 300 megahertz (MHz) and 15 gigahertz (GHz). Invented some 60 years ago, radar systems have been widely used for navigation, aviation, national defense, and weather forecasting. People who live or routinely work around radar have expressed concerns about long-term adverse effects of these systems on health, including cancer, reproductive malfunction, cataracts, and adverse effects for children. It is important to distinguish between perceived and real dangers that radar poses and to understand the rationale behind existing international standards and protective measures used today.
The power that radar systems emit varies from a few milliwatts (police traffic-control radar) to many kilowatts (large space tracking radars). However, a number of factors significantly reduce human exposure to RF generated by radar systems, often by a factor of at least 100:
- Radar systems send electromagnetic waves in pulses and not continuously. This makes the average power emitted much lower than the peak pulse power.
- Radars are directional and the RF energy they generate is contained in beams that are very narrow and resemble the beam of a spotlight. RF levels away from the main beam fall off rapidly. In most cases, these levels are thousands of times lower than in the main beam.
- Many radars have antennas which are continuously rotating or varying their elevation by a nodding motion, thus constantly changing the direction of the beam.
- Areas where dangerous human exposure may occur are normally inaccessible to unauthorized personnel.
In addition to the information provided in this document, there are other sources of information regarding RF energy and health effects. Some states maintain nonionizing radiation programs or, at least, some expertise in this field, usually in a department of public health or environmental control. The following table lists some representative Internet Web sites that provide information on this topic. The Health Physics Society neither endorses nor verifies the accuracy of any information provided at these sites. They are being provided for information only.
por Edgar Servita 18.856.338
No hay comentarios:
Publicar un comentario