Saturday, 19 February 2011
Extremely Low Frequency Radiation/Power Lines
Everyone is exposed to a complex mix of electromagnetic fields (EMF) of different frequencies that permeate our environment. Exposures to many EMF frequencies are increasing significantly as technology advances unabated and new applications are found. While the enormous benefits of using electricity in everyday life and health care are unquestioned, during the past 20 years the general public has become increasingly concerned about potential adverse health effects of exposure to electric and magnetic fields at extremely low frequencies (ELF). Such exposures arise mainly from the transmission and use of electrical energy at the power frequencies of 50/60 Hz.
Electromagnetic fields consist of electric (E) and magnetic (H) waves traveling together. They travel at the speed of light and are characterized by a frequency and a wavelength. The frequency is simply the number of oscillations in the wave per unit time, measured in units of hertz (1 Hz = 1 cycle per second), and the wavelength is the distance travelled by the wave in one oscillation (or cycle).
ELF fields are defined as those having frequencies up to 300 Hz. At frequencies this low, the wavelengths in air are very long (6,000 km at 50 Hz and 5,000 km at 60 Hz) and, in practical situations, the electric and magnetic fields act independently of one another and are measured separately.
Electric fields arise from electric charges. They govern the motion of other charges situated in them. Their strength is measured in units of volt per meter (V/m) or kilovolt per meter (kV/m). When charges accumulate on an object they create a tendency for like or opposite charges to be repelled or attracted, respectively. The strength of that tendency is characterized by the voltage and is measured in units of volt (V). Any device connected to an electrical outlet, even if the device is not switched on, will have an associated electric field that is proportional to the voltage of the source to which it is connected. Electric fields are strongest close to the device and diminish with distance. Common materials, such as wood and metal, shield against them.
Magnetic fields come from the motion of electric charges, that is, a current. Their strength is measured in units of ampere per metre (A/m) but is usually expressed in terms of the corresponding magnetic induction measured in units of tesla (T) or millitesla (mT). In some countries another unit, called the gauss (G), is commonly used for measuring magnetic induction (10,000 G = 1 T). Any device connected to an electrical outlet, when the device is switched on and a current is flowing, will have an associated magnetic field, the strength of which is directly related to the current drawn from the source. Magnetic fields are strongest close to the device and get lower with distance. Most common materials do not shield them.
Naturally occurring 50/60 Hz electric and magnetic field levels are extremely low, on the order of 0.0001 V/m and 0.00001 microtesla (μT), respectively. Human exposure to ELF fields is primarily associated with the generation, transmission, and use of electrical energy, for example, power lines. Electrical energy from power-generating stations is distributed to communities via high-voltage transmission lines. Transformers are used to lower the voltage for connections to residential distribution lines that deliver the energy to homes. Electric and magnetic fields underneath overhead transmission lines may be as high as 12 kV/m and 30 μT respectively. Around generating stations and substations, electric fields up to 16 kV/m and magnetic fields up to 270 μT may be found.
Electric and magnetic fields in homes depend on many factors, including the distance from local power lines, the number and type of electrical appliances in use in the home, and the configuration and position of household electrical wiring. Electric fields around most household appliances and equipment typically do not exceed 500 V/m and magnetic fields typically do not exceed 150 μT. In both cases, field levels may be substantially greater at small distances but they do decrease rapidly with distance.
In the workplace, electric and magnetic fields exist around electrical equipment and wiring throughout industry. Workers whose job it is to maintain transmission and distribution lines may be exposed to very large electric and magnetic fields. Within generating stations and substations, electric fields in excess of 25 kV/m and magnetic fields in excess of 2 mT may be found. Welders can be subjected to magnetic-field exposures as high as 130 mT. Near induction furnaces and industrial electrolytic cells magnetic fields can be as high as 50 mT. Office workers are exposed to very much smaller fields when using equipment such as photocopying machines and video display terminals.
The only practical way that ELF fields interact with living tissues is by inducing electric fields and currents in them. However, the magnitude of these induced currents from exposure to ELF fields at levels normally found in our environment is less than the currents occurring naturally in the body. Available evidence on the health effects of electric fields suggests that the effects of exposures of up to 20 kV/m are few and not of any health consequences. Electric fields have not been shown to have any effect on reproduction or development in animals at strengths over 100 kV/m.
There is little confirmed experimental evidence that ELF magnetic fields can affect human physiology and behavior at field strengths found in the home or environment. Exposure of volunteers for several hours to ELF fields up to 5 mT had little effect on a number of clinical and physiological tests, including blood changes, ECG, heart rate, blood pressure, and body temperature. Some investigators have reported that ELF field exposure may suppress secretion of melatonin, a hormone connected with our day-night rhythms. It has been suggested that melatonin might be protective against breast cancer so that such suppression might contribute to an increased incidence of breast cancer already initiated by other agents. While there is some evidence for melatonin effects in laboratory animals, volunteer studies have not confirmed such changes in humans.
There is no convincing evidence that exposure to ELF fields causes direct damage to biological molecules, including DNA. It is thus unlikely that they could initiate the process of carcinogenesis. However, studies are still underway to determine if ELF exposure can influence cancer promotion or copromotion. Recent animal studies have not found evidence that ELF field exposure affects cancer incidence. In 1979 Wertheimer and Leeper reported an association between childhood leukemia and certain features of the wiring connecting their homes to the electrical distribution lines. Since then, a large number of studies have been conducted to follow up this important result. Analysis of these papers by the U.S. National Academy of Sciences in 1996 suggested that residence near power lines was associated with an elevated risk of childhood leukemia (relative risk RR=1.5), but not with other cancers. A similar association between cancer and residential exposure of adults was not seen from these studies.
Many studies published during the last decade on occupational exposure to ELF fields have exhibited a number of inconsistencies. They suggest there may be a small elevation in the risk of leukemia among electrical workers. However, confounding factors, such as possible exposures to chemicals in the work environment, have not been adequately taken into account in many of them. Assessment of ELF field exposure has not correlated well with the cancer risk among exposed subjects. Therefore, a cause-and-effect link between ELF field exposure and cancer has not been confirmed.
The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has published guidelines on exposure limits for all EMF. The guidelines provide adequate protection against known health effects and those that can occur when touching charged objects in an external electric field. Since current scientific information is only weakly suggestive and does not establish that exposure to ELF fields at levels normally encountered in our living environment might cause adverse health effects, there is no need for any specific protective measures for members of the general public. Where there are sources of high ELF field exposure, access by the public will generally be restricted by fences or barriers so that no additional protective measures will be needed.
Protection from 50/60 Hz electric-field exposure can be relatively easily achieved using shielding materials. This is only necessary for workers in very high field areas. More commonly, where electric fields are very large, access of personnel is restricted. There is no practical, economical way to shield against ELF magnetic fields. Where magnetic fields are very strong, the only practical protective method available is to limit exposure of personnel.
Strong ELF fields cause electromagnetic interference (EMI) in cardiac pacemakers or other implanted electromedical devices. Individuals using these devices should contact their doctor to determine their susceptibility to these effects. Office workers may see image movement on the screen of their computer terminal. ELF magnetic fields around the terminal greater than about 1 μT (10 mG) can cause interference with the image on the screen. A simple solution to this problem is to relocate the computer to another part of the room where the magnetic fields are below 1 μT. These magnetic fields are found near cables that provide electric power to office or apartment buildings or around transformers associated with power supplies to buildings. The fields from these sources are generally well below the levels that cause any health concern.