Virginia Department of Health
Electric and magnetic fields, often referred to as electromagnetic fields (EMF), occur both naturally and as a result of generation, delivery, and use of electric power. In our society, where the use of electric power is pervasive, exposure to EMF is common from the vast array of electrical appliances and equipment, building wiring, distribution lines, and transmission lines.
EMF are fields of force and are created by electric voltage and current. They occur around electrical devices or whenever power lines are energized. Electric fields are due to voltage so they are present in electrical appliances and cords whenever the electric cord to an appliance is plugged into an outlet (even if the appliance is turned off). The strength of the electric field is typically measured in volts per meter (V/m) or in kilovolts per meter (kV/m). Electric fields are weakened by objects like trees, buildings, and vehicles. Burying power lines can eliminate human exposure to electric fields.
Magnetic fields result from the motion of the electric charge or current, such as when there is current flowing through a power line or when an appliance is plugged in and turned on. Appliances which are plugged in but not turned on do not produce magnetic fields. Magnetic fields are typically measured in tesla (T), or more commonly, in gauss (G) and milligauss (mG). One tesla equals 10,000 gauss and one gauss equals 1,000 milligauss. The strength of EMF decreases significantly with increasing distance from the source (1).
The Earth's natural electric field is essentially static (nonalternating) and is about 130 V/m. The Earth's magnetic field is also static and is about 0.5 G or 500 mG. In the United States, the electric power system uses alternating current (AC) that alternates back and forth (frequency) 60 times each second and is called 60-Hertz (60-Hz; cycles per second) power. In Europe and many other parts of the world, the frequency of electric power is 50 Hz.
There are basically three stages in generating electricity, or power, and moving the electricity from the electric stations to the end user. First, electricity is generated at an electrical generating station at about 20,000 volts or 20 kilovolts (kV). The power is then passed through a transformer which increases the voltage so that the power can be transported with minimum losses. In the second stage, electricity is transported over high-voltage transmission lines ranging from 69 to 765 kV. Transmission lines connect to substations where the voltage is reduced and power is transferred to lower-voltage distribution lines. In the third stage, distribution lines deliver power locally to individual users. The distribution system is composed of two voltage levels. One is a "primary" circuit (2 to 59 kV) that delivers power from a substation to a distribution transformer. From there the power flows through a "secondary" circuit to an end user. The "secondary" circuit voltage is low enough (120 to 240 volts) to operate household electrical appliances, lights, etc. The amount of power that a line transmits is the product of its voltage and current. Power systems are designed to hold voltages relatively constant, while currents increase and decrease depending on the power demand. For a given voltage the electric field remains relatively constant over time, but the magnetic field increases or decreases depending upon power demand (2).
Currently in the United States there are more than 300,000 miles of AC power transmission lines ranging from 115 to 765 kV. In Virginia, the highest voltage on transmission lines is 765 kV.
The EMF from power lines and appliances are of extremely low frequency (ELF) and low energy. They are nonionizing and are markedly different in frequency from ionizing radiations such as X-rays and gamma rays. As a comparison, transmission lines operate at a low frequency of 60 Hz while television transmitters operate at higher frequencies in the 55-890 million Hz (MHz) range. Microwaves operate at even higher frequencies, 1,000 MHz and above. Ionizing radiations such as X-rays and gamma rays have frequencies above 10(15) Hz. The energy from higher-frequency fields is absorbed more readily by biological material. Microwaves can be absorbed by water in body tissues and cause heating which can be harmful, depending upon the degree of heating that occurs. X-rays have so much energy that they can ionize (form charged particles) and break up molecules of genetic material (DNA) and nongenetic material, leading to cell death or mutation. In contrast, extremely low frequency EMF do not have enough energy to heat body tissues or cause ionization (3).
Over the past three decades, both public controversy and scientific uncertainty have surrounded the subject of potential adverse human health effects from exposure to power frequency EMF. The first studies of possible health effects of EMF exposure in an occupational environment were reported from the former Union of Soviet Socialist Republics (USSR) in the mid-1960s. Several subjective complaints, involving the cardiovascular, digestive, and central nervous systems, were reported by electric switchyard workers. Subsequent studies of electric utility linemen in the United States failed to observe the same adverse health effects reported by their counterparts in the former USSR. Since that time, enormous strides have been taken to explore the nature of any association between residential and occupational exposures to EMF and deleterious health effects.
Recently, there has been a growing concern about the possible carcinogenic effects of EMF associated with such exposures. This concern began with the 1979 publication of a report that children who had died from leukemia were two to three times more likely to have lived in homes within about 50-130 feet of high-current distribution lines compared to children who did not have leukemia. Subsequently, additional studies have appeared in the scientific literature that also found an association between residential exposure to EMF and increased risk of childhood cancer. Other epidemiologic studies have also examined increased incidence of leukemia and brain cancer among adults, especially with respect to occupational EMF exposure. In earlier studies there was an implicit assumption that the relevant risk factor was exposure to electric fields. However, virtually all recent epidemiologic studies of cancer have focused on magnetic field exposures as the possible etiologic determinant.