What are electric and magnetic fields?
Electric and magnetic fields are invisible areas of energy that are produced by electricity, which is the movement of electrons, or current, through a wire.
An electric field is produced by voltage, which is the pressure used to push the electrons through the wire, much like water being pushed through a pipe. As the voltage increases, the electric field increases in strength.
A magnetic field results from the flow of current through wires or electrical devices and increases in strength as the current increases. The strength of a magnetic field decreases rapidly with increased distance from its source.
Electric fields are produced whether or not a device is turned on, but magnetic fields are produced only when current is flowing, which usually requires a device to be turned on. Power lines produce magnetic fields continuously because current is always flowing through them.
Electric and magnetic fields together are referred to as electromagnetic fields, or EMFs. There are both natural and human-made sources of EMFs. The earth’s magnetic field, which causes a compass to point North, is an example of a naturally occurring EMF. Power lines, wiring, and electrical appliances, such as electric shavers, hair dryers, computers, televisions, and electric blankets produce what are called extremely low frequency (ELF) EMFs. ELF-EMFs have frequencies of up to 300 cycles per second, or Hertz (Hz); for example, the frequency of alternating current in power lines is 50 or 60 Hz. Cell phones produce radiofrequency EMFs above the ELF range. For more information about cell phones, see the fact sheet Cell Phones and Cancer Risk.
Electric fields are easily shielded or weakened by walls and other objects, whereas magnetic fields can pass through buildings, living things, and most other materials. Consequently, magnetic fields are the component of ELF-EMFs that are usually studied in relation to their possible health effects.
Why are ELF-EMFs studied in relation to cancer?
Any possible health effects of ELF-EMFs would be of concern because power lines and electrical appliances are present everywhere in modern life, and people are constantly encountering these fields, both in their homes and in certain workplaces. Also, the presence of ELF-EMFs in homes means that children are exposed. Even if ELF-EMFs were to increase an individual’s risk of disease only slightly, widespread exposure to ELF-EMFs could translate to meaningful increased risks at the population level.
Several early epidemiologic studies raised the possibility of an association between certain cancers, especially childhood cancers, and ELF-EMFs. Most subsequent studies have not shown such an association, but scientists have continued to investigate the possibility that one exists.
No mechanism by which ELF-EMFs could cause cancer has been identified. Unlike high-energy (ionizing) radiation, ELF-EMFs are low energy and non-ionizing and cannot damage DNA or cells directly. Some scientists have speculated that ELF-EMFs could cause cancer through other mechanisms, such as by reducing levels of the hormone melatonin. (There is some evidence that melatonin may suppress the development of certain tumors.) However, studies of animals exposed to ELF-EMFs have not provided any indications that ELF-EMF exposure is associated with cancer (1, 2).
What is the evidence for an association between magnetic field exposure and cancer in children?
Numerous epidemiologic studies and comprehensive reviews of the scientific literature have evaluated possible associations between exposure to ELF magnetic fields and risk of cancer in children (1, 3, 4). Most of the research has focused on leukemia and brain tumors, the two most common cancers in children. Studies have examined associations of these cancers with living near power lines, with magnetic fields in the home, and with exposure of parents to high levels of magnetic fields in the workplace.