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What are Electric and Magnetic Fields? Where Do They Come From?
Electric and magnetic fields (EMF) are invisible lines of force that are part of the natural and man-made environment. A natural source is the earth's magnetic field. Manmade sources include household or building wiring, electrical appliances and electric power transmission and distribution facilities. EMF strength decreases rapidly with distance from the source.
Electric fields are created around appliances and wires wherever a voltage exists, similar to the water pressure in a hose. Electric field strength is measured in units of volts per meter (V/m).
Health-related research around EMF focuses primarily on magnetic field exposures. Magnetic fields are created whenever electrical current flows, similar to the way water flows when the nozzle of a hose is opened. Magnetic field strength is measured in units of gauss (G) or more commonly in milligauss (mG)
What Are Typical Magnetic Field Exposures for People in the United States?
An individual’s magnetic field exposure varies throughout the day and depends on multiple factors including field levels in the home, vehicle, and work place as well as appliance use. Researchers from the Electric Power Research Institute (EPRI) surveyed 992 American homes in 1993 and found an all-room average magnetic field of 0.9 mG, though background field levels in some rooms exceeded 16 mG.
A follow-up EPRI study in 1998 characterized the 24-hour, personal magnetic field exposures of over 1000 people. An average 24-hour personal exposure of 1.25 mG was reported with a small number of people having average exposures over 10 mG.
Are Electric and Magnetic Fields a Health Hazard?
Significant research has been conducted internationally over the last 40+ years to evaluate the potential health impacts of EMF exposure. There is no definitive answer as to whether EMF exposures cause adverse health effects.
After reviewing more than two decades of research in this area, scientists from the National Institute of Environmental Health Sciences (NIEHS) concluded the evidence supporting EMF health risks was weak but still sufficient to warrant limited concern due to a possible weak association between increasing exposure to EMFs and an increased risk of childhood leukemia. In general, they concluded studies conducted on adult exposures do not support a link between residential EMF exposure and adult cancers.
A 2007 World Health Organization (WHO) report concluded that evidence for a link between magnetic fields and childhood leukemia is not strong enough to be considered causal but sufficiently strong to remain a concern. For all other diseases, the WHO classified the evidence of health effects at low exposure levels as inadequate.
The report emphasized that, given the weakness of the evidence for health effects, the health benefits of exposure reduction are unclear and policies based on the adoption of arbitrary low exposure limits are not warranted.
What Regulations Govern EMF?
In the absence of clearly established health risks, federal health authorities recommend that cost-effective measures combined with public education are appropriate to address public EMF exposure concerns. In its 1999 Report to Congress, the NIEHS stated: "the conclusion of this report is insufficient to warrant aggressive regulatory concern."
In 1993, the California Public Utilities Commission (CPUC) required investor-owned utilities to inform customers and employees about potential EMF health effects, provide free EMF measurements, and use EMF-lowering designs on new and rebuilt power lines and substations.
In 2006, the CPUC reviewed and updated its EMF policy. This decision reaffirmed that state and federal public health regulatory agencies have not established a direct link between exposure to EMF and human health effects. The policy directed that (1) use of numeric exposure limits was not appropriate in setting utility design guidelines to address EMF, and (2) existing “no-cost and low-cost” precautionary-based EMF policy should be continued for proposed electrical facilities. SCE stays in compliance with all applicable provisions of this CPUC decision.
What Magnetic Levels Are Found Near Power Lines?
Power transmission lines bring power from a generating station to an electrical substation. Power distribution lines bring power from the substation to customer homes. Transmission and distribution lines can be either overhead or underground.
Power transmission lines
Magnetic fields created by transmission lines vary with power line design, and how much electricity flows through the lines. However, at the edge of rights-of-way near 220 kV and 500 kV transmission lines, magnetic field levels in the 20-30 mG range can often be found. At a distance of 300 feet during average electricity demand, the magnetic fields from many transmission lines can be similar to typical background levels found in most homes.
Power distribution lines
Typical voltages for power distribution lines in SCE’s service territory range from 4 to 33 kV. Similar to transmission lines, the magnetic fields near distribution lines vary with power flow and design. According to the NIEHS, magnetic fields under main feeder distribution lines or over underground lines can create fields around 10 to 20 mG. For smaller distribution lines, field levels are often much less (below 10 mG to under 1 mG). At a distance of 100 feet, the magnetic field levels from distribution lines often drop to values similar to levels found in most homes.
Are There Official EMF Exposure Limits?
While there are no federal or state of California established limits for EMF exposure, some nongovernmental organizations have issued advisory limits. Exposure to magnetic fields from power lines, in addition to other common exposures in homes, schools, and offices, are far below the advisory limits.