Extremely low frequency magnetic (ELFM) fields are produced when an electric current flows from one point to another, such as along a power cord to an electrical appliance, or along a power transmission line. The surrounding magnetic field is present only when there is an electrical current (i.e. the electricity is being transmitted or appliances are on), and its strength varies with power consumption (increased current = increased magnetic field). ELFM fields are measured in units of tesla (T) or gauss (G).
ELFM fields arise from power frequencies in the range of 3 to 3,000 Hertz (Hz), and are mainly associated with man-made sources. North American and European electricity frequencies (60 and 50 Hz, respectively), fall within this ELFM power frequency range.
ELFM fields travel through most substances, but rapidly diminish in strength with increasing distance from the source. They induce circulating currents within the human body; the intensity of which are determined by the strength of the magnetic field outside the body.
There has been some debate as to whether long-term, low level exposure to ELFM can negatively influence health. Extremely low frequency magnetic fields are classified by the International Agency for Research on Cancer (IARC) as Group 2B, possibly carcinogenic to humans, based on limited evidence of an association with childhood leukemia. There is also inadequate evidence in animals for the carcinogenicity of ELFM.
Other health effects potentially associated with ELFM field exposure (but not consistently observed) include small variations in heart rate, perturbation to electrical brain activity during sleep, and melatonin suppression. Exposure assessment is a key shortcoming in developing epidemiologic associations between exposure to ELFM fields and adverse health outcomes.
ELFM field exposures occur in different settings (i.e. at home, school, work, while travelling, and outdoors), and levels may vary greatly between these environments. The ability to assess how these various sources contribute to exposure is limited.
In Canada, there are no national standards for limiting occupational or residential exposure to extremely low frequency fields (< 3000 Hz). Quebec, Ontario, and British Columbia have set voluntary standards for electric fields at high-voltage transmission line corridors (2 kV/m, 3 kV/m, and 5 kV/m, respectively). However, the purpose of these standards is related to electric shock, to ensure that an electric potential induced on large metal objects does not represent an electric shock hazard.
The table below summarizes international, short-term exposure guidelines for the general public and for occupational settings. Notably, the guidelines presented for the general public are much higher than 0.3 to 0.4 microtesla (µT), the lower levels of exposure linked to developing childhood leukemia. Long-term exposure standards for ELFM fields do not currently exist and are the subject of ongoing debate.
Reference Levels for Exposure to Magnetic Fields: General Public and Occupational Settings
ACGIH 2015 TLV
NOTE: All at 60 Hz
ACGIH = American Conference of Governmental Industrial Hygienists
ICNIRP = International Commission on Non-Ionizing Radiation Protection
IEEE = Institute of Electrical and Electronic Engineers
People working indoors may be exposed to ELFM fields generated by electronic devices and equipment such as computer monitors, air purifiers, photocopiers, fax machines, fluorescent lights, electric heaters, and electric tools (e.g. drills, power saws, lathes and welding machines).
Direct work with electrical systems can result in higher exposures. Jobs where this is a possibility include electrical engineers and engineering technicians, electricians, power line workers, power station operators, telephone line workers, TV repairers, and welders.
Workers in the electric power industry can be exposed to average ELFM fields ranging from 0.18 to 1.72 µT (in power stations); 0.08 to 1.4 µT (in substations); 0.03 to 4.75 µT (on lines and cables); and 0.2 to 18.48 µT (for electricians).
In 2002, a Quebec workplace study of women estimated exposure to ELFM fields by combining individual surveys of equipment use with published measurement data on ELFM fields associated with the specified equipment. Average ELFM field levels were estimated to range from 0.03 to 0.68 µT. Occupations with the highest mean exposures included sewing machine operators, electronics workers, kitchen workers, textile machine operators, and store managers. The highest mean exposure for an office worker was estimated to be 0.23 µT.
CAREX Canada has not prioritized ELFM for exposure estimate development. This is because there is a lack of exposure monitoring data in the Canadian Workplace Exposure Database on which to base an estimate.
The general population is exposed to ELFM fields from both indoor and outdoor sources. Individuals usually sustain the highest exposures to ELFM fields in the home rather than at work and outdoors.
The three major sources of ELFM fields in the home are current-carrying plumbing and/or electric circuits, appliances, and nearby power lines. The highest ELFM fields at home are associated with close proximity to domestic appliances incorporating motors, transformers, and heaters. Specific indoor sources include electrical wiring, fluorescent lighting, and electrical appliances and equipment such as refrigerators, stoves, radios, hair dryers, computers, etc. Generally, mean residential levels in the United States range between 0.055 µT and 0.110 µT.
North American transmission lines, which transfer electrical power from generating facilities to transformer stations, operate at voltages between 115 to 500 kilovolts (kV). Distribution lines transfer power from transformer stations to individual residences and operate at voltages between 4 to 24 kV. Although magnetic field does not depend on voltage, higher voltage lines usually carry higher currents and produce higher magnetic fields. At distances of 50 – 300 m from high-voltage power lines, magnetic fields generally fall to background strengths (depending on the power line design, current, and strength of background fields). Currents in power lines vary with the demand for electricity: over the course of a day, seasonally, and from year to year. Overhead power lines produce both electric and magnetic fields, whereas underground lines may produce only magnetic fields above ground. ELFM fields measured near transmission lines in the United States are reported by the National Institute of Environmental Health Sciences (see: Average ELFM Field Levels Measured Near Three Types of US Power Lines).
Our team has performed a detailed scan of exposure control resources and assembled a compilation of key publications and resources. These are organized by type of exposure (environmental or occupational) and by specificity (general or carcinogen-specific). Please visit our Exposures Reduction Resources page to view.
We also recommend exploring the Prevention Policies Directory, a freely-accessible online tool offering information on policies related to cancer and chronic disease prevention. Providing summaries of the policies and direct access to the policy documents, the Directory allows users to search by carcinogen, risk factor, jurisdiction, geographical location, and document type. Click here to learn more about policies specific to ELFM fields on the Directory. For questions about this resource, please contact a member of the Prevention Team at the Canadian Partnership Against Cancer at firstname.lastname@example.org.