Magnetic Fields
Full Profile
General Information
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 (↑ current = ↑ magnetic field).[2] The measurement of ELFM fields is expressed in units of tesla (T) or gauss (G).[1]
ELFM fields arise from power frequencies in the range of 3 to 3,000 Hertz, and are mainly associated with man-made sources.[3] North American and European electricity frequencies (60 and 50 Hertz, respectively), fall within this ELFM power frequency range.[2]
ELFM fields travel through most substances, but rapidly diminish in strength with distance from the source.[4] They induce circulating currents within the human body; the intensity of which are determined by the strength of the magnetic field outside the body.[2]
There has been some debate as to whether long-term, low level exposure to ELFM can negatively influence health. Many studies of ELFM field effects on humans have been performed, and while there is inadequate evidence for carcinogenicity at most sites, there is some evidence of a link with childhood leukemia.[3] One pooled analysis of nine studies showed no excess risk of leukemia in children at ELFM field exposures below 0.4 micro Tesla (µT), but a twofold excess risk for exposures above 0.4 µT. Another pooled analysis of 15 studies on childhood leukemia reported a relative risk of 1.7 for ELFM field exposures above 0.3 µT.[3] Extremely low frequency magnetic fields have been classified by IARC Group 2B, possibly carcinogenic to humans, based on the association with childhood leukemia.[3]
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.[1] Exposure assessment has been identified as a key shortcoming with respect to developing epidemiologic associations between exposure to ELFM fields and adverse health outcomes.[5]
Magnetic fields (extremely low frequency) have been ranked by CAREX Canada as Group A (immediate high priority) for environmental settings, particularly for children, and for occupational settings. Prioritization was based on the prevalence of exposure in Canada, and the feasibility of assessing exposure.
Regulations and Guidelines
ELFM field exposures accumulate 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 contributions to exposure from various sources is limited.[3]
In Canada, there are no national standards for limiting occupational or residential exposure to extremely low frequency fields (< 3000 Hz).[4] 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 is to ensure that an electric potential induced on large metal objects does not represent an electric shock hazard.[4]
A summary of international short-term exposure guidelines for the general public and in occupational settings is provided in the following table. Notably, the guidelines presented for the general public are much higher than 0.3 to 0.4 µT (the lower levels of exposure linked to the development of childhood leukemia). Long-term exposure standards for ELFM fields do not currently exist and are the subject of ongoing debate.[7]
Reference Levels for Exposure to Magnetic Fields: General Public and Occupational Settings[4]
| Jurisdiction | Public (µT) | Occupational (µT) |
|---|---|---|
| ACGIH1 | -- | 1,000 |
| ICNIRP2 | 83.33 | 416.67 |
| IEEE3 | 900 | 2,710 |
| Australia4 | -- | 1,000 |
| Bulgaria5 | -- | 1,200 |
| Russia6 | 507 | 100 |
Occupational Exposures
People working indoors may be exposed to ELFM fields generated by computer monitors, air purifiers, photocopiers, fax machines, fluorescent lights, electric heaters and electric tools in machine shops (e.g. drills, power saws, lathes and welding machines).[6]
Direct work with electrical systems can be associated with higher exposures; these job titles include electrical engineers and engineering technicians, electricians, power line workers, power station operators, telephone line workers, TV repairers, and welders.[1]
Average ELFM fields to which workers are exposed in the electric power industry have been found to range 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).[2]
In 2002, a Quebec workplace study of women produced exposure estimates by combining individual surveys of equipment use with published measurement data on ELFM fields associated with the specified equipment.[9] 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.
Environmental Exposures
The general population is exposed to ELFM fields from both indoor and outdoor sources;[6] individuals usually sustain the highest exposures to ELFMs in the home rather than at work and outdoors.[3]
The three major sources of ELFM fields in the home are current-carrying plumbing and/or electric circuits, appliances, and nearby power lines.[3] The total magnetic field consists of the background field with additions of “peaks” surrounding each appliance.[2] The highest ELFM fields at home are associated with close proximity to domestic appliances incorporating motors, transformers and heaters.[3] Specific indoor sources include electrical wiring, fluorescent lighting, and electrical appliances and equipment such as refrigerators, stoves, radios, hair dryers, computers, etc.[2]
North American transmission lines, which transfer electrical power from generating facilities to transformer stations, operate at voltages between 115 to 500 kilovolts (kV); while distribution lines transfer power from transformer stations to individual residences and operate at voltages between 4 to 24 kV.[1] Although magnetic field does not depend on voltage, higher voltage lines usually carry higher currents and usually produce higher magnetic fields.[2] 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).[3] Currents in power lines vary with the demand for electricity: over the course of a day, seasonally and from year to year.[2] Overhead power lines produce both electric and magnetic fields, whereas underground lines may produce only magnetic fields above ground.[1]
ELFM fields measured near transmission lines in the United States are summarized in the following table, as reported by the National Institute of Environmental Health Sciences:
Average ELFM Field Levels Measured Near Three Types of US Power Lines[3]
| Distance from centre point of power line | Mean µT, 1 m above ground | ||
|---|---|---|---|
| 115 kV line | 230 kV line | 500 kV line | |
| 0 m | 2.97 | 5.75 | 8.67 |
| Right-of-way edge (15 m) | 0.65 | 1.95 | 2.94 |
| 30 m | 0.17 | 0.71 | 1.26 |
| 61 m | 0.04 | 0.18 | 0.32 |
| 91 m | 0.02 | 0.08 | 0.14 |
A 1993 ELFM field study conducted in Quebec compared the personal exposures of 18 individuals residing within 76 meters of a 745 kilovolt (kV) transmission line, to the exposures of 17 people living more than 366 metres from a transmission line.[8] Exposure was similar for both groups during time spent away from home in a 24 hour period (0.07 to 0.25 µT). Exposure while at home was higher for people living within 76 m of a transmission line (0.59 to 0.86 µT), relative to those living more than 366 m away (0.08 to 0.29 µT).
Sources
- Wikimedia Commons Photo for Transmission Substation.
- World Health Organization, 2007. Environmental Health Criteria Monograph No. 238.
- IARC monograph summary, Volume 80 (2002)
- The Federal-Provincial-Territorial Radiation Protection Committee – Canada, 2005. Health Effects and Exposure Guidelines Related to Extremely Low Frequency Electrical and Magnetic Fields – An Overview. Prepared by the ELF Working Group, January 2005. (PDF)
- Kheifets, L. 2008. Future needs of occupational epidemiology of extremely low frequency (ELF) electric and magnetic fields (EMF): review and recommendations. Occupational and Environmental Medicine
- Health Canada: Electric and Magnetic Fields at Extremely Low Frequencies
- World Health Organization EMF Project, 2007. Rapporteur’s Report – Workshop on Developing and Implementing Protective Measures for ELF EMF, 20-21 June 2007, Geneva, Switzerland. (PDF)
- Levallois P, et al. 1995. Electric and Magnetic Field Exposures for People Living near a 735-Kilovolt Power Line. Environmental Health Perspectives, Vol. 103, No. 9, pp832-837.
- Deadman P. et al. 2002. Individual Estimation of Exposures to Extremely Low Frequency Magnetic Fields in Jobs Commonly Held by Women. American Journal of Epidemiology, Vol. 144, No. 4, pp 368-378.
Other Resources
- BC Centre for Disease Control website
- National Institute of Environmental Health Sciences – National Institutes of Health, Electric and Magnetic Fields website