Electromagnetic Fields - Exposure Limits
Exposure to time varying magnetic fields, from power frequencies to the gigahertz range can have harmful consequences. A lot of research has been conducted into identifying these effects and developing recommended safe exposure limits.
Electromagnetic Field Quantities
Before discussing exposure limits, we can do a quick review of how electromagnetic fields are measured. Electric fields are a result of the presence of charge. Magnetic fields are created when electrical current flows (the movement of charge).
Electrical Field - the strength of any electric field is a vector quantity and is measured in volts per metre [V.m-1].
Magnetic Field - can either be measured in terms of flux density, B in Tesla [T] or the field strength H in ampere per metre [A.m-1]. Magnetic fields are also a vector quantity. The relationship between flux density and field strength is given by:
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- µ is the magnetic permeability (for non-magnetic materials = 4π x 10-7)
Power Density - the power S in watts per unit area [W.m2]. Analysis of power density varies depending how close we are to the source of the source of electromagnetic emissions (near and far fields). Analysis of the extent of both these fields is complex, but as a rough order of magnitude the near field can be thought of as extending one wavelength λ [with λ = 300/fMHz]
For the far field, the power density in the direction of propagation is given by:
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- the ratio of E/H is also the characteristic impedance and in free space is a constant equal to 337 Ω.
For free space the power density can be expressed as:
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For the near field, the electric and magnetic fields are highly varying, and power density is not an appropriate quantity to use. Other measures such as current density J [A.m-1], current I [A] or energy absorption SA or SAR [J.kg-1 or W.kg-1] are more appropriate. A starting point for the calculation of these is Ohm's law:
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- σ is the electrical conductance [S.m-1]
Reference Limits
Given the nature of potential detrimental effects on the human body, reference limits are hard to define in a strict sense. Most countries define their own requirements in by publishing reference emission limits. Reference [1] describes a comparison of exposure limits for various countries (mainly European, but it does include the USA and Russia).
For example and to give some idea of how reference limits work, we can look at the UK guidelines. There are two separate sets of exposure limits:
- Public exposure - for members of the public who will be exposed by visiting an area
- Occupational exposure - for people who will be exposed as part of their job requirements
For both of these areas, the general UK exposure limits are[3]:
Reference limits for public exposure:
| Frequency | Electric field strength [V.m-1] | Magnetic field strength [A.m-1] | Magnetic flux density [µT] | Equivalent plain wave power density [W.m2] |
| Up to 1 Hz | - | 32,000 | 40,000 | - |
| 1 - 8 Hz | 10,000 | 32,000/f2 | 40,000/f2 | - |
| 8 - 25 Hz | 10,000 | 4,000/f | 5000/f | - |
| 0.025 - 0.8 kHz | 250/f | 4/f | 5/f | - |
| 0.8 - 3 kHz | 250/f | 5 | 6.25 | - |
| 3 - 150 kHz | 87 | 5 |
6.25 | - |
| 0.15 - 1 MHz | 87 | 0.73/f | 0.92/f | - |
| 1 - 10 MHz | 87/f½ | 0.73/f | 0.92/f | - |
| 10 - 400 MHz | 28 | 0.073 | 0.092 | 2 |
| 400 - 2000 MHz | 1.375f½ | 0.0037f½ | 0.0046f½ | f/200 |
| 2 - 300 GHz | 61 | 0.16 | 0.2 | 10 |
Reference limits for occupational exposure:
| Frequency | Electric field strength [V.m-1] | Magnetic field strength [A.m-1] | Magnetic flux density [µT] | Equivalent plain wave power density [W.m2] |
| Up to 1 Hz | - | 163,000 | 200,000 | - |
| 1 - 8 Hz | 20,000 | 163,000/f2 | 200,000/f2 | - |
| 8 - 25 Hz | 20,000 | 20,000/f | 25,000/f | - |
| 0.025 - 0.82 kHz | 500/f | 20/f | 25/f | - |
| 0.82 - 65 kHz | 610 | 24.4 | 30.7 | - |
| 0.065 - 1 MHz | 610 | 1.6/f | 2.0/f | - |
| 1 - 10 MHz | 610/f | 1.6/f | 2.0/f | 10 |
| 10 - 400 MHz | 61 | 0.16 | 0.2 | 10 |
| 400 - 2000 MHz | 3f½ | 0.008f½ | 0.01f½ | f/40 |
| 2 - 300 GHz | 137 | 0.36 | 0.45 | 50 |
Note: f = frequency indicated (ignore multiplier). For example the allowable occupational magnetic flux density at 900 MHz is 0.01 x 900½ = 0.3 µT
The following table derived from the above, gives reference limits for several important discrete frequencies.
| | | Public | Occupational |
| 50 Hz (ELF) | Electric field strength [V.m-1] | - | 10,000 |
| 50 Hz (ELF) | Magnetic flux density [µT] | - | 500 |
| 900 MHz (GSM) | Electric field strength [V.m-1] | 41 | 90 |
| 900 MHz (GSM) | Magnetic flux density [µT] | 0.14 | 0.30 |
| 900 MHz (GSM) | Equivalent plain wave power density [W.m2] | 4.5 | 22.5 |
| 1800 MHz (GSM) | Electric field strength [V.m-1] | 58 | 127 |
| 1800 MHz (GSM) | Magnetic flux density [µT] | 0.20 | 0.42 |
| 1800 MHz (GSM) | Equivalent plain wave power density [W.m2] | 9 | 45 |
| 2100 MHz (UMTS) | Electric field strength [V.m-1] | 61 | 137 |
| 2100 MHz (UMTS) | Magnetic flux density [µT] | 0.20 | 0.45 |
| 2100 MHz (UMTS) | Equivalent plain wave power density [W.m2] | 10 | 50 |
References
- Stam R. Comparison of international policies on electromagnetic fields (power frequency and radio frequency fields). Bilthoven, The Netherlands: National Institute for Public Health and the Environment,; 2011.
- ICNIRP Guidelines - for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300 GHz). ICNIRP, Health Physics 74(4):494-522; 1998.
- Radiation Safety Handbook, Leaflet 35, Radio Frequency Radiations. https://www.gov.uk; 2010.