Electromagnetic Fields - Exposure Limits 

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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

Image(52)
Differences between near and far field diffraction
Image Source: http://en.wikipedia.org/wiki/Near_and_far_field

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:

Image(53)
- 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:

Image(54)
- 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:
Image(55)

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:

Image(56)
- 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

  1. 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.
  2. 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.
  3. Radiation Safety Handbook, Leaflet 35, Radio Frequency Radiations. https://www.gov.uk; 2010.


Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

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