Exposure to time-varying magnetic fields, ranging from power frequencies to gigahertz frequencies, can be hazardous. A great deal of research has gone into identifying these effects and developing safe exposure limits.
Quantities of Electromagnetic Fields
Before we get into exposure limits, let’s take a quick look at how electromagnetic fields are measured. The presence of charge causes electric fields. When electrical current flows, magnetic fields are created (the movement of charge).
The strength of any electric field is a vector quantity that is measured in volts per metre [V.m-1].
The magnetic field can be measured in terms of flux density, B in Tesla [T], or field strength, H in amperes per metre [A.m-1]. Magnetic fields are a vector quantity as well. The following equation expresses the relationship between flux density and field strength:
Power Density – the power S measured in watts per square meter [W.m2]. The power density analysis varies depending on how close we are to the source of electromagnetic emissions (near and far fields). The analysis of the extent of both of these fields is complex, but the near field can be thought of as extending one wavelength [with = 300/fMHz].
The power density in the propagation direction of the far field is given by:
The electric and magnetic fields are highly varying in the near field, and power density is not an appropriate quantity to use. Other measurements, 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 preferable. Ohm’s law provides a starting point for calculating these:
Limits of Reference
Given the nature of potential harmful effects on the human body, strict reference limits are difficult to define. By publishing reference emission limits, most countries define their own requirements. A comparison of exposure limits for various countries is described in reference . (mainly European, but it does include the USA and Russia).
We can look at the UK guidelines, for example, to get a sense of how reference limits work. There are two types of exposure limits:
Public exposure – for members of the public who will be exposed as a result of visiting a location
Occupational exposure – for people who will be exposed as a result of their job.
The general UK exposure limits for both of these areas are:
The table below, derived from the preceding, provides reference limits for several important discrete frequencies.
R. Stam. International policies on electromagnetic fields: a comparison (power frequency and radio frequency fields). National Institute for Public Health and the Environment, Bilthoven, The Netherlands, 2011.
Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (ICNIRP) (up to 300 GHz). Health Physics 74(4):494-522, ICNIRP, 1998.
Radio Frequency Radiations, Leaflet 35 of the Radiation Safety Handbook. 2010. http://www.gov.uk/