Induced currents

The quantum energy of 50 Hz electromagnetic fields is too small to break chemical bonds. It is clear that power-frequency radiation does not cause ionisation in the same way that x-rays or alpha particles do. Instead, the main known way 50 Hz fields interact with people is by inducing currents.

What currents do magnetic fields produce?
What currents do electric fields produce?
What effects do induced currents have on the body?
What is a safe level of induced current?
Numerical calculations of induced currents

What currents do magnetic fields produce?
Any alternating magnetic field will induce an electric field, which in turn produces a current in a conducting medium. The human body is conducting and will therefore have a current induced in it – albeit, usually, a very small one. See figure 1 for the pattern of currents induced.

In power-frequency calculations, it is common to assume the human body has a radius of 0.2 m and a conductivity of 0.2 S m^-1. Using this model, a magnetic field of 160 microteslas (µT) induces a peripheral current density of 1 mA m^-2. More accurate numerical calculations can be done which take account of the actual shape of the body and the varying conductivities of different tissues. See figure 2 for an example of the results of such calculations.

What currents do electric fields produce?
Alternating electric fields also induce currents in the body. The calculation has to take account of the perturbation to the field caused by the body itself. For a typical person standing in a vertical field, a current of 1 mA through the body is induced by 70 kV m^-1 more on numerical calculations. See figure 3 for the pattern of currents induced by electric fields.

What effects do induced currents have on the body?
Within the body, currents induced by fields have the same range of effects as currents injected via electrodes, eg in an electric shock. But these effects depend entirely on the size of the current. Thus current densities of about 0.1 A m^-2 can stimulate excitable tissue and current densities above about 1 A m^-2 can cause ventricular fibrillation, as well as producing heating. However these current densities correspond to fields far larger than are ever encountered at 50 Hz.

At lower fields a range of possible effects have been reported. The established effect observed in humans at the lowest magnetic field is the magnetophosphene effect, where a flickering sensation is produced in peripheral vision by 50 Hz magnetic fields above about 10 mT (ie 10,000 µT). Magnetophosphenes are probably caused by induced current densities in the retina; the threshold at 20 Hz (the most sensitive frequency) is about 20 mA m^-2.

What is a safe level of induced current?
Taking account of all these effects, the NRPB exposure guidelines currently recommend that people should not be exposed to current densities in the head, neck and trunk of greater than 10 mA m^-2 (the "basic restriction").

Numerical calculations of induced currents
Detailed numerical calculations of induced currents in the body are performed by just a few groups round the world. We give more details of the latest results from such calculations here. In summary:

10 mA m is induced by 48 kV m^-1 or by 1800 µT
2 mA m is induced by 9.6 kV m^-1 or by 360 µT
(all figures for the central nervous system averaged over 1 cm2)

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