Induced currents and fields

The quantum energy of 50 Hz electromagnetic fields is too small to break chemical bonds. It is clear that power-frequency EMFs or 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.

Microshocks are a related but different phenomenon.

What currents do magnetic fields produce?

 induced current BAny 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. As shown on the right the current circulates round the body.

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.

What currents do electric fields produce?induced current E

Alternating electric fields also induce currents in the body. As shown on the right, for a vertical field, they run up and down 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.

 

Effects of induced currents on the body

Within the body, currents induced by fields have the same range of effects as currents injected via electrodes, e.g. 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 (i.e. 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.

Microshocks are a related but separate phenomenon, caused not by a continuous current but by a one-off discharge.

What is a safe level of induced current?

Exposure guidelines are usually designed to prevent all effects of induced currents, on the basis that any effect in the brain or nervous system is potentially harmful.  For example, the ICNIRP exposure guidelines currently recommend that people at work should not be exposed to current densities in the head, neck and trunk of greater than 10 mA m-2 (the "basic restriction") with a lower limit of 2 mA m-2 for the general population which may include people who are more sensitive because of medical conditions.

See more on how induced currents are calculated