Another type of impact on health which is the result of prolonged exposure (perhaps even years) to electromagnetic field levels even lower than those associated with the short-term effects, must be measured in the long-term. All that is known about the effects of electric and magnetic fields changing in time is in relation to the induction of fields and currents inside the exposed body.
Electric fields exert forces on any electrically charged particles, as for example ions in liquids. Accordingly, all particles covered by an electric field move to achieve an electrostatic balance, for which reason the field is null in the human body. In cases where the electric field is variable in time the electric charges, constantly trying to reach equilibrium, change their position according to the sign of the field, thus creating a fluctuating motion of charges on the surface (electric current induced by the electric field variable) which increases intensity with the increasing frequency with which the inducing field changes.
However, as far as a time-varying magnetic field is concerned, a different mechanism occurs as this generates an electric field in the surrounding space that changes over time. If the variable electric field is produced within the human body, it generates an electric current in accordance with Ohm’s law: J =σE.
While an electric field as a main source generates currents on the surface of the body, a magnetic field causes the movement of currents within the body thus affecting more delicate parts.
The electric field generated by a magnetic field has a variable spatial distribution that can be shown as lines of force which close in on themselves and link up with the lines of the magnetic field strength (see Figure 1).
The induction of electric fields and currents inside the human body causes two biological effects, both potential health issues. The first one relates to the electrical stimulation of muscles and nerves, while the second is the so called Joule thermal effect.
When the effects of these two phenomena occur immediately after exposure to the fields, one can speak of short-term effects. When they occur after a number of years after a prolonged exposure to lower field values we talk about long-term effects.