Electric fields occur when positive or negative electrical charges are present and apply forces on other charges within the fields. The fields and their strength are measured in volts per meter (V/m). Active electrical wires produce electric fields, as do almost all electric powered devices. The field exists even when there is no current flowing. When the voltage increases, so does electric field surrounding the device. When moving away from the source of the field, the field strength is reduced.

Magnetic fields are created from electric charges. Their strength is measured in amperes per meter (A/m). Electromagnetic Fields (EMFs) also have related measurements which identify the flux density, expressed in microtesla, µT. When current is increased, the strength of the magnetic field increases. Magnetic fields are not commonly blocked with everyday materials.