Electromagnetic fields (EMF) are a combination of invisible
magnetic fields of force. They occur both
naturally and due to human activity.
Naturally occurring EMF are for example, the earth static
magnetic field to which we are constantly exposed, electric fields caused by
electrical charges in the clouds or by the static electricity produced when two
objects are rubbed together as well as sudden electric and magnetic fields
caused by lightning, etc.
Man-made electromagnetic fields (EMF) are for example
generated by extremely low frequency (ELF) sources, such as power-lines, wiring
and appliances as well as by higher frequency sources such as radio and
television waves and, more recently, cellular telephones and their antennas.