EMF Overview
 EMF (Electric and Magnetic Fields or Electromagnetic Fields) can be classified or described in two ways. One is by frequency or frequency band. Examples are ELF, VHF and UHF. Another classification is by type: "Ionizing" and "non-ionizing". Following this overview section, we will tend to use designations according to frequency band and will concentrate on the frequencies substantially below visible light - those frequencies most associated with communications or instrumentation - DC, AC/ELF and RF/Microwave.
Ionizing Radiation
Ionizing radiation is so named because electromagnetic radiation at these high frequencies has the energy to break a chemical bond in a molecule, destabilizing or "ionizing" it. We speak of these bands as different types of "rays". Included in this range are Ultra-violet (above violet) rays, X-rays, Gamma rays and Cosmic rays.
Non-ionizing Radiation
Non-Ionizing radiation are those frequencies at and below visible light. Included are the bands of most interest to us: DC (direct current or static), ELF (including 50/60 Hz or AC fields from commercial power), the VHF and UHF television/radio frequencies (RF), and higher radio frequencies, including microwaves.
Electric and Magnetic Fields
Electromagnetic waves are composed of two different but related fields -- electric, known as the "E-field", and magnetic, called either "H-" or "B-field". Electric fields are created by voltage and measured in volts per meter (v/m). The higher the voltage, the stronger will be the resultant field. An electric field will exist even when there is no current flowing. Magnetic fields are created when electric current flows: the greater the current, the stronger the magnetic field. Magnetic fields are measured in gauss (G) and milligauss (mG) or Tesla and microtesla (mT). 10 milligauss equals 1 microtesla.
Natural Sources of EMF
Electromagnetic fields are present everywhere in our environment Electric fields are produced by the build-up of electric charges associated with thunderstorms. The earth's static DC magnetic field is present everywhere on the planet and far out into space.
Man Made Sources of EMF
There are numerous man made EMF sources: subway rail systems and medical MRI equipment produce strong DC fields; passing electric distribution lines and building electrical equipment emit AC magnetic fields; broadcasting and communication transmitters and radar systems generate powerful RF magnetic fields.
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