Electric smog is a term commonly used to describe all artificially generated electric (E field) and magnetic (H field) fields. These occur everywhere where a voltage is present or a current flows. All types of radio and TV transmitter emit electromagnetic fields. Fields are also generated in industry, business, and the home; they affect us without us noticing them.
Units of electric field strength: Volts per meter (V/m).
Units of magnetic field strength: Amperes per meter (A/m) or magnetic flux density (B) in Tesla (T) or Gauss (G).
Electromagnetic fields propagate as waves at the speed of light (c). The wavelength depends on the frequency (f), measured in Hertz (Hz). If the distance from the field source is less than three wavelengths, this is generally considered to be the near field region (practically always the case at low frequencies up to 30 kHz). Far field conditions are present at distances greater than three wavelengths.
The distinction between near and far field is important for measurement. The relationship between the electric field strength (E, V/m) and magnetic field strength (H, A/m) is not constant in the near field, so they have to be measured separately. In the far field, the situation is different: Only one of the field quantities has to be measured, since the other can be calculated because of the constant relationship between them. The electric field can be screened easily e.g. using a thin, earthed metal sheet. In contrast, the magnetic field penetrates practically all known building materials.
Electromagnetic fields that could exceed permitted limits occur in the following sectors, above all:
- Transmitters for radio, TV and telecommunications; radar installations
- Industrial equipment, such as spark erosion, induction heating, plastic welding, microwave applications, aluminum processing
- Medical equipment for diathermy, electro surgery, hyperthermy, nuclear magnetic resonance tomography
Currents that can irritate the sensory, nerve and muscle cells are induced in the body in low frequency fields.
The higher the field strength, the stronger the effects. However, the field strength decreases with increasing distance from the field source.
High frequency fields heat up the body.
The degree to which the electromagnetic waves are absorbed depends on the frequency and intensity of the field and the type of tissue. Parts of the body with poor circulation, such as the eyes, are particularly vulnerable. In contrast, the heart and brain can easily deal with heat as they are well supplied with blood.
As well as obvious injuries such as burns, the long term effects are under discussion; these include increased cancer risk, hormonal imbalances, cell growth, and te immune system.
For human safety, basic limit values for current density (J) in milliamperes per square meter (mA/m2) for low frequencies and for the specific absorption rate (SAR) in Watts per kilogram (W/kg) for high frequencies have been specified by international agreement.
Measurement of the fundamental limit values is extremely complex and practically impossible to do. For this reason, measurements of quantities for electric and magnetic field strength derived from the fundamental units are made instead.
The field strengths recommended by the ICNIRP (International Commission for Non-Ionizing Radiation Protection) are recognized internationally and are used as the basis for the national standards and laws in many countries.
In the far field, the relationship between electric and magnetic field strength is fixed. That is why you only need to concentrate on measuring one field component - usually the electric - and calculate the other from thisFar field conditions can generally be assumed when the distance from the field source is more than three wavelengths. The far field of long wave transmitters does not start until several kilometers from the source. For mobile communications antennas that operate in the gigahertz band with wavelengths of around 30 cm, the near field begins at a distance of about one meter.
In the low fequency range up to about 30 kHz, the electric and magnetic field must always be measured separately. The electric field is measured by the field strength in V/m or kV/m (Volts per meter or Kilovolts per meter). The magnetic field is described by the magnetic induction in units of T or mT (Tesla or Millitesla) or in G or mG (Gauss or Milligauss), as well as by the magnetic field strength in A/m (Amperes per meter). Magnetic induction can be converted to magnetic field strength using a constant, (magnetic permeability).
For high frequency range in fields oscillating in the MHz or GHz range (1 million or 1 billion cycles per second), a distinction is made between an upper and lower range. The electric and magnetic field strengths are measured separately in V/m and A/m. In the upper range, the power flux density in mW/cm2 or W/m2 is measured; this derives from the electric and magnetic fields but only the electric or the magnetic field component is actually measured.