Electric and magnetic fields that vary over time interact with the electrically charged particles which matter is made of. Of particular interest is the interaction with biological systems ranging from basic cellular structures to complex organisms such as plants and animals.
To properly quantify the energy absorbed by a material, especially by human tissue, dosimetric quantities are used. Dosimetry expresses the current and power density and the energy absorbed per unit area or volume as defined below:
• Current density ‘J’: is the current flowing through a cross section of a conductor such as the human body or a part of it. It is measured in terms of A/m2.
• Density of power ‘S’: is used for very high frequency types of current where depth of penetration is small. It is calculated as the radiant power perpendicular to a surface divided by the same surface area and is expressed in W/m2.
• Specific energy absorption ‘SA’: is defined as the energy absorbed per unit of mass of biological tissue and is expressed in Joule/kg.
• Specific Absorption Rate of energy ‘SAR’: This is the rate of absorption of energy per unit mass of body tissue averaged over the entire body or specific parts of it. It is used to assess and eventually limit excessive energy deposition in small parts of the body resulting from particular exposure conditions. Both SAR, averaged over the whole body, as well as local body part values are used. It is measured in W/kg.
The quantities mentioned above are used as references to measure the effects on the human body and to define exposure limits. These, however, cannot be measured directly on the individual exposed to assess the intensity of radiation. Instead, measurable physical quantities such as magnetic field and induction are used. This limitation means that the quantities, which are obtained through mathematical models simulating the behaviour of the human body, are defined in terms of modules of magnetic flux density and magnetic field.
At low frequencies the tissue of the body are able to shield and so mitigate the electric field, by contrast, magnetic fields or magnetic induction are not shielde by the tissues and therefore does not attenuate its effect. Consequently, it is clear that at low frequencies the magnetic field is the main pollutant factor as far as affecting biological properties is concerned. The direct, short-term or acute effects due to electromagnetic fields are well represented by current density (A/m2).
|Current density J [mA/m2]||Symptoms|
|J > 1000||Extrasystoles and fibrillation|
|100 < J <1000||Tissues stimulation: possible risks|
|10 < J < 100||Possible symptoms on the nervous system|
|1 < J < 10||Minor effects|