Exposure: Arises when a person is subject to electric, magnetic or electromagnetic fields or conduct currents, different from those which originate from physiological activities of the human body or other physical effects.

Exposure standard: Regulations, recommendations or a standard which were legislated by a responsible Institution and refer to the limits of allowed exposure.

Non ionizing radiation: Any kind of electromagnetic radiation which cannot extract electrons from atoms or molecules in order to produce directly or indirectly ions or ionized molecules.

Far field area: The area of an antenna field where the angular field distribution is essentially independent from the distance of the antenna. In that area, the field’s character is predominantely that of a plane wave, that is with uniform local distributions of the electric and magnetic field in planes which are transverse to the direction of propagation.

Near field area: This area lies generally near an antenna, or any other radiating  structure, where the angular field distribution depends on the distance from the antenna. In that area, the electric and magnetic fields are not those of a plane wave. The near field area can be further divided into the reactive near field area, which is closest to the radiating structure and contains the biggest, or almost the total amount of the stored energy, and the radiating near field area, where the radiating field dominates over the reactive field, but has a complicated structure and does not have the characteristics of a real plane wave.

Power density: It is the radiant power incident perpendicular to a surface, divided by the area of the surface and is expressed in watts per square metre (W/m²). For the case of plane waves, the power density S, the effective electric field strength E and the effective magnetic field H are related to the free space impedance via the following formula: S=E²/377=377H²

Current density J: It is defined as the current flowing through a unit cross section perpendicular to its direction in a volume conductor such as the human body or part of it, expressed in amperes per square metre (A/m²).

Electric field strength (E): It is the vector quantity that corresponds to the force exerted on a charged particle regardless of its motion in space. It is expressed in volts per meter (V/m).

Magnetic field strength (H): It is the vector quantity (H), which, together with the magnetic flux density, specifies a magnetic field at any point in space. It is expressed in amperes per meter (A/m).

Magnetic flux density (B): It is a vector quantity, resulting in a force that acts on moving charges and is expressed in teslas (T). In free space and in biological materials, magnetic flux density and magnetic field strength can be interchanged using the equivalence 1 A/m = 4π 10^–7 T.

Specific energy absorption (SA): It is defined as the energy absorbed per unit mass of biological tissue, expressed in joules per kilogram (J/kg).

Specific energy absorption rate (SAR): Averaged over the whole body or over parts of the body, it is defined as the rate at which energy is absorbed per unit mass of body tissue and is expressed in watts per kilogram (W/kg). Whole body SAR is a widely accepted measure for relating adverse thermal effects to RF exposure. Besides the whole body average SAR, local SAR values are necessary to evaluate and limit excessive energy deposition in small parts of the body resulting from special exposure conditions. Examples of such conditions are: a grounded individual exposed to RF in the low MHz range and individuals exposed in the near field of an antenna.

Safety Index (SI) or Total Exposure Quotient: It is used when the exposure to multiple electromagnetic sources with frequencies above 1MHz is examined . The SI is defined as the sum of the ratios of the measured power density at each frequency, to the value of the reference level for that particular frequency. A value of SI lower than unit denotes compliance to the exposure guidelines with respect to thermal effect circumstances.

Single-Frequency Exposure Quotient (SFEQ): It is defined as the ratio of the power density at a specific frequency, to the value of the reference level for that particular frequency. A value of SFEQ lower than unit denotes compliance to the exposure at this frequency.