Wireless communication systems, medical devices and power transmission infrastructure emit electromagnetic (EM) fields that interact with the human body on a continual basis. It is important to know this interaction in order to assess biological effects and provide safety compliance. The human tissue acts like a lossy dielectric material in which electromagnetic waves cause currents and power uptake, which cause thermal action. The concepts of electromagnetic field theory (EMFT) which are used in this paper include propagation of waves in lossy media, boundary conditions, power losses, etc. The Specific Absorption Rate (SAR) is the most important safety measure that is presented to control exposure. The paper puts emphasis on the fact that EMFT offers a theoretical basis on designing safer electronic systems, as well as restricting biological hazards.
In modern environment, mobile communication systems, Wi-Fi networks, radar, and medical imaging devices are spreading widely because of the presence of electromagnetic fields. Such systems can work in a very wide frequency range between very low frequency (ELF) to microwave and millimeter-wave frequency. The interaction of these electromagnetic waves on a biological structure when present in the human body interact with the tissues and stores energy depending on the frequency, field strength and properties of tissues.
The dielectric properties of human tissues, including skin, muscle, fat, and bone, are described in terms of permittivity ( ε), permeability ( μ) and conductivity ( σ ). Biological tissues are lossy dielectric, as the conductivity of biological tissues is finite. Electromagnetic waves as they enter the tissue cause induction currents that result in energy dissipation. This consumed energy is then converted to heat and the major source of concern in safety is therefore thermal effects.
The theory of electromagnetism Field Theory offers mathematical techniques to examine the penetration of the waves, attenuation, reflection and the absorption of energy. These principles are imperative in assessing the exposure of the devices like mobile phones held in proximity to the head, which is one of the scenarios of a real-world application.
The electromagnetic (EM) field interaction with the human tissue can be discussed in terms of basic principles of the electromagnetic field theory (EMFT). Permittivity, permeability and conductivity are the characteristics of biological tissue (e.g. skin, muscle, brain). Biological tissues are lossy dielectric media due to the finite conductivity of biological tissues. When electromagnetic waves are transversed through such media, some of the energy and energy is lost in the form of heat.
Wave propagation through lossy media is one of the Direct EMFT concepts. It occurs when motion of an EM wave by the mobile phone antenna reaches the human head that the wave first meets the air tissue boundary. The variations in the intrinsic impedances of air and tissue reflect a certain part of the wave and the rest is transferred to the tissue. This is known to be controlled by boundary conditions based on Maxwell equations according to which the tangential component of the electric field is continuous across the interface.
Induced conduction current within biological tissue is another conceptual EMFT phenomenon. The electric field causes current density to be generated in the medium as tissue is nonzero conductive. Such induced currents cause power loss and increase of temperature into the tissue. It is due to this process that one gets the thermal effects as a result of the exposure to electromagnetism.
Also, there is electromagnetic wave attenuation with increasing depth of the wave penetration into the tissue. Tissues with a large amount of conductivity absorb the most energy and therefore less penetration depth occurs. Thus, fast electromagnetic waves are more apt to be absorbed at the surface whereas low frequency goes deeper into the body.
The most commonly used parameter to determine the energy absorption within a biological tissue is the Specific Absorption Rate (SAR). SAR is directly proportional between the strength of an electromagnetic field and the power absorbed per unit mass and is utilized in both international safety standards to control the amount of exposure. All of these EMFT concepts describe the interaction of electromagnetic radiation with the human body and provide the theoretical foundation of the analysis of the biological safety.