Features of Electromagnetic Fields

Electromagnetic fields also exist in nature. Electric fields are generated by the formation of thunderclouds in the atmosphere. Static electricity is a phenomenon caused by electric fields. Magnetic fields exist around magnets. The magnetic field (geomagnetism) created by the Earth allows a compass to indicate the direction of north, and enables birds and fish to navigate.

As the frequency of the electromagnetic field increases, the electric and magnetic fields intertwine, and the electromagnetic field exhibits stronger wave-like properties, transmitting one after another over a distance. These waves are referred to as "electromagnetic waves".
The term "radio wave" is sometimes used to describe the use of high-frequency electromagnetic fields (radio frequency electromagnetic fields) as waves, such as in wireless communications.

Electromagnetic fields (low-frequency electromagnetic fields) generated by transmission lines and home appliances have a wavelength of 5,000 km to 6,000 km, so that in ordinary living space, the wave properties can almost be disregarded. Therefore, from a physical point of view, it is more accurate to describe them as "electromagnetic fields" rather than "electromagnetic waves".

The strength of the electric field is measured in volts per meter (V/m) as it depends on the electric voltage. Generally, the intensity of electric field decreases rapidly with the distance from the source.

A magnetic field is not generated until the switch is turned on and electric current begins to flow.
Since the magnetic field depends on electric current, the unit of A/m (ampere per meter) is used. However, magnetic flux density is more commonly described using the unit T (tesla). Typically, µT (microtesla) is used to represent the strength of the magnetic field around us, where 1 µT is one millionth of a T.
Generally, the intensity of the magnetic field decreases rapidly with the distance from the source.

Extremely high frequency electromagnetic fields, such as X-rays and gamma rays, are called ionizing radiation because they possess extremely high energy that ionizes atoms and molecules (i.e., separate electrons from the nucleus).

Ionizing radiation can induce mutations (including cancer) due to of its ability to damage DNA strands, which constitute genes, and also to damage tissues.

Non-ionizing radiation does not have sufficient energy to ionize materials as ionizing radiation does. Therefore, exposure to non-ionizing radiation does not adversely affect the genes in cells.
The energy of electromagnetic fields decreases with increasing wavelength, as shown in the table below.

The value of the geomagnetic field varies on the location. Around the equator it is about 30 µT (microtesla), while at the North and South Poles it is about 60 µT. However, the geomagnetic field is a static magnetic field, meaning that the direction of the magnetic field is always constant, unlike the magnetic fields generated by electric power facilities, where the direction of the magnetic field fluctuates. Additionally, the electric discharge current from a lightning strike can reach tens to hundreds of kA (kiloamperes), creating a very large magnetic field instantaneously in the vicinity of the strike.
On the other hand, electric fields are created by electrically charged particles (electric charge). The static electricity that causes a crackling sound when you try to take off a sweater on a dry winter day is due to the action of the electric field.

However, magnetic field strength is generally expressed as magnetic flux density (the amount of magnetic flux lines per unit area), in which case T (tesla) and G (gauss) are used.

• The relationship between Tesla and Gauss is as follows:
  • 1 G = 100 µT (1 Gauss = 100 microtesla)
  • 1 mG = 0.1 µT (1 milligauss = 0.1 microtesla)
  The strength of a magnetic field in the SI unit system is indicated in T (tesla).

When the whole body is exposed to radio waves, thermal effects can have harmful impacts on the human body if the whole-body average SAR reaches approximately 4 W/kg or more. Therefore, radio wave protection guidelines set the standard value for whole-body average SAR at 0.4 W/kg in the working environment, incorporating a safety factor of 10. In the general environment, the standard value for whole-body average SAR is set at 0.08 W/kg, applying a safety factor five times greater than that used for the working environment

In cases where radio wave energy is concentrated and absorbed in a localized area of the body (such as, in the head when using a cell phone), the standard value for local SAR in the general environment is set at 2 W/kg.

Power density (W/m²) = Electric field strength (V/m)² / 377 = 377 * Magnetic field strength (A/m)²

When the human body is exposed to very strong radio waves with a frequency of approximately 100 kHz or higher, some of the energy is absorbed by the body and converted into heat, leading to an increase in body temperature. This phenomenon is known to as the thermal effect.

As an indicator of heat generation within the body, the amount of energy absorbed by body tissues per unit mass over a specific period is expressed as SAR (Specific Absorption Rate), which is expressed in W/kg (watts per kilogram).