Magnetic Fields and Human Health
When evaluating whether there might be a connection between cancer and static electric or magnetic fields, can all electromagnetic fields be considered the same?
No. The nature of the interaction of an electromagnetic source with biological material depends on the frequency of the source, so that different types of electromagnetic sources must be evaluated separately.
X-rays, ultraviolet (UV) light, visible light, MW/RF, magnetic fields from electrical power systems (power-frequency fields), and static magnetic fields are all sources of electromagnetic energy. These different electromagnetic sources are characterized by their frequency or wavelength.
The frequency of an electromagnetic source is the rate at which the electromagnetic field changes direction and/or amplitude and is usually given in Hertz (Hz) where 1 Hz is one change (cycle) per second. The frequency and wavelength are related, and as the frequency rises the wavelength gets shorter. Power-frequency fields are 50 or 60 Hz and have a wavelength of about 5000 km. By contrast, microwave ovens have a frequency of 2.54 billion Hz and a wavelength of about 10 cm, and X-rays have frequencies of 10^15 Hz and, and wavelengths of much less than 100 nm. Static fields, or direct current (DC) fields do not vary regularly with time, and can be said to have a frequency of 0 Hz and an infinitely long wavelength.
The interaction of biological material with an electromagnetic source depends on the frequency of the source. We usually talk about the electromagnetic spectrum as though it produced waves of energy. This is not strictly correct, because sometimes electromagnetic energy acts like particles rather than waves; this is particularly true at high frequencies. The particle nature of electromagnetic energy is important because it is the energy per particle (or photons, as these particles are called) that determines what biological effects electromagnetic energy will have [62].
At the very high frequencies characteristic of hard UV and X-rays, electromagnetic particles (photons) have sufficient energy to break chemical bonds. This breaking of bonds is termed ionization, and this part of the electromagnetic spectrum is termed ionizing. The well-known biological effects of X-rays are associated with the ionization of molecules. At lower frequencies, such as those characteristic of visible light, RF, and MW, the energy of a photon is very much below those needed to disrupt chemical bonds. This part of the electromagnetic spectrum is termed non-ionizing. Because non-ionizing electromagnetic energy cannot break chemical bonds there is no analogy between the biological effects of ionizing and nonionizing electromagnetic energy [62].
Non-ionizing electromagnetic sources can still produce biological effects. Many of the biological effects of soft UV, visible, and IR frequencies also depend on the photon energy, but they involve electronic excitation rather than ionization, and do not occur at frequencies below that of IR (below 3 x 10^11 Hz). RF and MW sources can cause effects by inducing electric currents in tissues, which cause heating. The efficiency with which an electromagnetic source can induce electric currents, and thus produce heating, depends on the frequency of the source, and the size and orientation of the object being heated. At frequencies below that used for broadcast AM radio (about 10^6 Hz), electromagnetic sources couple poorly with the bodies of humans and animals, and thus are very inefficient at inducing electric currents and causing heating .
Thus in terms of potential biological effects the electromagnetic spectrum can be divided into four portions:
The ionizing radiation portion, where direct chemical damage can occur (X-rays).
The non-ionizing portion of the spectrum, which can be subdivided into:
The optical radiation portion, were electron excitation can occur (visible light, infrared light)
The portion where the wavelength is smaller than the body, and heating via induced currents can occur (MW and higher-frequency RF).
The portion where the wavelength is much larger than the body, and heating via induced currents seldom occurs (lower-frequency RF, power frequencies, static fields).
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