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Electromagnetic Fields > ... > Biological effects > Thermal effects
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Biological effects due to energy absorption and heating
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Radiofrequency electromagnetic fields are absorbed by the body mass of living organisms and may subsequently provoke various effects. The absorption of energy depends mainly on the intensity and the frequency of the electromagnetic fields but also on the properties and structure of the biological tissues. Field forces as well as thermal effects of radiofrequency fields have been identified and are physically defined.
Effect Mechanisms
Living systems including human beings contain a lot of electrically charged particles and polar molecules. Polar molecules as, for example, water molecules are electrically neutral as a whole but carry a partial charge which is negative at one end and positive at the other end.
Electric and magnetic fields displace electrically charged or polar molecules by exerting force on them. Under the influence of electromagnetic fields these particles move very fast in time with the frequency. In this way they rub against each other and heat is produced.
If the intensity of the fields is very strong, whole cells can move as a consequence of field forces. They orientate or migrate in the field. Such athermal effects can, however, not be generated by fields of mobile communication as they are not strong enough.
Heat Effect
The heat effect is the crucial factor for possible health effects of radiofrequency fields in humans. If radiofrequency fields act on the human body it has the possibility to compensate for this additional heat via the so-called thermo-regulatory response:
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If the heating is limited locally this additional heat can in general be dissipated via the blood flow.
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If the whole body is warmed up, the blood supply to the skin raises and heat is delivered through evaporation at the skin surface (sweating).
Health detriments are not anticipated until certain threshold values are exceeded and the thermo-regulatory response of the body is impaired. Adverse health effects in animal experiments have been observed if the body temperature increased significantly higher than 1 degree Celsius for a long period of time:
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Metabolic processes were impaired,
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behavioural changes occurred and
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adverse effects on embryonic development were observed.
Long-lasting hyperthermia in the eye region promotes the development of cataract and other diseases of the eye.
Specific energy absorption rate (SAR)
Most relevant for biological effects of radiofrequency fields is the absorbed energy in the body. The reference quantity is the specific absorption rate (SAR, measuring unit: Watt per kilogram – W/kg). It defines the power (energy per unit of time) absorbed per kilogram of body mass.
A whole body exposure to radiofrequency electromagnetic fields which corresponds to SAR-values of 4 Watt per kilogram on the average results in a temperature rise in humans of approximately 1 degree Celsius. For comparison: normal muscle activity corresponds to 3 to 5 Watt per kilogram.
Penetration depth
The penetration depth is an important influence factor for the effect of radiofrequency radiation on living organisms. It strongly depends on the frequency:
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Electromagnetic fields in the range of 1 megahertz (MHz), used for amplitude modulated broadcast (AM) penetrate about 10 to 30 centimetre into the body.
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In the frequency range used for mobile communication networks, that means around 1 gigahertz (GHz), electromagnetic fields penetrate only a few centimetres into biological tissue.
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For frequencies above 10 gigahertz as used in radar equipment, the penetration depth is below 1 millimetre.
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At still higher frequencies energy absorption of electromagnetic fields occurs only at the skin surface.
Resonance
Resonance is another phenomenon to be considered when effects of radiofrequency radiation are evaluated. In this context the body size is important because the body is acting as a receiving antenna. If the body size is approximately half the radiation wavelength it is in the “resonance range”, where energy absorption is highest.
The frequency range of resonant absorption depends on body size and the orientation of a person in the electromagnetic field. For adults it is about 70 and for children it is about 100 megahertz. This frequency range is used for very high frequency (VHF) broadcasting. Resonance effects were taken into account when limit values were defined.
Many animal experiments are conducted with mice whose resonant frequency is in the range of several gigahertz. Energy absorption of a mouse at its resonant frequency of approximately 2 gigahertz is much higher than that of a human being at the same frequency. This has to be considered when results from animal experiments are extrapolated to humans.
“Microwave hearing”
A special effect of radiofrequency electromagnetic fields is produced by short heavy pulses. On certain conditions these pulses can be heard as hum or click. This effect is called “microwave hearing”. Its currently accepted mechanism is based on the thermoelastic properties of the tissue. Due to short high-power radiofrequency signals tissue areas of the brain are heated and expand as a consequence. Mechanical waves in the tissue lying in the audible region are excited thereby and the inner ear is stimulated.
In order to generate perceptible acoustical stimuli a very high energy per single pulse is necessary. If the energy per pulse is limited such effects cannot occur. The electromagnetic fields of radio and television transmitters as well as of mobile telecommunication cannot evoke “microwave hearing”.
Athermal effects could not be proven up to now
In the evaluation of effects of radiofrequency fields there is generally consent on the thermal effects explained above.
However, so-called athermal effects give rise to discussion in the public. Up to now possible health impacts due to athermal effects at low intensities of radiofrequency fields could scientifically not be proven.
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