Biological Mechanisms of the Effects of Non-Ionizing Radiation
Over the past few years there has been much controversy regarding the effects of non-ionizing radiation on cells and on living organisms. Scientists studying the effects of non-ionizing radiation on health have proposed a number of possible mechanisms through which radiation may affect the living body:
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A number of organizations, such as ICNIRP, IEEE and FCC , as well as health authorities in various countries have issued recommendations for safety limits, in an attempt to protect employees and the general public from the adverse effects of non-ionizing radiation. These recommendations were formulated according to the highest levels of radiation, above which the proven thermal effects of non-ionizing radiation are observed. However, there is general agreement that there may be additional health effects unrelated to heating of tissues (non-thermal effects). Such effects may occur even if the source of radiation has lower power density than that recommended by the safety limits. Therefore, many countries have formulated more stringent recommendations. In Israel, the safety exposure limit is one tenth of that defined by ICNIRP. This stringency is in accordance with the precautionary principle. This principle advocates reducing exposure while striking a balance between the needs of evolving technology and protecting human health.
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Thermal effects (heating)
The best-known proven effect of non-ionizing radiation is heating of tissues. This effect occurs mainly in the range of 30MHz to 300 GHz (radio frequency).
Infrared thermography image showing the heat
of Notebook and smartphone
Biological tissues are composed mainly of water and electrically-charged particles. Each tissue has a specific capacity for absorbing energy, according to its characteristics. Energy from electromagnetic fields in the radiofrequency and microwave ranges, when absorbed by biological tissues, causes movement of the charged particles in the tissues leading to a local increase in the tissue temperature. The tissue can usually cope with a slight increase of its temperature, by increasing the blood flow to the area that absorbed the radiation. If the temperature increase is high the body cannot overcome and dispel the excess heat, and the tissue may be damaged (e.g. a burn). However, at the power with which most personal devices emit electromagnetic radiation, the heating effect is minimal and does not carry the risk of significant damage. For instance, exposure to a power of 600 milliwatts (mW) for 50 minutes would raise brain temperature by 0.08oC - 0.190C, whereas nerve cell damage would be observed only after an increase of 4.50C for 30 minutes. One study found that a mobile phone, with a power of 250 mW would raise brain temperature by only 0.10C under prolonged exposure.
Effects of induced voltage gradients and electric currents
Within the extremely low frequency (ELF) range of 0 Hz - 3 kHz (electricity grids), in the presence of a strong magnetic or electric field, induced voltage gradients and currents may cause electrocution (electric shock). Induced voltage gradients and currents may also cause stimulation of biological tissues or a change in their electric or nervous conduction. This phenomenon may also occur at higher frequencies - within the radiofrequency range - under exposure to sources whose power is higher than the power that causes heating of tissues.
Non-thermal effects
Non-thermal effects of non-ionizing radiation are those not directly related to an increase in tissue temperature (the thermal effect), but rather to other changes that occur in the tissue as a result of exposure to electric or magnetic fields. Such changes may be caused by a sequence of biological processes and may affect the living organism at different levels (from the molecular to the cellular, the organ, and the whole living organism) activating various mechanisms that affect health.
Epidemiologic studies pointing to an association between non-ionizing radiation and the development of cancerous tumors led to a working hypothesis that living cells in the body are able to 'sense' non-ionizing radiation and react to it without undergoing heating. To verify this hypothesis, the effects of different frequencies of non-ionizing radiation on intracellular processes in tissue cultures of various cell types were studied. As expected, non-ionizing radiation did not have a significant effect on the DNA sequence or on its structure. Conversely, slight but reproducible effects were observed on intracellular mechanisms of signal transduction, including of free radical formation, phosphorylation and protein breakdown. These findings showed that cells are able to 'sense' non-ionizing radiation; however to date there is no evidence that the 'sensing' leads to physiological changes, including cell proliferation or death. It is interesting to note that the 'sensing' of radiation has also been observed in cells originating in whole organisms (e.g. flies), but here too the response was not translated into any measurable physiological processes, possibly because in these experiments the radiation did not exceed the dose required to bring about physiological effects. In order to reveal the mechanisms whereby radiation affects living cells, additional research on more sensitive systems (such as mutation -bearing or DNA-deficient cells) is needed.
References
- Non-ionizing radiation, Part II: Radiofrequency electromagnetic fields / IARC Working Group on the Evaluation of Carcinogenic Risks to Humans (2011: Lyon, France)
- Bernardi P, Cavagnaro M, Pisa S, Piuzzi E. Specific absorption rate and temperature increases in the head of a cellular-phone user. IEEE Trans. Microwave Theory Tech. 2000; 48:1118-1126.
- Van Leeuwen GM, Lagendijk JJ, Van Leersum BJ, Zwamborn AP, Hornsleth SN, Kotte AN. Calculation of change in brain temperatures due to exposure to a mobile phone. Phys Med Biol. 1999; 44:2367-2379
24.9.2015
