radiation, non-ionizing

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radiation, non-ionizing Electromagnetic fields and radiation have, perhaps through their historical associations with magnetic lodestones and electrical storms, been linked to forces of nature that are not readily understood. Early students of the effects of electromagnetic phenomena on living organisms include the noted eighteenth-century Italian scientists Luigi Galvani and Count Alessandro Volta. The responsiveness of tissue to electrical stimulation described by these scientists no doubt provided some inspiration for Mary Shelley when writing the novel Frankenstein.

The foundation of modern electromagnetic theory, which accounts for the way in which electromagnetic radiation interacts with the human body, was laid down by the great Scottish scientist James Clerk Maxwell. His treatise on Electricity and Magnetism, published in 1873, was built on the experimental observations of another major British scientist, Michael Faraday, who, amongst other things, championed the concept of electric and magnetic ‘lines of force’. It was Maxwell's theoretical understanding, however, that paved the way for Einstein's special theory of relativity, eventually leading to Max Plank's formulation of the quantum theory. The latter underpins our present understanding of the interaction between electromagnetic radiation and matter, and the subdivision of the electromagnetic spectrum into ionizing and non-ionizing regions.

The electromagnetic spectrum (see figure) extends from static (or DC) fields such as those generated by permanent magnets, through the extremely low-frequency electric and magnetic fields generated by the supply and distribution of mains electricity, to radiofrequencies used for radio and TV transmission and now for mobile phone communications. Higher frequencies extend to infra-red, visible, and ultraviolet optical radiations and to X-rays and gamma radiation. The quantum energies of these latter 2, highly energetic, short wavelength (>100 nm) radiations are sufficient to be able to eject electrons from an atom, and thus they comprise the ionizing part of the electromagnetic spectrum. Non-ionizing radiations are less energetic and are conventionally subdivided into electromagnetic fields and radiations and optical radiations.

Electromagnetic fields and radiations

Life originated in the static, geomagnetic field of the Earth. It is widely believed that migratory animals, which include species of birds, butterflies, and fish, can utilize this (and other) information for navigation. Surprisingly, however, human beings seem relatively unaffected by exposure even to the large static magnetic fields, some 20–40 000 times natural background levels, that are used in magnetic resonance imaging systems. This equipment is used to provide images of the soft tissues of the body, derived from radio signals emitted mostly by transiently excited hydrogen nuclei, for clinical diagnosis. Above these levels, the electrical potential generated across the aorta by the flow of blood in the static field becomes substantial. In addition, people working in such fields have occasionally reported feelings of vertigo and nausea, possibly because of electrical stimulation of the vestibular organs.

Static and power frequency electric fields, such as those encountered under overhead powerlines, generate an electrical charge on the surface of the body. Human volunteer studies, carried out at the Electrical Power Research Institute in Palo Alto, California and elsewhere, have shown that many people can perceive the hair vibration and other effects that result from exposure to electric fields typically greater than those likely to be encountered even under high voltage overhead powerlines. The time variation of the surface charge, in the case of power frequency electric fields, will induce the flow of small electrical currents; these are however tiny compared to the thresholds for nerve stimulation. In contrast, the most sensitive and well-documented response of the human body to extremely low frequency magnetic fields is the induction of flickering visual sensations — the magnetic phosphenes — which are thought to result from the interaction of induced eddy currents with cells in the retina. One early investigator, the English physiologist Sylvanus Thompson, described them thus: ‘On inserting the head into the interior of the coil, in the dark, or with the eyes closed, there is perceived over the whole region of vision a faint flickering illumination, colourless, or of a slightly bluish tint.’ However, such high magnetic flux densities are unlikely to be encountered except in very specialized situations.

Radiofrequency currents are not able to stimulate nerve and muscle tissue in the same way, as the reactance of cell membranes renders them invisible to such high frequencies. The French physiologist, Arsène d'Arsonval, demonstrated around the turn of the century that the passage of a radiofrequency current through his colleagues sufficient to light a bulb resulted in a sensation of warmth, an observation that led to the development of short-wave and later microwave therapy for the treatment of injured tissue. Today, the most well-established effect of exposure to radiofrequency and microwave radiation from whatever source is that of heating through increased molecular kinetic energy, either heating of the whole body or localized heating of parts of the body, depending on various factors including the proximity of the exposed person to the transmitter. Much work on human physiological responses to radiofrequency and microwave radiation has been carried out by the American physiologist Eleanor Adair at the John B. Pierce Laboratory and at Yale University. Most people, however, encounter levels of radiofrequency and microwave radiation that are many orders of magnitude below those likely to induce measurable heating.

The pervasiveness of man-made electromagnetic fields and radiation, combined with some uncertainties about the possible existence of low level health effects, particularly in relation to childhood cancers, have generated much public concern over the past few decades. A large number of biological and epidemiological studies have now been carried out, centering mostly on the possible effects of exposure to power frequency magnetic fields. Generally speaking, however, there is at present no convincing evidence of any risk to health from such exposure; if there is a risk, most authorities agree that it is likely to be very small. The rapid expansion of mobile phone use has, however, led to some disquiet about the possible effects of exposure to the low levels of radiofrequency and microwave radiations emitted by these devices. Further biological and epidemiological research is being undertaken in order to address these concerns.

Optical radiations

The sun is the most important source of optical radiation, creating through its action on the environment the conditions necessary to sustain life. Light, which is only the visible part of the spectrum of optical radiation, drives photosynthetic processes in plants, generating food and oxygen, and enables our perception of the world through visual processes in the eye and associated parts of the brain. However, prolonged gazing at bright sources of light such as the sun can result in a loss of visual acuity through photochemical damage to the light-sensitive receptor cells in the retina. Such damage has been reported for example in anti-aircraft gunners and plane spotters during World War II, and in people viewing a partial eclipse of the sun.

Infra-red radiation, which is less energetic and of longer wavelengths than light, heats tissue through increasing molecular vibrational energies. It is mostly absorbed by the skin; we feel the warmth when we sit in bright sunlight or around a fire. With regard to the eye, however, mid-wavelength infrared-B radiation is absorbed mostly by the crystalline lens; heat-induced opacities (cataracts) have historically been associated with work in industries handling hot materials. H. M. Chief Inspector of Factories, Thomas Legge, reported in 1907 that ‘persons exposed to incandescent molten glass or to continual furnace glare in the flattening of glass suffer unduly, more than ten times as frequently as other people, from cataract.’ ‘Heat cataract’ became a recognized industrial disease, although modern working practices, including the wearing of protective goggles, have virtually eliminated this risk.

Ultraviolet radiation is the most energetic of the non-ionizing electromagnetic radiations; the shortest, most energetic wavelengths are however attenuated by the atmosphere, particularly the ozone layer. Ultraviolet radiation is able to damage DNA and other biologically important molecules through direct absorption, and to generate highly reactive oxygen radicals, which can result in adverse health effects; tissues of the skin and eye are most at risk. In the absence of dietary supplements, however, we require some limited daily exposure to ultraviolet radiation in order to produce sufficient vitamin D. Short-term damaging effects resulting from acute over-exposure include sunburnt skin, which becomes painful and red, and may blister and peel, and inflammation of the cornea (keratitis) and conjunctiva (conjunctivitis) of the eye; the last two effects are also known collectively as ‘snowblindness’ or ‘welder's eye’.

In the longer term, repeated exposure to excessively high levels of ultraviolet radiation may lead to photoageing of the skin and to an increased risk of cataract and skin cancer. Malignant skin tumours are the most severe health effect; their incidence has increased by about 50% over the last decade, possibly reflecting behavioural changes relating to sun exposure. Non-melanoma skin cancers are relatively common but rarely fatal; experimental studies carried out by Frank de Gruijl and his colleagues at Utrecht using mice have been invaluable in showing that both the mid-wavelength (UVB) and long-wavelength (UVA) regions are carcinogenic. Cutaneous malignant melanoma occurs much less frequently but accounts for the majority of skin cancer deaths. In contrast to the non-melanoma skin cancers, the aetiology of malignant melanoma is not clear; there are no good animal models. However, short-term, intermittent exposure to high levels of solar radiation, particularly during childhood, is thought to be a contributing causal factor.

Richard D. Saunders

See also free radicals; magnetic brain stimulation; sun and the body.

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