Pressures on the environment may occur through chemical, physical and biological agents. The subject of this study is physical agents, where physical fields are examined.
Physical fields are part of the natural environment. Depending upon their properties and the surrounding conditions, physical fields are transmitted through the spaces in which we live, potentially affecting human health and nature. Human activity can add to, modify, and enhance and reduce the intensity of these fields. Furthermore, changes in physical fields can be linked to global environmental problems such as stratospheric ozone depletion (causing a potential increase in ultraviolet-B radiation) and climate change.
Physical fields arising from non-ionising and ionising radiation are additional agents exerting pressure on organisms in the environment. Box A explains the concepts and terminology associated with such fields. Some parts of the electromagnetic spectrum, such as gamma rays and X-rays from nuclear sources and electron beams, which have wavelengths less than 10-8 metres, are said to be ionising (ie, radiations and fields that have enough energy to produce ionisation of matter such as gases and biological matter). Radiations with a wavelength longer than 10-8 metres are said to be non-ionising. These include electromagnetic fields and non-ionising ultraviolet (UV) radiation, which are discussed in this study. The effects of low-frequency electric and magnetic fields are included (especially below power transmission lines) as well as exposure to UV-B radiation. Microwaves are not covered in this overview.
Human-induced sources of electromagnetic fields are generally small when compared with natural sources. However, suspicion has been directed towards certain human-made frequencies and wavelengths which happen to be far greater than those occurring naturally, although evidence of health effects from EMF is insubstantial.
With respect to ultraviolet light, the evidence of biological damage from exposure is clear. Some epidemiological evidence suggests that certain frequencies and/or wavelengths may be associated with human health effects. However, much of this exposure for humans appears to be related more to choice of lifestyle (such as sunbathing) than to environmental factors. This may yet be exacerbated by the depletion of the stratospheric ozone layer, whose equilibrium has been disrupted by emissions of CFCs and halons.
There are a few other types of non-ionising radiation which have demonstrated human health effects, including high-frequency fields, ultrasound and laser light. Medical applications of all three of these have demonstrated clear diagnostic or therapeutic benefits. In human terms, exposure is presently significant only in the occupational context, which is beyond the general scope of this overview.
Sources Natural Electromagnetic fields (EMFs) are present naturally. The Earth has a 'static' (steady) magnetic field strong enough to turn a compass needle. EMF of various strengths also occur in thunderstorms, in the static electricity associated sometimes with clothes and furnishings, and in the electrical activity of the human body itself. The Earth's core is thought to be responsible for massive electric currents inside the Earth, which in turn are thought to be the source of most of the Earth's static magnetic field. This field has a strength of about 40 A/m (50 µT). A natural electrical field is also normally present at the Earth's surface due to the electrical charges in the upper atmosphere and from solar activity. At ground level during good weather this field is about 100 V/m, but during severe weather it may rise to many thousands of volts per metre.
Artificial EMF are produced by power transmission lines and by most equipment that uses a mains supply, including everyday home appliances. Table 2 provides a basis for comparison of the field strengths associated with various sources in the environment. It is clear from this that the human-induced sources of EMF generally occur at much higher intensities than the naturally occurring fields, especially at the power frequencies of home appliances (50 Hz). Some of the fields to which humans are occasionally subjected are extremely strong. There is a general but unproven feeling, however, that DC (also referred to as 'static') fields, such as the Earth's magnetic field and that involved in magnetic resonance imaging (MRI), are less hazardous or problematic than AC (or 'non-static') fields. Human exposure to non-static fields generally comes through electricity use.
Effects of EMF EMF may have direct or indirect effects on living organisms. Direct effects arise when electric fields induce a surface charge on an exposed body. This results in a distribution of electric current in the body, depending on exposure conditions, size, shape and position of the exposed body in the field. Observable biological effects may include vibration of body hair, stimulation of sensory receptors and cellular interactions. Indirect effects of EMF are defined as those that may occur when an electric current passes completely through the body when in contact with a conducting object. Observable effects may include the occurrence of visible discharges, sometimes known as 'sparks', or interference with medical implants such as cardiac pacemakers.
EMF act in a very different manner from most other environmental agents. A straightforward measure of field strength, analogous to a measure of the concentration of a chemical in the air or water, may not provide an accurate measure of exposure or risk. The exposure to EMF can be characterised by several different parameters (field strength, field direction, field orientation in relation to the body exposed, field complexity, and so on), though it is not known which of these parameters are associated with risk to human health and the organisms.
The WHO recently summarised the results of research studies on the impact of EMF in the laboratory on animals, on humans, at the cellular level, and on the biological effects of Extremely Low Frequency (ELF) fields (WHO, 1993b). Evidence for thermal effects of exposure to EMF is identified. There are also some epidemiological studies which claim increased incidence of cancer in children and adults, as well as workers occupationally exposed to magnetic fields of 50 to 60 Hz over long time periods (see reviews in UK NRPB, 1992). The greatest concern relates to children reportedly exposed to field strengths of 0.1 to 0.4 µT living in proximity to power lines.
Most studies have been criticised as no biological mechanism has been suggested to explain these effects and because of the low statistical significance of the results due to the low case numbers and inadequate controls. Consequently, there is still great uncertainty surrounding the possible carcinogenic effects of magnetic fields. Two recently published studies, from Sweden and Finland, of children living close to power lines (Verkasalo et al, 1993; Feychting and Ahlbom, 1993) concluded that electric and magnetic fields of transmission lines do not constitute a major public health problem regarding childhood cancer. The increased risk of childhood leukaemia noted in the Swedish study (one extra case per year) was indicated to be insignificant from a public health point of view.
At its meeting in May 1993, the International Commission on Non-Ionising Radiation Protection (ICNIRP) reviewed all available data on the possible carcinogenicity of 50 to 60 Hz magnetic fields. They concluded that there is no firm evidence of the existence of a carcinogenic hazard from exposure to ELF fields, but that the findings justify the proposal of a specific programme for further research (WHO, in press).
Policy and research Present public safety standards for electromagnetic fields are based primarily upon laboratory experiments and modelling studies. These focus on exposure conditions known to cause physiological or chemical effects associated with the development of cancerous cells.
Some international standards have been set by, for example, the European Committee for Standardisation (CEN) and its subsidiary the European Committee for Electrotechnical Standardisation (CENELEC). Limits for electric and magnetic fields at power frequencies of 50 to 60 Hz in some European countries are shown in Table 3, together with the guide limits of the International Radiation Protection Association. Some individual countries such as Belgium, the former Czechoslovakia and the former USSR have set limit values for overhead power lines in different settings (general purpose, road crossings, accessible or inhabited areas and edge of right of way). However, there are no internationally agreed limits for overhead power lines (Maddock, 1992). See WHO (1993b) for details on other frequency ranges.
There are currently no proposals to legislate on the exposure of the general public to EMF in the EU. Attention in the EU is currently centred on concerns about occupational exposure of workers to the risks arising from physical agents (CEC, 1993b) in the case of EMF, this includes primarily the exposure of those working on high-tension lines and is further restricted to the thermal effects of such exposure.
The majority of European countries have protection standards which regulate occupational exposure to EMF, but the standards differ from each other in the permitted exposure levels, coverage of EM spectrum, permitted exposure time, and technical standards for EMF sources. These differences arise from disparate research findings consequential from questions on what constitutes a health hazard, and methodological differences in collection, processing and interpretation of results. An international research project at European level, in the framework of the European Commission's COST programme (COST 244 'Biomedical Effects of Electromagnetic Fields') aims to coordinate European research in this field. Its objective is to ensure the scientific background for developing common European protection standards in the whole spectrum of EMF, oriented both to the general population and to occupationally exposed personnel. The project entered the research phase in 1994 and is planned to end in 1997.
Outlook and conclusions
Public concern about EMF continues. The WHO predicts increases in EMF exposure in coming years as increased electrification accompanies socio-economic development (WHO, in press). This general scenario might perhaps be tempered in some respects by the following considerations.
There are differences between Western Europe and Central and Eastern Europe in the trends for electricity consumption, and in the use of electrical appliances in the home. While trends put forward in the WHO socio-economic development scenario may well apply to Central and Eastern Europe for the next two decades, it is doubtful that anything similar will apply in Western Europe. In Central and Eastern Europe, most future electricity expansion could be accompanied by more modern equipment and appliances and more efficient generation, distribution and consumption practices, which may reduce EMF exposure. The spread of information and concerns about EMF in Western Europe will lead to prudent avoidance of EMF where possible, particularly through burying cables, as well as appliance designs that seek to limit EMF, improved energy efficiency, and so on. Such activities place in doubt an expected increasing trend in EMF as witnessed in Western Europe over the past decades, and may result in exposures beginning to decrease overall. The problem of understanding EMF effects on health might be more rapidly resolved through epidemiological studies of electrical workers than through studies of the general public, who generally have much lower exposure.
The existing situation in Europe could be considered adequate, if the measures and limits internationally agreed for the protection of workers are properly implemented. Until further research results suggest otherwise, the IRPA (International Radiation Protection Association) limit for continuous exposure of the general public of 5 kV/m for electrical field strength (or a magnetic flux density of 0.1 mT at 50/60 Hertz) may be considered to provide substantial protection from possible health effects.
Sources Natural Ultraviolet (UV) radiation is emitted by the sun with wavelengths between 10-7 and 10-8 metres as well as by various human-induced sources. The UV radiation band can be divided into three smaller bands: UV-A (320 to 400 nm), UV-B (280 to 320 nm) and UV-C (<280 nm). About 5 per cent of the total solar radiation that reaches the Earth's surface is ultraviolet. The amount and intensity (of UV-B especially) varies with the angle of the sun (time of day, season and latitude), the weather (clouds, mist, fog, etc) and the altitude. At sea level, about 95 per cent of the UV radiation is UV-A, 5 per cent is UV-B.
Most solar UV radiation which would otherwise reach the Earth's surface is absorbed by the atmosphere. However, human activities have been recognised to be changing the composition of the atmosphere, including the ozone layer. Such a change could permit a small but significant increase in the amount of UV radiation reaching the Earth's surface, with repercussions for human health and ecosystems.
Although ozone layer measurements have indicated a decrease in levels of stratospheric ozone, there is little evidence of increases in UV flux at the Earth's surface, with the exception of a large increase recorded in the South Pole region (see Box B). It has been suggested that the most common monitors (Robertson-Berger (RB) meters), designed before there was a perceived need for detailed knowledge of the UV spectrum, are not sensitive enough to detect the changes, which are occurring most importantly in the shorter wavelengths recorded by the meter (CEC, 1993c).
A recent Canadian study (Kerr and McElroy, 1993, cited in WHO, in press) showed that levels of UV-B in Toronto during the winter have increased by more than 5 per cent per year from 1989 to 1993, as ozone levels have decreased. At the time it was produced, this study was the most reliable of its kind with regard to the situation in the northern hemisphere.
Artificial Technology has also introduced new sources of UV radiation and visible light which can have deleterious effects on human health. Much of the working population, and more and more of the general population, of Europe is now exposed to the UV radiation emanating from fluorescent and high-intensity quartz tungsten-halogen lamps over long periods. Likewise, industrial and medical uses of UV radiation are widespread, including such diverse applications as photo-polymerisation, disinfection and sterilisation, welding, and the widespread use of lasers.
Changing fashion and human behaviour patterns have also led to increased exposure to the sun and UV radiation, which has already demonstrated profound effects on human health, in particular skin cancers. The use of sun-lamps and sun-beds, most frequently for cosmetic reasons, is a common source of additional UV exposure. Prior to the mid-1970s, mercury lamps were generally used as sun-lamps, sources of high levels of both UV-B and UV-C. While some of these devices are surely still around, since the late 1970s fluorescent lamps, generating almost exclusively UV-A (the most important for tanning) and visible light, have mostly been produced for this application. A clear link has not been proven between the use of sun-lamps or sun-beds and the incidence of skin cancer. However, the use of artificial tanning equipment is widespread and largely uncontrolled. In addition, it should be noted that a 'safe' level of exposure has not been defined. It is estimated that 10 per cent of people in the Western world use sun-lamps or sun-beds (WHO, in press).
There is also an increasing use of UV light in working environments (Moseley, 1988, and WHO, in press):
Fluorescent lamps consist of low pressure mercury vapour lamps generating a strong emission at a wavelength of 254 nm. Depending upon the nature of the phosphor coating which is excited by this wavelength, there may be emissions of UV radiation. Other low pressure mercury vapour lamps are used for disinfection and sterilisation of drinking water, swimming-pool water, sewage, and so on. No conclusive studies have linked these UV emissions with adverse effects, but further studies have been called for (IARC, 1992). High-intensity (several kilowatts) quartz-halogen lamps may be used to induce polymerisation during the production of protective coatings or printed circuit boards, or the curing of special inks. Large xenon arc lamps are used to promote weathering of test samples, or to illuminate broad areas such as football fields. These efficient lamps generate significant amounts of UV radiation, which is usually designed to be attenuated by the glass housing of the lamps; however, some UV-B usually escapes. Electrical arc welding also gives rise to UV-B, which has been associated with specific health effects (Mariutti and Matzue, 1987). Lasers and other high-power sources of UV and visible radiation are used in the chemical and electronic industries, and in clinical medicine. Such high intensities can cause not only immediate burns, but also risks of stray or scatered light. Effects Effects on humans UV light is sufficiently energetic to rupture chemical bonds or to energise molecules into excited states which can initiate a variety of chemical and biological processes. UV-C is especially lethal to many living organisms since it interacts particularly with proteins and DNA. However, UV-C is not an environmental problem from natural sources, because it is almost completely absorbed by oxygen and ozone (even at reduced levels) in the atmosphere before it reaches the Earth's surface (CEC, 1993c). Artificial sources, on the other hand, deserve attention. The longer wavelengths represented by UV-A are little absorbed by the ozone or the atmosphere, but, compared with UV-C and much of UV-B, they are also relatively harmless to most living things.
UV-B, by contrast, which is partially absorbed by ozone in the atmosphere, is of special concern because it may have damaging effects on the biosphere. Unfortunately, slight changes in the ozone layer, which is especially suited to blocking UV-B wavelengths around 300 nm, may have a significant effect on the amount of UV-B that reaches the surface of the Earth. In fact, a reduction in ozone permits a very specific increase in UV-B radiation between 290 and 315 nm precisely that waveband where solar irradiance is normally reduced by 10 000 times due to absorption by ozone. Extensive studies on humans have demonstrated that UV-B wavelengths of 290 nm are 1000 to 10 000 times as effective in producing damage to DNA, killing cells, and causing skin erythema and skin cancer than are visible light wavelengths longer than 300 nm (Moan et al, 1989, and McKinley and Diffey, 1987).
Many biological reactions are particularly sensitive to the 290 to 315 nm waveband, which may result from evolutionary adaptation because of the protection provided by the Earth's ozone layer. An instantaneous (in geological terms) change in this protection will not permit time for a similar modification to take place. Even the low levels of UV-B radiation that normally reach the Earth's surface can damage DNA, and cause sunburn, eye cataracts and some skin cancers (IARC, 1992), not to mention suppressing the normal human immune responses (Morrison, 1989). It is possible, therefore, that if ozone depletion or any other mechanism leads to significant increases in UV-B fluxes at the Earth's surface, this could have an important effect on the human immune response to certain allergens or infectious agents, as well as on the development of some cancers. The 'International Programme on Health, Solar UV Radiation and Environmental Change' (ITERSUN), a joint IARC/UNEP/WHO study which involves measurement of UV radiation at the surface, cancer incidence, ocular cataracts and immune suppression, may help resolve some of these uncertainties.
Effects on non-human targets UV radiation can penetrate sea water to a depth of up to 50 metres, which is sufficient to cause various ecological effects. UV-B damage to phytoplankton in the laboratory has been reported, which goes beyond DNA effects to include impairment of cellular metabolism and motility (Smith, 1989). Increased levels of UV-B may bring irreversible damage or death to zooplankton, which are an essential element of the food-chain for all higher marine life forms. Other marine organisms in the upper levels of the sea include the eggs and larvae of fish, which would certainly be susceptible to damage from raised levels of UV-B (Smith, 1989).
The effects of enhanced levels of UV-B on plants varies between species, as well as between different populations of the same species, indicating the additional influence of reflections from the soil or other local variables (Tevini and Teramura, 1989). Most plants respond to some extent through adaptation, including photo-repair, accumulation of UV-absorbing pigments and delay in growth. Various food crops such as soya beans and cereals are especially sensitive to UV radiation. The causes are not well understood, but have been shown to involve an impairment of the photosynthetic mechanism. In general, increased levels of UV radiation are expected to lead to an increase in the production of less economically valuable plants, and a decrease in the production of the more valuable plants.
Most animals have developed a certain coat or pigmentation which protects them from the harmful effects of UV radiation, but many (cattle and sheep are the most widely studied) seem susceptible to ocular cancers at a rate which is directly proportional to UV exposure (Kopecky, 1978). Any significant increase in UV-B is expected to have significant effects on food-producing animals, and these effects may be assumed to be greater at higher latitudes. A question which now arises, however, is what impact ozone depletion, through increasing levels of UV-B radiation, has already had on the welfare of the biosphere (SCOPE, 1993).
Policies and strategies Policies for reducing UV exposure have been determined, until now, through strategies for protecting the ozone layer. These have focused on two areas: reduction and eventual elimination of emissions of ozone-depleting substances, and 'active adaptation' to an environment of greater exposure to UV radiation. The Montreal Protocol (concluded in September 1987) and the London and Copenhagen amendments (June 1990 and November 1992, respectively) address the first of these. In many cases, controls already in force or being considered within the EU and some other European countries are even stricter than those agreed at Copenhagen (CEC, 1993c). Even if all countries comply with these agreements, however, ozone depletion may exceed any natural variations in the ozone layer that humans have had to cope with until now.
Outlook and conclusions There is clear evidence from research involving both animals and humans of a direct causal relationship between UV radiation exposure and skin cancers. The rapid increase in European skin cancer cases in recent years is undoubtedly connected to social and recreational behaviour the key factors as well as to other less clear causes. Increased public awareness of the hazards of UV exposure is necessary, especially for individuals with sensitive skin types and for young children.
To date, there is little direct evidence in Europe that depletion of the stratospheric ozone layer has led to increases in terrestrial UV-B levels. It is widely suspected, however, that the consequences of ozone depletion may be substantial, including a broad range of effects on human health. The need to counter further depletion of the ozone layer is clear.
Efforts are continuing to ensure that the Montreal Protocol and subsequent agreements are respected. Meanwhile, active adaptation would involve behavioural change to avoid excessive exposure to the sun. Over the medium to long term, active adaptation could imply, for example, that biotechnology might contribute to the development of food crops with greater resistance to UV radiation.
A broad range of measurements, programmes and research is already being carried out in Europe. This interest is motivated in large part by the scientific interest and broad policy implications of ozone layer depletion. However, this research requires increasing coordination, or at least harmonisation, so that definitions, measurements, procedures and data will be comparable between European countries.