The energy level of some types of radiation is high enough so that when they interact with matter they cause the formation of electrically charged particles or ion-pairs and break molecular bonds. These so-called ionising radiations are by their nature potentially harmful to life; at high doses they can be lethal and at lower doses can cause genetic damage. Ionising radiation occurs naturally, and life on Earth has always been exposed to it. Human activities can, however, enhance exposure, and new sources have been created. The toxicity of radionuclides derives almost exclusively from the effects of the radiation which emanates from them, although some radionuclides, such as the isotopes of plutonium, are also highly chemically toxic.
Doses can be received from external radiation (the radiation source is outside the body in air, soil, etc) and from internal radiation (from radionuclides which enter the body by food, by drinking water and through inhalation). Terrestrial radionuclides are almost entirely responsible for internal exposures.
Natural sources include: primary and secondary cosmic radiation; radon and thoron in the air; and diverse radionuclides in the soil which may enter water and food.
Cosmic rays originate both in outer space and from the sun. Those coming from space remain fairly constant, but those from the sun are produced in bursts during solar flares. The intensity and number of cosmic rays which enter the Earth's atmosphere are affected by the earth's magnetic field; more cosmic rays enter near the poles than near the equator, so exposure decreases with decreasing latitude. Exposure to cosmic radiation increases with altitude due to the shielding effect of the atmosphere. Furthermore, some differences in indoor exposure to cosmic radiation arise due to variations in the shielding factor of different building designs. Other naturally occurring radionuclides acting as sources of ionising radiation are those continuously being produced by the interaction of cosmic rays and the atmosphere, eg, tritium (H-3).
All materials in the Earth's crust contain trace amounts of uranium, thorium and other naturally radioactive elements (terrestrial radiation). These nuclides include the long-lived isotopes potassium-40, uranium-238 and thorium-232, and, by radioactive decay, the medium to short-lived radionuclides radium-226 and radon-222, as well as their short-lived decay products. The activity level depends on the type of rock or soil there is relatively high activity in granites, low activity in some sedimentary rocks and intermediate activity in soils. The gamma rays produced by these materials irradiate the whole body more or less uniformly. Since most building materials are extracted from the earth, they also contain natural radioactivity (particularly potassium-40, uranium-238 and thorium-232). Exposure to terrestrial radiation is therefore experienced both indoors as well as outdoors as a function of geology and the structure and type of the buildings.
Apart from exposure of the lungs to radon, thoron and their daughter products discussed below, the human body is internally exposed from natural radionuclides such as beryllium-7, potassium-40 and members of the uranium-238 decay series (especially lead-210 and polonium-210) and the thorium-232 decay series. These radionuclides enter the body by air, food and water. Exposure due to potassium-40 is essentially constant over a given population, but lead-210 and polonium-210 exposure are more variable with diet. Overall doses from all of these sources together do not vary much between individuals, but of course the cumulative dose increases with age.
Radon-222 is a radioactive gas which emits alpha particles. It is the direct daughter product of radium-226 formed during the radioactive decay of naturally occurring uranium-238. As this occurs in the Earth, radon eventually escapes to the open air, where it is quickly dispersed. When the gas enters a dwelling, however, either through the floor or from the walls, dispersal is much slower and the concentration may become higher than outdoors. As the gaseous radon itself decays (half-life 3.8 days), alpha- and beta-emitting daughter radionuclides are formed. These normally attach themselves to small particles or aerosols in the air and, when inhaled, deposit in the respiratory tract and lungs, irradiating these internal tissues. Concentrations of radon may vary appreciably from one locality to the next (frequently by a factor of ten or more), and they may vary dramatically from one house to another (sometimes by a factor of 100), particularly in relation to the ventilation rate which may have been reduced to conserve energy. Thoron (another name for radon-220) is produced by the decay of thorium-232, but levels and doses are generally much lower than those of radon-222 (about 10 per cent). The relative magnitude of the most important natural sources of radiation exposure in Western European countries is illustrated in Figure 2. Comparable data are not currently available for Central and Eastern Europe.
Humans have increased their exposure to radiation from the environment through a number of activities. These include industrial activities where raw materials containing naturally occurring radionuclides at low concentrations are processed on a large scale (eg, the production of phosphate fertilisers, the extraction of oil and gas and the burning of fossil fuels), mining, the extraction of water from deep wells, the testing of nuclear weapons, the generation of nuclear power, and the use of radiation or products with high radioactivity in medicine, other industry and research. Ionising radiations can be made artificially either by the generation of appropriate electromagnetic fields, as in X-ray apparatus, or by the creation of artificial radionuclides produced in accelerators or by nuclear fission (atomic weapons or nuclear reactors). The general population is exposed to these ionising radiations either directly (eg, from medical uses) or through their presence in the environment (eg, discharges from the nuclear industry). Table 2 compares doses resulting from such artificial human-induced sources to an equivalent period of exposure to natural radiation sources (UNSCEAR, 1993). This perspective indicates that the increased exposure from most of these activities is, on the whole, not a great cause for concern. However, artificial radiation may in fact contribute a significant fraction of the dose over which there is some control. Particular attention is best devoted therefore to such sources of exposure, some of the most important of which are discussed below
With respect to the various artificial sources of exposure to ionising radiation, medical exposure is by far the greatest for the average individual and probably represents the area in which reductions in exposure would be most cost-effective. In any given year, probably a third of the Western European population is exposed to X-rays, with a dose up to 42 mSv/year (Vaas et al, 1991). In 1986, close to 50 per cent of the Russian population received X-rays for medical reasons (Nikitin et al, 1991); the increased exposure compared with Western Europe was caused largely by technical problems.
Nuclear medicine (diagnostic examination with radio-pharmaceuticals) contributes additional exposure of an estimated 1 to 2 per cent of the Western European population, with doses up to some 100 mSv/year (Vaas et al, 1991). Comparable figures are not available for Central and Eastern Europe.
Radiation therapy gives rise to very high (though usually localised) exposure of patients, but in this case such high levels of exposure are necessary to attain the objective of destroying cancerous cells.
Radioactive emissions and waste disposal
Most radioactive wastes are, or are planned to be, disposed of by concentration and confinement in surface, near surface or deep underground licensed facilities. Disposal by discharge to and subsequent dilution and dispersion in the environment is severely regulated at national and international level; its use is limited to some low-active liquid or gaseous effluents resulting from the operation of nuclear facilities (electronuclear, medical, etc). The authorised and controlled releases and discharges to water (lakes, rivers, and seas) and atmosphere from such facilities contributes directly to the dispersion of the radionuclides to the environment. In Western Europe, the spent fuel reprocessing plants (Sellafield, UK, and La Hague, France, and, to a lesser degree, Marcoule, also in France) are the most significant points of controlled release of liquid and gaseous effluents, the contribution from electronuclear power plants being individually much less. As a result of improved working practices, the level of radiologically significant discharges to the Irish Sea from Sellafield is now about 1 per cent of peak levels in the 1970s, when they were by far the most important from any single site in Western Europe (CEC, in press).
The practice of dumping radioactive waste at sea has been regulated since 1975 by the London Convention (more correctly referred to as the International Convention on the Prevention of Marine Pollution) by dumping of waste and other matter. Up to 1983 dumping of low-level radioactive waste packages complying with specific criteria has been authorised and practised by some Western Europe countries in the northeast Atlantic under the aegis of the Nuclear Energy Agency of the OECD. No environmental contamination has been detected up to now. This practice was suspended in 1983, following a voluntary moratorium. Since 1994 the prohibition of all sea dumping has been accepted by all signatories to the convention other than the Russian Federation, which has not as yet been able to implement alternative approaches. Potential unquantified threats of environmental contamination are found in the Barents and Kara seas around the Novaya Zemlya archipelago. They result from the dumping of radioactive objects, including highly radioactive material, by the former USSR (Yablokov et al, 1993).
Also in the former USSR, emissions into the Techa river (near Chelyabinsk and Sverdlovsk in the southern Ural mountains) in the 1940s and 1950s and associated operations of the Mayak nuclear facility (also the site of the Kyshtym radiation accident see below) has resulted in severe contamination of the area. Furthermore, a small lake (Lake Karachay) continues to be used to store high-activity waste material and is probably the worst site of radioactive waste accumulation on earth (Burkart and Kellerer, in press).
Uncontrolled release of radionuclides to the environment can occur as a result of accidents during the handling, transport, use and disposal of radioactive materials and during the operation of nuclear facilities. From an environmental point of view a distinction should be made between abnormal nuclear events having consequences limited to a nuclear plant or site, and abnormal nuclear events causing off-site radioactive pollution. According to the International Nuclear Events Scale (INES), first introduced on a trial basis by the IAEA in 1989 and now formally adopted with some revisions (IAEA, 1992), only these latter should be called accidents, the former being anomalies or incidents. Accidents have been few, the most prominent being the Windscale reactor fire accident (UK, 1957), and, considerably worse, the severe accidents at the Kyshtym military reprocessing plant (USSR, 1957) and the Chernobyl power plant (Ukraine, 1986). The Chernobyl accident had the most widespread effect in Europe for increasing exposure to ionising radiations. No accident has been reported since 1987 in Western Europe. Even relatively minor abnormal nuclear events have to be reported to the national safety authorities according to the provisions of national law. A number of international reporting systems are also in use on a voluntary basis, such as that directly associated with INES under the aegis of the IAEA.
Atmospheric testing of nuclear weapons has been a major source of artificial radioactivity in the environment. The extensive testing in the 1950s and 1960s released enormous quantities of radionuclides into the upper atmosphere which have now dispersed throughout the whole environment. In terms of radiation exposure, the most significant radionuclides from this source are strontium-90 and caesium-137 with half-lives of approximately 30 years. Since the signing of the Test Ban Treaty in 1963, atmospheric tests have almost been eliminated (the last atmospheric test was conducted by China on 16 October 1980). However, fall-out continues to provide a certain component of routine radiation exposure, although annual doses have become very small compared with the variations in natural background radiation.
Effects and risks
Whether natural or artificial, the effects of ionising radiation on matter are the same transferring energy which can break and rearrange the bonds holding molecules together. In biological materials, such chemical changes can have various types of consequences: effects can be deterministic (with a threshold dose above which the effect can be anticipated) or stochastic (with a statistical probability of occurrence decreasing with the dose but with no lower limit). Radiation sickness or organ and skin damage are deterministic effects. They occur only at high radiation doses at dose rates above a threshold, and the severity is related to the dose. Cancer and hereditary disorders are stochastic effects. Their probability, but not their severity, is assumed to be proportional to the dose even at very low doses (see below; ICRP, 1991).
Different types of radiation vary in their ability to penetrate matter and thus in the efficiency with which they transfer energy and induce biological effects. The conversion of absorbed doses to a common 'effective dose' permits different types of radiation exposure to be aggregated on a common basis. This effective dose is expressed in sievert (Sv) or sub-units thereof.
Because of its high rate of energy transfer, alpha radiation is one of the most damaging types of radiation. Internal exposure, primarily through the inhalation or ingestion of alpha emitting radionuclides, is of most concern for alpha radiation. Gamma radiation looses its energy over greater distances, causing less biological damage but being more penetrating.
In 1977, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) assessed the risk of a fatal cancer from ionising radiation at 2.5 per cent per dose of 1 Sv (UNSCEAR, 1977). Since then, with the availability of additional data from Japan and improved methods of risk evaluation, the estimated risk has been increased to 5 per cent per Sv for the general population, ie, a cumulative dose of 1Sv gives a one in twenty chance of a fatal cancer occurring (ICRP, 1991). To apply this coefficient to the ordinary circumstances of exposure in order to develop a risk factor for protection purposes, allowing for the overall potential detriment to health, the risks of non-fatal cancers and hereditary defects suitably weighted for lethality and severity are also taken into account. This results in an all-inclusive risk factor of about 7.3 per cent per Sv. Applied to the worldwide (somewhat higher in Europe) average annual dose rate of 2.4 mSv per year, this implies a theoretical lifetime risk to the average individual of premature death from natural radiation-induced malignancy of about 1 per cent. In order to put this risk in perspective, it can be compared to an overall probability of death by cancer. For the population of the USA it has been estimated that 30 per cent of fatal cancers are induced primarily by smoking, 35 per cent by substances ingested in food, and 3 per cent by alcohol consumption (Doll and Peto, 1981). These effects have meant that even large-scale epidemiological studies have not been able to identify an influence on the incidence of cancer attributable to natural radiation.
Policies and strategies
The main areas of action have been for occupational safety (particularly in the medical and nuclear industries) and exposure to the general public. The non-occupational standard of 1 mSv/year applies to populations which may be exposed to artificial ionising radiation. ICRP (1991) recommends lifetime dose for the public from planned activities should not exceed 70 mSv, with a maximum of 5 mSv in any one year. The dose averaged over 5 years should be less than 1 mSv/year. This does not apply to natural background or medical exposure.
Little can be done to prevent exposure to terrestrial and cosmic radiation. With regard to radon exposure, however, some authorities, including the European Commission, have recommended that high exposure to radon in the home should be avoided. There are also legal requirements in some European countries to limit exposure to radon in the workplace (Green et al, 1993). Public understanding of radon risk must be improved, and home owners should be encouraged to undertake the necessary measures and remedies.
Radiation exposure of patients is assumed to provide direct benefits to these patients which more than offset the associated risks, and hence it would be counter-productive to impose general limits, especially in radiation therapy applications. In diagnostic applications, however, there is certainly scope for using more efficient X-ray techniques giving a reduction in exposure without affecting the benefits.
Outlook and conclusions
Exposure to radiation in general, and especially the role of the nuclear industry, attracts a great deal of consideration in politics and society as a whole, disproportionate to scientific claims or evidence. Therefore, there is a significant challenge to weigh these considerations properly together with the valid scientific and socio-economic evidence.
Little can be done about the risks of exposure from natural sources of ionising radiation; indeed, humans have always had to cope with such risks. Therefore, much of the potential future problem lies in the proliferation of different kinds of artificial radiation sources, and in particular from medical applications and the nuclear industry.
In medical sciences in particular, changes in tissue weighting factors proposed by ICRP (Clarke, 1991) may make for important changes in exposure assessments, due to the proposed increase in specific values for abdominal organs. This would render average medical exposures nearly as significant as natural exposures for humans, and create even more pressure for appropriate monitoring and control of these exposures. New recommendations have also recently been published for radon (ICRP, 1993).
Finally, estimates of average exposure of humans cannot reflect accurately the wide-ranging exposure of individuals. A margin of safety must be incorporated to ensure that particular groups, such as miners, airline staff, and those living in high-radon areas, are not exposed beyond the recommended control levels (see also WHO, in press).