Zbigniew Jaworowski

Central Laboratory for Radiological Protection

Ul. Konwaliowa 7,
03-194 Warszawa, Poland jaworo@clor.waw.pl

After ionizing radiation and radioactivity were discovered at the end of the 19^th century their social status has oscillated between enthusiastic acceptance and rejection. This was in concurrence with recognition of their three basic aspects: 1) usefulness for medical applications and for technical and scientific aims; 2) beneficial effects of their low levels; and 3) harmful effects of high levels. In the first part of the 20^th century the acceptance prevailed, in the second the rejection. The change of the public mood, that occurred rather abruptly after the World War II, was not due to discovery of some new danger of radiation, but was caused by political and social reasons, not related to real radiation effects.

The possibility of the use of ionizing radiation for medical diagnostics was first demonstrated by W. K. Roentgen, one month after his discovery, by publishing in Nature in January, 1896 an x-ray photograph of the hand of his wife. In 1902 Pierre Curie together with two physicians, C. Balthazard and V. Bonchard, discovered that radium rays are efficient in the cancer therapy. The theoretical basis for this therapy was posed in 1906 by as the result of their experiments with rats. They coined the following law: " X-rays are more effective on cells which have a greater reproductive activity". "From this law, they commented perhaps too optimistically, it is easy to understand that roentgen radiation destroys tumors without destroying healthy tissues".

The beneficial or hormetic effects of low doses of ionizing radiation were found two years after Roentgen and, independently, A.H. Beckerel announced the discovery of ionizing radiation. First such effects in algae were reported by Atkinson . He noticed an increased growth rate of blue green algae exposed to x-rays. This particular observation was followed by thousands of publications on hormetic effects, and it was repeated and confirmed 82 years later .

That ionizing radiation can be hazardous for man was first announced in a German Medical Weekly . The early students and users of radiation voluntarily or unknowingly exposed themselves to high radiation doses. Among the pioneers of radiation and radioactivity from 23 countries, scientists, physicists, medical doctors, nurses, and x-ray technicians, about 100 persons died by 1922, and 406 died until 1992, with afflictions that could be related to radiation. The first fatal victim of ionizing radiation was in 1900 a German engineer, F. Clausen. The names of victims are all recorded in "Book of Honor of Roentgenologists of All Nations", published in Berlin in 1992 . This experience sounded alarm, and the need for protection against high doses of radiation was quite early realized.

In the 1920s the concept of "tolerance dose" was introduced, defined as a fraction of a dose causing reddening of the skin. This fraction corresponded originally to an annual dose (in modern units) of 700 mSv, in 1936 it was reduced to 350 mSv, and in 1941 to 70 mSv. The concept of tolerance dose, which was effectively a statement of threshold, served as the basis for radiation protection standards for three decades , until in 1959 International Commission on Radiological Protection based its recommendations on the linear no-threshold principle (LNT) . Introducing the LNT principle to radiological protection was stimulated by an undue concern in the 1950s of disastrous genetic effects on the human population of ionizing radiation produced by man. Quite often at that time one could see in the literature geneticists' statements on radiation similar to the following one: "...we have reached a stage where human mistakes can have a more disastrous effect than ever before in our history - because such mistakes may drastically change the course of man's biological evolution" . The later years, and especially the observations of the progeny of survivors of nuclear attacks on Hiroshima and Nagasaki, demonstrated that this concern was an overreaction, tuned with strong emotions, evoked by the menace of nuclear war. However, the feelings are not the best basis for regulations. Professor W.V. Mayneord, the late chairman of the ICRP Committee IV, commented on using LNT as the regulatory basis: " I have always felt that the argument that because at higher values of dose an observed effect is proportional to dose, than at very low doses there is necessarily some 'effect' of dose, however small, is nonsense" . Mayneord's worry about the values of ICRP recommendations was "the weakness of the biological and medical foundations coupled with a most impressive numerical façade".

During the past several decades there was a tendency to decrease the standards of radiation protection to ever lower values, which in the 1980s and the 1990s reached 20 mSv per year for occupationally exposed people and 1 mSv per year for general population. For an individual, who receives no direct benefit from a source of radiation, a maximum dose of 0.3 mSv in a year was recently proposed , and for some instances an exemption level of 0.01 mSv per year . Justification for such low levels is difficult to imagine as no one has been identifiably injured by radiation while exposed within the hundreds or thousands of times higher standards, set by the ICRP in the 1920s and the 1930s . The life expectancy of the survivors of nuclear attacks on Hiroshima and Nagasaki was found higher than in control groups , no adverse genetic effects were found in the progeny of survivors . There is also an ample evidence of beneficial effects of low doses of radiation in people occupationally, medically or naturally exposed to doses much higher than the current radiation protection standards (see e.g. ). For adherence to regulations based on such low standards society pays hundreds of billion of dollars, with no detectable benefits. Each human life hypothetically saved by implementing these regulations costs about $2.5 billion . Such spending is morally questionable, as (1) the limited resources of the society are spent on prevention of an imaginary harm, instead on real advancement of health, and (2) because low radiation doses are beneficial for the body. Because of these two reasons, such expenditures may have actually an adverse effect on the population.

In this presentation I wish to compare the levels of radioactivity and radiation in various environmental situations, influenced by natural processes and human practices. Such comparison may help to see radiation standards in a realistic perspective.

RADIOACTIVITY

When the life began some three and half billion years ago, the natural level of ionizing radiation at the planet's surface was about three to five times higher than it is now . At that time, the long-lived potassium-40, uranium-238, and thorium-232 had not yet decayed to their current levels. Their content in the contemporary Earth's crust is still quite high, and it is responsible for the highest radiation exposure of every living being. One ton of average soil contains about 1.3 x 10⁶ Bq of potassium-40, thorium-232 and uranium-238 and their daughters. This corresponds to 2.6 x 10¹⁵ Bq per cubic kilometer (Table 1). Decay of these natural radionuclides present in 1 kilometer thick soil layer produces 8000 calories per square meter annually .

TABLE 1. Average activity (Bq) of the whole chains of natural radionuclides in the continental crust and soil, and total activity of wastes from nuclear power. After and .

We can compare the natural, extremely long-lived activity of potassium-40 (T1/2 = 1.28 x 10⁹ years), thorium-232 ( T1/2 = 1.4 x 10¹⁰ years) and uranium-238 (T1/2 = 4.47 x 10⁹ years) in soil, with activity of much less long-lived radioactive wastes from the nuclear power cycle. In 1997 the total annual production of electricity in nuclear reactors was 254.5 GW . Assuming that annual production of wastes in nuclear power reactors is 8.8 x 10⁹ Bq per MWe , the global production of radioactive wastes from this source amounts to 2.2 x 10¹⁵ Bq per year, with the longest lived plutonium-244 (T1/2 = 8.26 x 10⁷ years). Such amount of average natural activity is contained in a relatively small block of soil 0.9 by 0.9 km wide and 1 km deep. None of the man-made component of these wastes has appreciably higher radiotoxicity (expressed as Sv/Bq) than the natural thorium-232 .

No special barriers prevent the natural radionuclides from migration from, say, a depth of 1 km to the surface of the ground. They can be transported by mechanical actions, or move in solution. Thorium is not susceptible to leaching under most geological conditions and its principal mode of occurrence is in refractory minerals. Uranium is highly mobile, and may migrate with ground water to distances of several tens of kilometers or more. Radium is mobile in sulfate-free neutral or acidic solutions. The average volcanic injections of alpha emitting ²¹⁰Po into the global atmosphere during non-eruptive activity amount to about 5 x 10¹⁵ Bq per year, i.e., almost twice as much as the 1997 production of radioactive wastes from nuclear power reactors (Table 2). Geochemical differences between uranium, thorium and radium may lead to drastic changes in their radioactive equilibrium .

In contrast, for man-made radioactive wastes many effective, sophisticated barriers are provided in deep underground depositories. At a first glance, one can see in Table 1 that it would take about 3 billion years of such a global production of wastes from nuclear power reactors as in 1997, to double the total activity of natural radionuclides in the Earth's continental crust. The activity of wastes accumulated until the end of 2000 from the whole global civilian nuclear fuel cycle is much greater. It amounts to 200 000 tones of "heavy metals", which after 10 years cooling corresponds to activity of about 7 x 10²¹ Bq . Disposal of high level wastes and spent fuel in geologic repositories may result in doses to population that do not begin to accumulate until well after 500 years . After 500 years activity of all high level wastes accumulated until now will decrease to about 7.4 x 10¹⁵ Bq , corresponding to natural activity contained in a block of soil about 1.7 by 1.7 km wide and 1 km deep, and consisting about 1 billionth part of the natural activity present in the Earth's crust.

It is interesting to compare the annual flows into the global atmosphere of radionuclides from natural sources with flows from nuclear weapon explosions and production, nuclear power cycle, coal burning, and the Chernobyl catastrophe. Except for the Chernobyl catastrophe, the flows of nine radionuclides, with the greatest potential impact on public health, were compared by . Here I present only the highest flows of activity from particular sources (Table 2). To account for various energy emissions by different nuclides, the flows of radiation energy are also given.

Table 2 demonstrates that the flow of activity from the natural sources into the global atmosphere is 2 to 5 orders of magnitude higher than from particular man-made sources, and the flow of radiant energy is 3 to 5 orders of magnitude higher. It appears that at the global scale, the anthropogenic emissions of radionuclides and their impact are dwarfed by the natural ones. In the case of nuclear power the highest flow of activity is that of ³H (5.6 x 10¹⁶ Bq per year), but the highest flow of radiation energy is that of ²²²Rn, because of its decay energy (5.5905 MeV) higher by a factor of 300 than the decay energy of ³H; ²²²Rn activity flow is only 1.5 x 10¹⁶ Bq per year.

TABLE 2. Most important annual flows of activity of radionuclides and of their radiation energy into the global atmosphere.

This might not necessarily be the case at the local scale, especially in military practices. The widest civilian contamination of the ground surface occurred after the Chernobyl accident. According to data in , on the first day after this, probably greatest possible, civilian nuclear catastrophe, a high ground contamination consisting of two patches with a lethal dose rate of 1 Gy per hour, covered in an uninhabited location an area of about 0.5 km², and reached a distance of 1.8 km from the burning nuclear reactor. Several hundred meters outside the 1 Gy isolines the dose rate dropped by 2 orders of magnitude (Figure 1). Fortunately, this situation did not pose immediate danger for general population. This can be compared with an isoline of 1 Gy per hour after a 10 MT surface nuclear explosion, reaching (at calm weather) to a distance of 440 km , and covering with lethal fallout tens of thousands square kilometers. In the localities remote from the Chernobyl power station deposition of radionuclides was much lower, and did not reach levels which could led to acute radiation health effects, or to chronic effects, such as genetic disturbances, leukemia or solid cancers . The only exception might be the increase of registration of thyroid cancers in children and adults . Until now only one girl died from radiation-related thyroid cancer after the Chernobyl accident (see also However, the increase of registration of thyroid cancers may be a result of causes other than Chernobyl radiation, most probably among them being the screening effect.

RADIATION DOSES

The global distribution of radionuclides in the biosphere, and the use of radiation are reflected in the radiation doses received by the population from various sources. During the past several decades UNSCEAR was collecting data on doses from radionuclides in the environment and from their medical and other uses. Although far from being complete, the UNSCEAR compilation of data is probably the most comprehensive one, and enable estimation of the temporal changes in average annual radiation doses received by the global population from particular sources. In its reports to the General Assembly of the United Nations UNSCEAR refrained from presenting in graphic form the results of such estimations expressed in units of rems or sieverts. I present them in Figure 2, based on internal documents of UNSCEAR (for a part of medical and natural exposure), and on the UNSCEAR data published or approved for publication .

The highest annual radiation dose is received from natural sources. The average value for the external and internal exposure of the global population currently estimated by UNSCEAR is 2.4 mSv per year. The natural dose ranges widely in particular regions of the world. UNSCEAR estimate for a part of East Asia and part of Europe suggest that a 39% fraction of the population receives annual doses from terrestrial gamma radiation lower than 1.5 mSv, 30% doses of 1.5 - 1.99 mSv, 18% doses of 2.0 - 2.99 mSv, 6.3% doses of 3.0 - 3.99 mSv, and only 0.4% doses higher than 10 mSv. However, this estimate is not covering the areas of high natural radiation background, such as in Iran, India or Brazil. For example, in the State of Kerala, India the annual radiation dose reaches up to 76.4 mGy (lifetime dose of >5 Gy), and it is not associated with an increased cancer incidence or cytogenetic aberrations . In the area of Araxa, Brazil (74 000 inhabitants) the average annual radiation dose is 2800 mGy. In the city of Ramsar, Iran the absorbed dose rate in air reaches up to 17 500 mGy per year . In some parts of Ramsar people are living in houses where the annual radiation dose is up to about 700 mGy , what is similar to the value of the tolerance dose from the 1920s, and corresponds to a lifetime dose of about 50 Gy. In the area of Ramsar people are exposed to so high radiation levels since several generations. The cytogenetic studies have shown differences between these people and the controls, but incidence of cancers and leukemia was not increased.

Compared with the apparently non-harmful annual doses in the high natural radiation areas, the average doses received by the global population from man-made sources seem to be of no importance. This statement if valid also for about 4.8 million people leaving in areas contaminated by the local fallout from the Chernobyl accident , where the average annual radiation dose is about 6 mSv. The highest average dose to the global population from Chernobyl fallout of 0.045 mSv was in 1986. The global exposure from medical diagnostics was rapidly growing from the 1950s, probably due to steadily increasing access to x-ray technology in the developing countries. Since the 1980s this exposure seem to be stabilized. Even at the heyday of the nuclear weapon tests at the beginning of the 1960s, the average global exposure from this source (0.113 mSv in 1963) was much smaller than the medical exposure. The exposure from the civilian nuclear power cycle was steadily growing since 1955 reaching in 2000 a trifle value of 0.002 mSv.

CONCLUSIONS

Man's contribution to the contents and flows of radionuclides and of radiation energy in the particular compartments of the environment consist a tiny fraction of the natural contribution. In some areas in the world the natural radiation doses to man and to other biota are many hundreds times higher than the currently accepted dose limit for general population. No adverse health effects were found in humans, animals and plants in these areas. In the future reconstruction of the edifice of radiation protection, that now stands on the abstract LNT foundations, down-to-earth approach will be necessary, taking into account apparently safe chronic doses in the high natural radiation areas, rather than the statistical variation around an average global value. It seems, therefore, that studies of these areas deserve a special attention and support in the coming years.

The twentieth century witnessed the dawn of man-made ionizing radiation and radioactivity, the use of the highest human knowledge to kill the people in Hiroshima and Nagasaki, and the greatest nuclear catastrophe in Chernobyl. This catastrophe claimed only about 30 fatal occupational victims and probably none among the public, proving that nuclear energy is a comparatively save means of power production. It was also found that high semi-acute doses of radiation can cure cancers, and that small chronic doses of radiation are beneficial for health. It seems that discovery of "new" radiation and of radioactivity, that opened the gate for unlimited energy, has similar meaning for man as discovery of fire some 500 000 years ago. Fire made man the most ubiquitous species and enabled expansion of life outside the Earth's biosphere. Our ancestors were mentally adapting to fire for many thousands of years, sometimes even deifying it. It seems that one century was not enough for such adaptation to ionizing radiation and radioactivity. But there is some hope - all things go now faster than in the past.

ACKNOWLEDGEMENTS

Thanks are due to Prof. S. Chwaszczewski, Prof. L. Dobrzynski and Dr. A. Strupczewski for helpful discussions and assistance.

REFERENCES

FIGURE 1. Measured exposure rates in air on 26 April 1986 in the local area of the Chernobyl reactor. Units of isolines are R h^-1. After

FIGURE 2. Annual trends in average individual global radiation doses. Based on

TEHRAN, IRAN OCTOBER 18-20, 2000