§ 1. The debris in the stratosphere resulting from test explosions of nuclear weapons contains radioactive materials, of which strontium 90 merits the most immediate attention. Their effect on the body has been discussed in authoritative reports, notably in the Medical Research Council's report entitled "The Hazards to Man of Nuclear and Allied Radiations" (Cmd. 9780, June, 1956) and in the report issued in August, 1958, by the United Nations Scientific Committee on the Effects of Atomic Radiation. The purpose of the present statement is to review the situation and to comment on inferences that have been made concerning the residence time of radioactive materials in the stratosphere.
§ Rate of Deposition of Fall-out
§ 2. For several years, authorities in the United Kingdom and in many other countries have been making regular measurements of the rate of deposition of strontium 90 and other isotopes. Measurements since May, 1958, have indicated a general rise, involving an approximate doubling of the rate of deposition in rainfall in the United Kingdom.
§ 3. The estimation of the fall-out likely to occur in the future depends, amongst other factors. on an assessment of the total amount of radioactivity in the "stratospheric reservoir". Such an assessment can be attempted in a number of ways, for example by:—
- (a) direct measurement of the quantity of debris in the stratosphere. The United States Department of Defence has carried out an extensive series of investigations, using balloon-carried measuring equipment for this purpose; but the results so far obtained are difficult to interpret and the method is not yet considered to be satisfactory.
- (b) estimating the amount of fission products which have been injected into the stratosphere by nuclear explosions, and measuring the world-wide deposition which has already taken place. It is considered that knowledge of these factors is insufficiently precise to give reliable estimates.
- (c) measuring the relative proportions of shorter and longer-lived fission products in fall-out at the time of deposition, so as to give a broad indication of the average length of time of residence of fission products in the stratosphere, and measuring the fission product fall-out rate. The estimate of the size of the reservoir obtained in this way is directly proportional to the assumed length of the residence time.
4. The Report of the United Nations Scientific Committee on the Effects of Atomic Radiation contains, in paragraph 97 of Annex D, the following observations on this subject:Analysis of fall-out material has shown that strontium 90 can remain in the stratosphere for many years before being deposited on the earth. The depletion mechanism of the stratospheric reservoir is not yet adequately known. It has been estimated from measurement of fall-out rate and stratospheric content that the annual strontium 90 fall-out is about 12 per cent. of the stratospheric content. This annual fraction corresponds to a mean residence time of about 8 years which is in agreement with a value of 10 plus or minus 5 years derived from unpublished data. The concept of a constant fractional removal per year of the stratospheric content is inconsistent with meteorological principle. However, nothing better can be offered at present. If the concept is to be used, a mean residence time of about 5 years appears to be the best value and a reasonable upper limit is about 10 years. The latter value has been used in the calculations to follow, since it tends to yield results on the pessimistic side.
§ 5. It has recently been suggested, notably by the U.S. Department of Defence, on the basis of an interpretation of their measurements, that the residence time of radioactive materials in the stratosphere has been overestimated. If this were true, it would confirm that the estimates made by the United Nations Scientific Committee were unnecessarily pessimistic, since there would be less of this material at present in the stratosphere than they estimated. There is some evidence to suggest that debris may leave the stratosphere more quickly from explosions in high latitudes than from explosions nearer the Equator.
§ 6. A shorter residence time would mean that the radioactivity would have had less time to decay before the fall-out was deposited; but strontium 90 decays so slowly that the difference would be very small. There would also be an increased amount of shorter-lived fission products in the fall-out, but the increase in the close to body tissues from these would also be relatively small.
§ 7. Once radioactive materials have left the stratosphere and entered the troposphere the local rate of deposition depends very largely on rainfall. Heavy rain tends to produce enhanced fall-out. This is certainly one of the factors responsible for the higher recorded measurements of fall-out in the summer of 1958. However, in subsequent months in which the rainfall in the United Kingdom was more nearly normal, the data so far available indicate that the rate continued high. There is evidence from the nature of the radioactive 1102 material deposited that this is likely to have been due to recent tests, presumably those conducted in the Autumn by the U.S.S.R., the fall-out from which possibly occurred more rapidly because they took place in high latitudes. The so-called "Argus" experiments conducted about the same time by the United States are understood to have been three low yield nuclear explosions at great heights; they therefore resulted in only a very small addition to the radioactivity already in the stratosphere
§ Monitoring of strontium 90 in diet
§ 8. In this country, regular measurements are made of strontium 90 in food, milk, drinking water and air, and in soil and vegetation. We thus have much information on the rate at which strontium 90 enters the diet and reaches bone. The investigations of radioactivity in food were considerably expanded in 1958. Milk receives special attention because much of the total strontium 90 in diet comes from dairy produce. Representative samples are collected regularly from over two hundred depots handling some forty per cent. of the total milk production in the United Kingdom. So far as is known, no equally comprehensive scheme is in operation in any other country. Other foods which introduce considerably less strontium 90 into the diet are also examined; for example, cereals, both imported and home-produced, and vegetables.
§ 9. The measurements of very low levels of strontium 90 involve lengthy and complex processes of analysis, using specialised techniques and facilities. The results of the food survey for 1958 are now being prepared for publication. They indicate that, although the extent of contamination varies between different parts of the country, the amounts of strontium 90 in food and drinking water are well below those which are likely to give rise to concentrations in human bone which, in the words of the M.R.C. report would require "immediate consideration".
§ 10. The quantity of strontium 90 in diet at the present time depends mainly on what has been deposited in the preceding few months, rather than on the total deposition since fallout began. Most of the strontium 90 taken up by cattle is that freshly deposited on herbage before it has had a chance to be washed into the soil, The quantity of strontium 90 in human diet in the United Kingdom rose much less in 1958 than did the rate of deposition; this may have been largely due to heavy rain washing strontium 90 from herbage into the soil. Once strontium 90 has entered the soil it may still be absorbed through the roots of grass and other crops, but the amount at present taken up in this way is relatively small.
§ 11. If the enhanced rate of fall-out which began in the middle of 1958 continues in 1959, an increase in the strontium 90 content of diet is to be expected. Because the contamination of food is dependent on the rate of deposition in the immediate past, no precise prediction of the future situation can be made; but the evidence now available leads to the view that the concentration in human bone of strontium 90 attributable to what has already been 1103 injected into the stratosphere is likely to remain below that at which immediate consideration would be necessary.
§ Strontium 90 in human bone
§ 12. A recent United Kingdom report has given the results of further measurements of strontium 90 in samples of human (mainly children's) bones from the first few months of 1958. The measurements on bones of 59 children (stillborn or dying under the age of 5 from various causes) showed only a slight increase as compared with corresponding results for 1957. These results accord with what was to be expected, since the level of strontium 90 in the diet in the relevant period had increased very little as compared with the previous year.
§ 13. The irradiation of human bone and bone marrow attributable to strontium 90 is at present small in relation to that from the natural radioactivity to which they are exposed; the variation in natural radiation levels from place to place in the U.K. (and elsewhere) is far greater than the additional radiation likely to arise from tests already conducted.
§ Other radioactive materials
§ 14. Attention is given also to other radioactive materials, for example caesium 137, plutonium and carbon 14. The extent of the effort devoted to the study of these substances is kept under regular review in the light of the changing fall-out situation.
§ 15. Results of measurements of caesium 137 in water, milk and the human body in the United Kingdom have been published. Caesium 137 is not readily taken up from the soil by plants and, unlike strontium 90, is not retained in the body for long periods. Its biological significance arises from genetic effects, the magnitude of which depends on the gonad dose incurred. This has been discussed in the report of the United Nations Scientific Committee on the Effects of Atomic Radiation (Chapters III, VI and VII). The dose to the gonads delivered by caesium 137 is small in relation to variations in natural radiation between different parts of this country.
§ 16. The incorporation into the body of plutonium from fall-out is small in importance compared with that of strontium 90, since plutonium is only very slightly absorbed from food. Both materials can be absorbed through inhalation, but this is a minor source of intake.
§ 17. For several years, measurements have been made of the concentration of carbon 14 in air and in biological material. Carbon 14 occurs naturally from the effects of cosmic radiation, but some also comes from weapon tests. This subject has been under consideration by the United Nations Scientific Committee (Document A/AC.82/INF.3, 23rd July, 1958). The concentration of carbon 14 is being kept under review because of its long persistence, notwithstanding the very low rate of irradiation that it causes in the human body.