J. TUZO WILSON November 18 1961


J. TUZO WILSON November 18 1961


A quick, reliable guide to the little scientists now know about

Radioactive fallout 500 times greater than “normal” descended on vast tracts of Canada during the third week of September, 1961—a few weeks ago. The federal health department scientists who made the measurements at twentyfour air monitoring stations across the country almost certainly compared the September fallout level with “normal” levels uneasily, for they really had no idea what normal levels might be. They began taking measurements only in 1959, and what their report actually meant was that September, 1961, fallout levels were 500 times greater than the average of the measurements they made between 1959 and late 1961.

It was in 1958, as scientists and many other people vividly remember, that the U.S. and the U.S.S.R. declared their short-lived moratorium on nuclear testing. How extensively their previous tests had already contaminated the air is any scientist’s guess. In any case, even before the Canadian scientists had relayed their late September measurements to the people of Canada, the Russians had already detonated subsequent nuclear devices far dirtier than any that contributed to the abnormal rain that fell during the third week of September.

While the Canadian scientists actually had no idea how much fallout is normal, they had learned something during the months since the Russians resumed nuclear testing. They had learned that Hudson Bay seems to be a funnel for fallout; winds that bring the nuclear refuse across the Arctic are channeled by the Bay into a stream that brings them down on the most heavily populated parts of Canada.

At Toronto, the fallout level during the third week of September was an average of 144.4 disintegrations per minute per cubic meter of air. A UN report says that a daily average of 6.6 of these units is the highest level that can be said to be completely free of medical risk. The Toronto readings, however, are not as alarming as they may

seem, Health and Welfare Minister Waldo Monteith has said. They will not become a health hazard, Mr. Monteith and radiation experts in his department say, unless they are maintained for a full year. These experts explain that a high proportion of short-lived radioactive elements are contributing to the Canadian fallout level this autumn. The real danger, they say, lies only in long-lived elements like strontium-90 and cesium-137. While the department is testing for the presence of these elements, the results of the tests will not be known for several months. Meanwhile, according to a department announcement, “the short-lived particles disintegrate rapidly. The long-lived ones remain in the air, and eventually find their way into human bones.” Like almost everything that has been said on this subject by government scientists of whatever country, the things the federal health department scientists say are evidently meant to be reassuring. In fact, there has been strikingly little said or written about fallout that has not obviously been intended to ease people’s minds about nuclear debris, or to inflame their consciences over it—almost nothing, in speech or print, that has set out simply to place as many facts as are known about fallout in front of as many people as care to read them. Such an account is included in a book by J. Tuzo Wilson, IGY : The Year of the New Moons, which will be published soon by Longmans Green. Professor Wilson is an unusual scientist: an explorer and mountain climber, one of the architects of a current theory of continent formation, president of the International Union of Geodesy and Geophysics during the International Geophysical Year. During the IGY Professor Wilson traveled over 100,000 miles to observe the work of participating scientists. One aspect of this work was to establish most of the facts that are now reliably known about fallout. An account of these facts is one chapter in Professor Wilson’s book; that chapter is published HERE.-THE EDITORS


Professor of Geophysics, University of Toronto

THE RADIOACTIVE ISOTOPES included in the study of nuclear radiation are formed in four ways. Some were formed at the time of creation, along with other elements; some are produced by cosmic-ray bombardment; some are made artificially in nuclear reactors; and some are generated in atomic explosions.

About a dozen isotopes of different elements in nature have been found to be feebly radioactive, and to decay very slowly into isotopes which are stable. The continued existence of some of these radioactive isotopes in spite of the fact that they are constantly wasting away

is only possible because they decay very slowly. Nevertheless, the implication is that they were formed at a not infinitely remote time, probably about five billion years ago. This is now considered to be the probable date of the creation of all the elements in the solar system, although the elements in stars other than the sun and in other galaxies may have been formed at different, unknown, times. The solar system. including the earth, took shape out of these elements at a slightly later date, about four and a half billion years ago.

Of the dozen long-lived isotopes found in nature, five are quantitatively important: uranium-238, uranium-235, thorium-232, rubidium-87, and potassium-40. These isotopes arc widely scattered in small concentrations and occur almost everywhere—in rocks, soil, water

and air, and even in human beings. None of these elements is dangerous to handle, and because the atmosphere shields us from most of the radiation emanating from outer space, over three quarters of all the radiation which affects us comes, and has always come, from these five isotopes. These isotopes arc useful for determining the time when rocks were deposited, and provide the chief clocks for our time scale of the earth.

One of them, potassium-40, is the source of most of the radiation that humans and other animals receive because it is a constituent of their bodies. Uranium and thorium do not occur in normal tissues in appreciable amounts, but uranium, one of the elements formed by the decay of uranium, is found in small quanti-

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Soviet bomb fallout did not settle evenly. It fell fast and heavily on the northern hemisphere

Natural radiation is also produced by the unstable isotopes produced by cosmicray bombardment. Cosmic rays are a form of radiation coming from outer space which can hit and disrupt other atoms, creating radioactive isotopes which decay later. The following are the isotopes so far discovered, with their average duration: beryllium-10. 2.700.000 years; carbon-14, 5.600 years; silicon-32. 700 years: hydrogen-3 (tritium). 12.5 years; sodium-22, 2.6 years; sulphur-35, 87 days; beryllium-7, 53 days; phosphorous-33. 25 days: phosphorous-32. 14 days; chlorine-39, one hour.

The most important isotope produced by cosmic rays is carbon-14. (According to J. R. Arnold and E. A. Martcll. it causes only about one percent of the feeble internal radioactivity present in humans, of which potassium-40 is the chief contributor.) In the whole atmosphere, in oceans and in living matter there are about 100 tons of this isotope, mostly in the form of carbon dioxide gas dissolved in the oceans. There is about one ton in the atmosphere. Next in importance among products of cosmic rays is tritium, as the highly radioactive isotope, hydrogen-3, is called. Before atomic bombs were exploded, there were only about twenty pounds of this on earth. The fact that such a small quantity could be detected even though it is thoroughly mixed with the atmosphere is an indication of the powerful nature of radioactive poisons.

Until 1932 all radioactivity was formed naturally, in one of the above two ways. In that year Frédéric and Irène JoliotCurie produced the first artificial radioisotopes. but they were not created in large

numbers until the first nuclear explosion in 1945. The third kind of nuclear radiation. then, is that artificially created in controlled laboratory machines and in nuclear reactors. Many of the radioactive isotopes so formed, like cobalt-60, arc useful for the treatment of disease and for industrial

purposes. Some of them are suitable for release in the sea or air as markers and can be used like a dye to tag bodies of water or masses of air so that movements and rates of dispersion can be measured.

Had these three been the only kinds of nuclear radiation, their study might not have been included in the IGY. but there is a fourth kind—that resulting from explosion of nuclear bombs. The descent to earth of the abundant artificial radioactive

isotopes created in atomic explosions is known as fallout. The isotopes generally fall in clouds of very fine dust.

Although fallout was already under investigation. much of the work was secret. During the IGY additional data were collected and made available to the public. In spite of the political implications, no less than forty countries announced they would participate in the program, and data from over half of them, including some results from communist countries, had reached World Data Centre A in Washington by the end of 1959. The U.S.S.R. did not participate in this program.

The enormous controversy that has been associated with discussions of fallout is of course a result of its potentially lethal effects. But these effects can only be guarded against if the physical phenomena with which fallout is associated are understood.

After any nuclear explosion tiny solid particles are carried up into the atmosphere. This fine dust is radioactive, for it contains about a hundred different radioactive isotopes, products of nuclear fission, created during the explosion. Their abundance can tell much about the bomb that made them. If the explosion takes place on the ground, the radioactive isotopes are more numerous than if it takes place high in the air or over water. If the explosion is large, such as that caused by a megaton H-bomb, a heavy fallout occurs near the site during the first few hours, but a fireball carries the rest of the isotopes into the stratosphere, which begins at a height of from 30,000 to 55.000 feet. The particles then are above the sources of rainfall. At one time it was thought that they would

remain there as long as five years; this is now less certain.

If, on the other hand, the explosion is smaller, such as one from a kiloton Abomb. then the fireball does not rise as high but remains in the lower atmosphere, or troposphere, and the radioactive dust is all brought down with rain and snow in a few months. Due to the nature of tropospheric circulation, little of this fallout crosses the equator; most of it comes down in the hemisphere in which the explosion took place. Stratospheric fallout is more significant because, being generated by large explosions, it is more abundant, and because it may reach all parts of the world.

The IGY program concentrated on collecting data about fallout due to tiny solid particles. This was done either by exposing sheets of gummed paper to the air and measuring their radioactivity, or by pumping measured volumes of air through suitable filters to trap the particles; the apparatus we saw in operation on Mauna Loa was an example of this procedure. One unexpected result of these studies has been a great increase in our knowledge of the circulation of the upper air.

At first it was supposed that the main fallout from large explosions would settle down from the stratosphere slowly and at a uniform rate all over the world. But it settled much more rapidly than had been envisaged. In particular, large and small Soviet test explosions in mid-latitudes caused much heavier falls in the northern than in the southern hemisphere. On the other hand, the fallout from large American and British explosions in the equatorial

regions came down at a slower rate. It was then realized that in mid-latitudes mixing might occur between the troposphere and stratosphere in the vicinity of the strong currents known as jet-streams. Such mixing in mid-latitudes would explain the faster rate of fallout from Soviet explosions in Siberia, and the absence of mixing over the equator would account for the slower rate of fallout there. The matter was explored further in experiments during the last American tests. Tungsten and rhodium were incorporated into the bombs to produce unusual radioisotopes, and the particles thus tagged confirmed earlier theories.

Other measurements have shown that nuclear explosions have doubled the amount of carbon-14 in the atmosphere from one to two tons, and increased the amount of tritium from twenty to 100 pounds. But the effects of these increases are not yet serious: the total radiation so produced is still far less than the radioactivity of natural uranium, thorium, and potassium.

Dangerous for a lifetime

The particular danger to humans of fallout is that out of about a hundred products of fission, four tend to be trapped in the human body. Hence, they can easily become concentrated in amounts sufficient to produce dangerous radioactive radiation within humans. Two of these radioactive isotopes, iodine-131 and barium-140. have short lives and are only dangerous for a few weeks or months after an explosion. Strontium-90 and cesium-137. however, collect in bones and flesh respectively and remain dangerous for periods comparable with the span of human life.

Fallout is not entirely new: it is an addition to a natural phenomenon that has always existed. We must consider separately the dangers to living creatures and to future generations. We must also distinguish between the dangers of the present situation, which arc not generally regarded as serious, and the dangers that may arise if nuclear explosions and atomic waste are allowed to continue to contaminate the air and water. Natural radioactivity is not dangerous, but exposure to massive doses of radiation, such as have been released in a few accidents in nuclear physics laboratories. is quickly lethal. Between these extremes is a very wide range, and we have

not yet had enough experience to fix the safety limits.

Natural radiation causes genetic changes by occasionally damaging the very complex molecules through which characteristics are transmitted. Most genetic changes, or mutations, are harmful, and we know that natural radiation already creates some imperfect offspring. But no one knows the extent to which these effects would be increased by more radioactivity. If a certain increase in radioactivity produces congenital defects in one birth in a million, or cancer in one person in a million, then, some have argued, this is not very important and the necessity of testing weapons for defense purposes justifies the increased suffering. Others emphasize that there are 2.500 million people in the world and that to cause 2.500 more children to be deformed or 2,500 more people to suffer from cancer is intolerable.

Biologists are now hard at work on these problems, for it is certain that we need more information not only in the event of other nuclear explosions, but also in order to be able to control the hazards produced by natural radioactivity and by the operation of nuclear reactors for peaceful uses.

In July, 1959. a study by the New York Operations Office of the United States Atomic Energy Commission was published. comparing the effects of the existing level of fallout with natural radioactivity. The report states that “the maximum foreseeable dose from strontium-90 in the New York area is thereby estimated to be about five percent of the dose due to natural radioactivity.” This does not sound like a very serious increase until one realizes that the distribution is not uniform. J. L. Kulp has made extensive studies of the variations and concludes that whereas many' children will be subject to less than the average increase of five percent, some will be exposed to more, and a few to much more, as much as double the natural amount. According to Kulp. even if no more atomic bombs are exploded, the dose in most children will continue to increase until 1966, as they absorb more and more of the strontium-90 already formed. After that time, the natural decay of strontium90 will exceed the amount eaten and absorbed. The present situation, it would seem, is not alarming, but additional explosions would be certain to increase the hazard, unless they were confined below ground. ★