Man and Strontium 90

Just how dangerous to the human race is the radioactive fallout from nuclear-weapons tests? The subject is enormously complex, and to understand all aspects of it requires expert knowledge of many sciences, including genetics and medicine as well as physics. A beginning is being made to answer the question.

To find out how much damage mankind should expect from strontium 90, one of the fallout isotopes, the U.S. Atomic Energy Commission financed a study by Drs. J. Laurence Kulp, Walter R. Eckelmann and Arthur R. Schulert of Columbia's Lament Geological Observatory. Last week the team made a report in Science.

Strontium 90 is probably the most-feared fission product. Chemically similar to calcium, it is absorbed along with calcium by the human system and deposited in the bones, where its persistent radioactivity (half-life 28 years) may cause cancer. Collecting 500 samples of fresh human bone from widely separated parts of the world, the Columbia men analyzed them delicately and concluded that "at the present time, strontium 90 can be found in all human beings, regardless of age or geographic location s . ." The amount is not large. Averaging all the results together, they reckoned that the human race now has .12 micromicrocuries* of strontium 90 for each gram of body calcium. This is about one ten-thousandth of "the presently accepted maximum permissible concentration."

In their small sample, however, the researchers found a good deal of variation between individuals. The rapidly growing bones of young children averaged three to four times as much strontium 90 as the bones of adults. Even certain adults had ten times more than others. One sample of adult shinbone from Vancouver, B.C. had 75 times as much as the average.

Bomb to Bone. The Columbia men did more than analyze bones; they also traced the path of strontium 90 from the nuclear reaction to the human body. Most of it was produced by the biggest thermonuclear explosions, U.S. and Soviet, and most of it rose high into the stratosphere. The particles are so small that they fall very slowly until they reach the lower atmosphere. Then rain washes them quickly down to the surface. This process takes time; strontium 90 is now spread all over the earth, with somewhat less in the Southern than in the Northern Hemisphere.

When any kind of strontium gets into soil, it is taken up by plants as if it were calcium. Since plants do not like it as much as calcium, those grown in calcium-deficient soil generally contain more strontium 90 than plants that get plenty of calcium. It is not established, however, that all plants behave in this way.

Cows & Grass. When animals, including milk cows, eat plants containing strontium 90, they reject it selectively in favor of calcium. Therefore milk contains less strontium 90 in proportion to calcium than the grass or alfalfa that the cows eat. This means that humans who get most of their calcium from milk will collect less strontium 90 than people who get their calcium direct from vegetable sources.

Most of the strontium 90 created by past bomb tests is still in the stratosphere or in the soil, but it will tend to move for years into human bones. If no more large tests are made, the Columbia men figure, the average human bone should contain, by 1970, about 1.3 micromicrocuries of strontium 90 per gram of calcium. This is eleven times the present amount.

The Columbia men do not consider their work complete. It measured only one of the many fission products. It had nothing to do with the genetic perils of radioactivity. It paid no attention to areas (such as the U.S. Southwest) where "local" fallout has been heavy. It used a very small sample: 500 cases out of 2.5 billion humans.

The Columbia men are concerned about such individuals as the Vancouver man who have a lot more strontium 90 than the average, and about people who get most of their calcium from vegetables that were grown in calcium-deficient soil. Such people may come much closer to the "permissible" level. The permissible level itself is still considered debatable. It was derived principally from a small amount of experience with the cancer-causing effects of radium in the bones; at that time no strontium 90 existed in the world. When more is known, the permissible level for strontium 90 may have to be lowered sharply.

* The curie is the unit of radioactivity. One curie is 37 billion (3.7 times 10^10) atomic disintegrations per second. One micromicrocurie is one-millionth of one-millionth of a curie.

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