Well, then, the thought arises, or should arise—can these ‘spontaneous’ cases be due to the potassium-40 beta particles careering around within us all? This has been thought of, to be sure, and chemists have calculated the probabilities of beta particles (or free radicals produced by them) just happening to strike a gene and damaging it.

  The results of these calculations seem conclusive. The effect is insufficient! The radiation to which a human being is subjected as a result of potassium-40 atoms within him is about the same as the radiation to which he is exposed as a result of cosmic ray bombardment from outer space. Both together cannot possibly account for more than a tiny fraction of the ‘spontaneous’ cancers and mutations.

  But are we through? Have we exhausted the possibilities of explosions within us?

  The answer is ‘No’ to both questions. In the last chapter, I mentioned that the bombardment of the atmosphere by cosmic rays results in the continuous production of carbon-14. Carbon-14 is radioactive and has a half-life of only 5,570 years, but its presence is maintained at an even level by cosmic ray production at a rate that just balances its rate of breakdown.

  This ‘even level’ is certainly not very high. Carbon atoms exist in the atmosphere as part of the molecules of the gaseous substance, carbon dioxide. Only 0.04 per cent of the atmosphere is carbon dioxide. Only one of the three atoms in the carbon dioxide molecule is carbon. And only one carbon atom out of every eight hundred billion (800,000,000,000) is carbon-14. Certainly, the total amount of carbon-14 present in the atmosphere doesn’t seem to be overwhelming. Let’s see!

  The weight of the atmosphere is 14.7 pounds on every square inch of the Earth’s surface. Take into account all the square inches there are on the Earth’s surface (a little over eight hundred thousand trillion [800,000,000,000,000,000] if you’re curious) and the total weight of the atmosphere turns out to be five thousand nine hundred trillion (5,900,000,000,000,000) tons.

  From this we see that the total weight of carbon dioxide in the atmosphere is two trillion three hundred and sixty billion (2,360,000,000,000) tons; the total weight of the carbon atoms themselves is seven hundred and fifty billion (750,000,000,000) tons; and the total weight of the carbon-14 is 0.9 tons, or 1,800 pounds. Indeed, not an overwhelming amount, but then not an insignificant one either.

  Eighteen hundred pounds of carbon-14 contain thirty-five thousand trillion trillion (35,000,000,000,000,000,000,000,000,000) atoms. If these are spread evenly through the atmosphere, each cubic inch of air (at room temperature and sea-level pressure) would contain 120,000 atoms of carbon-14. In the air contained in a moderately-sized living-room there would be over three hundred billion atoms of carbon-14.

  Put it another way. Every time you breathe, 30 cubic inches of air moves in and out of your lungs. That means that each time you breathe, you pump three and a half million atoms of carbon-14 into your lungs. In an average lifetime, you will have breathed over two thousand trillion atoms of carbon-14.

  Not insignificant at all!

  But do any of these atoms become part of living tissue? They certainly do; that is the crux of the whole thing.

  Living matter is approximately 10 per cent carbon by weight. Every bit of that carbon, whether the organism is large or microscopic, plant or animal, of the sea or of the land, came originally from the carbon dioxide of the air. Green plants incorporate carbon dioxide of the air into larger carbon-containing molecules. (Microscopic plants in the ocean may use carbon dioxide molecules that have dissolved in the sea and reacted with the water molecules found therein, but that came originally from the air, too.) Animals eat the plants (or other animals that have already eaten the plants) and use the carbon atoms as hand-me-downs for their own purposes.

  Since living tissue makes no distinction (or practically none) between carbon-14, which is radioactive, and carbon-12 and carbon-13, both of which are stable, the proportion of carbon-14 in living matter, including your own personal living matter, is the same as it is in air.

  As long as you (or any living creature) is alive, the proportion of carbon-14 in your body remains constant. You remain in constant balance with the atmosphere where the production of carbon-14 by cosmic rays just balances its rate of breakdown. The result is that carbon isolated from any recently living object contains enough carbon-14 to cause the liberation of 450 beta particles a minute for every ounce of carbon present.

  In the last ten years or so, this fact has become important to archeologists.

  You see, as soon as an organism dies, it stops incorporating carbon dioxide (either directly from the atmosphere or indirectly, by way of its food) into its tissues. Therefore, it stops collecting carbon-14. Whatever carbon-14 was in its tissues at the moment of its death remains, but that slowly breaks down. At the end of 5,570 years, the carbon-14 of its remains is half gone. Carbon isolated from those remains would produce only 225 beta particles per minute per ounce.

  It is fairly easy to count the beta particle production, using appropriate devices, and so we have a method for telling how old any object of living origin might be. The age of a mummy might be told from the ‘counts’ given off by his mummified flesh or by his linen wrappings. The wood of some old prehistoric Indian abode or some ancient parchment could be tested.

  This method of ‘radiocarbon dating’ is quite objective and doesn’t depend upon anyone’s historical theories or anyone’s interpretation of ancient inscriptions. It has been used to estimate the time when mankind first arrived in the Western Hemisphere. By and large, its results have been in accord with decisions previously reached by historians.

  To get an idea how the number of counts correlates with age, look at Table XII.

  Naturally, the fewer the counts, the more chance there is of error. All mathematical treatment of radioactive breakdown is statistical in nature and statistics works more poorly as the numbers grow smaller. By the time you get down to just a few counts per ounce, you’re on rocky ground. Furthermore, there are always stray counts coming from radioactive atoms of all sorts that happen to be close at hand. This is called ‘background radiation’. The amount of this is small, but as the carbon-14 counts get fewer and fewer, even a small amount of background radiation can throw results badly awry. For these reasons, radiocarbon dating has its limitations and can only be pushed back in time (with increasing shakiness) some thirty thousand years.

  But we mustn’t stay away from our main subject too long. What about the effects of the carbon-14 in our body?

  The hundred and fifty pound man I mentioned earlier in the article contains about three hundred trillion trillion (300,000,000,000,000,000,000,000,000) carbon atoms. Of these, some three hundred and fifty trillion (350,000,000,000,000) are carbon-14 atoms. If we omit the mineral matter of the body, and assume the carbon-14 atoms are spread out evenly otherwise, then each cell contains just about 11 atoms of carbon-14.

  This is quite a small figure when you compare it with even the rarest trace element in the body. There are 40,000 times as many cobalt atoms in a cell, at the very least, as carbon-14 atoms. The comparison with potassium-40 is even more extreme. There are over 700,000 times as many potassium-40 atoms in a cell as carbon-14.

  If it has been decided, then, that potassium-40 is quite harmless to the body, it would certainly seem as though carbon-14 ought to be harmless thousands of times more emphatically.

  But wait! Carbon-14 has a much shorter half-life than has potassium-40. An equivalently larger proportion of its atoms ought to be breaking down per second, and after all it is the number of beta particles being produced, and not the number of atoms, that counts.

  Well, knowing the number of carbon-14 atoms present and the half-life of carbon-14, we can calculate that each second there are some twelve hundred beta particles produced within the body by carbon-14 breakdowns.

  The proportion of potassium-40 is still greater, but no longer so one-sidedly. Potassium-40 produces nearly 30 times as many beta particles as does carbon-14.

  But there is another point that must be considered, though, in addition to mere numbers. The energy of the beta particles produced by potassium-40 is some ten times as great as those produced by carbon-14. The potassium-40 atom, as it breaks down, can therefore do ten times the damage that carbon-14 atoms can do as they explode. That swings the pendulum in the other direction again, for it would now appear that potassium-40 in the body does, on the whole, 300 times the damage carbon-14 does.

  Certainly, it seems that no matter what we do, or how we slice it, carbon-14 remains badly out of the running.

  Ah, but carbon-14 has a trump up its sleeve!

  The key molecules of the cells are the genes I mentioned earlier. It is change in these which can bring about mutations and cancer. These genes contain no potassium atoms in their molecule. Any potassium-40 atoms present in the cell are located elsewhere than in the gene. Beta particles that shoot out of an exploding potassium-40 atom must strike a gene molecule just right or must give rise to a free radical which will strike it just right. It is as though you were standing in a globular shooting gallery, blindfolded, and aimed at random in the hope of hitting a few tiny targets placed here and there on the walls, floor and ceiling.

  All in all, the chances of a beta particle from a potassium-40 atom doing damage to a gene, either directly or indirectly, is very low; perhaps only one out of many millions.

  But now let’s consider carbon-14. The gene is not merely being shot at by beta particles from exploding carbon-14 atoms. It contains carbon-14 atoms.

  The genes make up about 1 per cent of the average cell. (Only of the cells now; there are no genes in the extracellular material.) This means there are some twenty-four trillion trillion atoms in the genes of which half, or twelve trillion trillion (12,000,000,000,000,000,000,000,000) are carbon atoms. Of these, some fourteen trillion (14,000,000,000,000) are carbon-14 atoms.

  It comes to this, then: There is, on the average, one carbon-14 atom in the genes of every two cells.

  The number of these that break down can be calculated. It turns out that each second, in your body as a whole, 50 carbon-14 atoms, located in the genes, are exploding and sending out a beta particle. These may be weaker and fewer than the beta particles sent out by exploding potassium-40 molecules, but every one of these fifty scores a hit!

  Even if you suppose that the beta particle from an exploding carbon-14 atom within a gene might plow through the remainder of the gene without hitting any of its atoms squarely enough to do damage (this is possible) the fact still remains that the exploded carbon-14 atom has been converted to a stable nitrogen-14 atom. By this change of carbon to nitrogen, the gene is chemically altered (only slightly perhaps, but altered nevertheless). Furthermore, the carbon-14 atom, having shot out a beta particle, recoils just as a rifle would when it shoots out a bullet. This recoil may break it away from its surrounding atoms in the molecule and this introduces another change, and an even more important one.

  In order for potassium-40 to do as much damage to a gene as the gene’s own carbon-14 will do, the chances of a beta particle from a potassium-40 atom (or its free radical product) striking a gene hard enough to do damage must be at least as good as one in eight thousand. The chances just aren’t that high or anything like it, so I end by concluding that carbon-14 is much more likely to be responsible for ‘spontaneous’ cancer and mutations than potassium-40 is.

  And if that is so, there is precious little that can be done about it unless someone turns off the cosmic rays, or unless we build underground cities.

  However, the situation isn’t as serious as you might think. Fifty explosions per second within your genes may sound as though you couldn’t last very long without developing cancer or having deformed children, but remember, a change in a gene may mean any kind of a change whatever. In the vast majority of cases, any serious change (and we don’t really know how big a change must be before it’s ‘serious’) simply results in that particular gene in that particular cell refusing to work at all. Probably only a vanishingly small percentage of the changes result in cancer. (And if we only knew what the details of that change was!)

  Then again, some cells are more important than others. Only the cells that give rise to ova and spermatozoa can be responsible for mutated children, and they form only a small percentage of the total number of cells in the body.

  Fifty explosions per second over the entire body doesn’t mean much to the individual cell (any more than the fact that twenty stars in our Galaxy blow up each year should cause us to worry much about our Sun). If you were to consider a single particular cell in the body, chosen at random, then an average of 18,000 years must pass before a single carbon-14 explodes in its genes.

  This is the same as saying that if you live to be 70, the chances that a particular cell in your body will ever have experienced even a single carbon-14 breakdown in its genes is only one in 260.

  So sleep in comfort!

  NOTE

  This article was written, originally, in November, 1956. Some two years earlier I had written a short article on the same subject which appeared in the February 1955 issue (pages 84-85) of the Journal of Chemical Education. That, I believe, was the first mention in print of the relationship of carbon-14 to genetics.

  In 1958, when atmospheric testing of nuclear bombs still went on wholesale, Linus Pauling (my favorite chemist) published a paper in Science (November 14, 1958) which went into the matter in a careful and systematic wav and pointed out the manner in which testing would increase the carbon-14 content of the atmosphere and therefore the incidence of undesirable mutations.

  I have a letter from Professor Pauling, dated 11 February 1959. which refers in most kindly fashion to my article and—well, I just thought I’d mention it.

  CHAPTER THREE—HEMOGLOBIN AND THE UNIVERSE

  Even the purest and most high-minded scientist finds it expedient sometimes to assault the fortress of truth with the blunt weapon of trial and error. Sometimes it works beautifully. As evidence and as a case in point, let us bring to the front of the stage the hemoglobin molecule.

  Hemoglobin is the chief protein component of the red blood cells. It has the faculty of loosely combining with molecular oxygen to form oxyhemoglobin. That combination takes place in the small blood vessels of the lungs. The oxyhemoglobin there formed is carried by the blood stream to all the cells of the body; it gives up its oxygen to these cells and becomes hemoglobin once more. It is then ready to make its way to the lungs for another load.

  Because of hemoglobin’s vital function in life and because of its ready availability in fairly pure form, the protein has been subjected to the closest scrutiny on the part of chemists. It was found, for instance, that the hemoglobin molecule is approximately a parallelepiped in shape, with dimensions of 6.4 by 4.8 by 3.6 millimicrons. (A millimicron is one-billionth of a meter; a meter is forty inches.) The bulk of this molecule is ‘globin’ which, by itself, is an unstable protein. It makes up 97 per cent of the whole. Attached to the globin, and rendering the whole more stable, are four iron-bearing groups called ‘heme’ (see Figure 1).

  Hemoglobin can be split apart into a heme fraction and a globin fraction without very much difficulty, and the two can be studied separately. Heme, being simpler in construction and quite stable in addition, has been naturally the more intensively investigated of the two.

  The heme molecule is flat and approximately circular in shape. In the very center of heme is an iron atom. Surrounding that iron atom are twenty carbon atoms and four nitrogen atoms—plus some hydrogens—arranged in four small rings that are themselves connected into one big ring. This wheels within wheels arrangement occurs in numerous compounds other than heme—notably in chlorophyll—and is called the ‘porphyrin ring’. Establishing the structure of the porphyrin ring itself took some fancy footwork, but was a relatively straightforward matter.

  Now, however, there enters an additional refinement. There are eight points in the porphyrin ring where groups of atoms called ‘side-chains’ can be, and are, attached. In the heme molecule, the eight side-chains are of three varieties: four of one kind, two of another, and two of a third. Porphyrin rings to which are attached that particular combination of side-chains are called ‘protoporphyrins’.

  Now this is the ticklish point. Which side-chains are attached to which positions in the porphyrin ring? To illustrate the difficulty, let’s draw some pictures. Since this chapter concerns itself not with chemistry—despite appearances so far—but merely with some simple arithmetic, there is no need to make an accurate representation of the porphyrin ring. It will be sufficient to draw a ticktacktoe design (Figure 2). Topologically, we have achieved all that is necessary. The two ends of each of the four lines represent the eight positions to which side-chains can be attached.

  If we symbolize the side-chains as a, b, and c (four a’s, two b’s, and two c’s), several arrangements can be represented. Two of these are shown in Figures 3a and 3b. Altogether fifteen different and distinct arrangements can exist. Each arrangement represents a molecule with properties that are in some respects different from those of the molecules represented by every other arrangement. Only one of the fifteen is the arrangement found in heme.

  Which one?

  A German chemist, Hans Fischer, was faced with that problem and he solved it in the most straightforward possible manner. He wrote down the fifteen possible arrangements on pieces of paper, numbering them arbitrarily from one to fifteen. He then, in effect, called out sixty graduate students, separated them into platoons of four apiece, and gave each platoon one of the arrangements. Instructions were for each to synthesize the protoporphyrin with the particular arrangement pictured.

  The students got to work. As each protoporphyrin was formed, its properties were compared with those of the natural protoporphyrin obtained from hemoglobin. It turned out that only one of the synthetic protoporphyrins matched the natural product. It was the one that Fischer had happened to assign the number 9, and it has the side-chain arrangement shown in Figure 4. Since then, generations of medical students and biochemists have memorized the formula of the natural product and learned to call it ‘Protoporphyrin IX’. (It is my. personal experience that few students show any curiosity at all as to why the IX.)