During the first weeks after a nuclear explosion some radioactive iodine may fall out of the cloud and contaminate grazing land. A cow eats hundreds of pounds of grass in a few days time. Now iodine is found in the cow’s body or in the body of any mammal mainly in one spot. This is the thyroid gland located in man near the Adam’s apple. The thyroid gland is important because it secretes a chemical which regulates many of the body functions. In man, these include how we burn up our food and in what mood we are. About twenty per cent of all the iodine which is taken up, whether radioactive or natural, is concentrated in this one rather small gland. Such a concentration is precisely the kind of danger for which we must watch. Shortly after nuclear tests, cows that graze on range land have been found with abnormally large amounts of radioactive iodine, although not so large as to be harmful. In human beings, however, the measured levels of radioactive iodine are less than a hundredth of what they are in the cows because by the time this radioactive isotope has reached man, it has mostly decayed into a stable, harmless variety of xenon gas. There are many potentially dangerous isotopes in the radioactive debris of a nuclear explosion. But most of them decay too soon to affect man. Isotopes which live an extremely long time compared to the human life-span are also not dangerous to man. A radioactive particle in the body is not harmful unless it disintegrates and releases its energy while the individual is still alive. Two examples of long-lived radioactive isotopes, which are used as fuel in the bombs and which may be left over from the explosion in large quantities, are: uranium²³⁵ and plutonium²³⁹. Uranium²³⁵ has a half-life of 710 million years, which is much too long to be dangerous. Plutonium has a half-life of 24,000 years and is somewhat more dangerous. The danger from plutonium arises because it emits an energetic alpha ray. The danger from radioactivity depends on the kind of particle emitted—alpha, beta, or gamma rays—and whether these rays attack the body from the inside or the outside. From the outside the gamma rays are the most dangerous and the alpha rays the least dangerous. From the inside the order is just reversed. To cause damage from the outside the radiation must be very penetrating. Gamma rays can go through the whole body. Beta rays are stopped in the skin tissue. Alpha rays cannot even penetrate the outer layer of non-living, protective skin. On the inside, however, in the sensitive organs, the short range of the alpha rays makes them exceedingly dangerous. Their energy is concentrated in a small amount of tissue to which damage is severe. The beta rays cause a slightly less concentrated damage, and the gamma rays the least concentrated of all.