Having said what the age is of the various kinds of fallout, we are in a position to say which radioactive species are still present when the radioactivity is deposited on the ground. The close-in fallout, being only a few hours old, still includes many short-lived isotopes, which disintegrate before there is a possibility of ingestion or inhalation into the body. Consequently the danger from the close-in fallout results from external exposure, mainly to gamma radiation on the whole body, and to a lesser extent to energetic beta rays on the skin. Clothes and ordinary housing provide relatively little shielding against gamma rays. Special protective shelters are needed. During a war if the enemy were to bomb our cities with super-megaton weapons surface-burst, the close-in fallout would be a far greater agent of destruction against an unsheltered populace than either blast or thermal radiation. In the stratospheric world-wide fallout, however, all of the short-lived radioactivity has disappeared, since a period of many years has elapsed since the explosion. After a year or so the only gamma emitter which is left in appreciable quantity is cesium¹³⁷, with a half-life of 30 years. Its gamma ray, however, is not very penetrating. In spite of this fact cesium¹³⁷ is considered to be the second most important hazard for the long term fallout. The first is strontium⁹⁰, which is a beta emitter with a half-life of 28 years. This is long enough so that most of these nuclei will still be present even after spending a long time in the stratosphere. Since strontium is chemically similar to calcium, it contaminates our foodstuffs and is easily incorporated into our bodies. Once inside it stays for long periods of time, deposited in our bones. We shall see in a later chapter how serious this danger may be. The tropospheric fallout, and to a lesser extent, the stratospheric, includes some other radioactive species besides cesium¹³⁷ and strontium⁹⁰, and we shall discuss these in the next chapter. But by and large they are of little consequence (with the possible exception of iodine¹³¹) either because they are not easily absorbed in the body or else because their radiation is not very energetic. The world-wide hazard is thus narrowed down to just two isotopes, an internal beta emitter and a weak gamma emitter. CHAPTER XI From the Soil to Man There is a bewildering variety of radioactive products deposited in the fallout. Given certain conditions all of them could be dangerous to man. Actually, very few are. An example of a radioactive isotope which is produced in large quantity by the fission process and about which there is some reason to worry, but actually is not dangerous to man, is iodine¹³¹. This isotope in the fallout is not dangerous because it has a rather short half-life: eight days.