The answer depends on a fact which we have emphasized throughout this book: that isotopes of the same element are chemically and biologically indistinguishable. The radioactive variety of strontium will behave exactly like the stable natural variety. In particular, the ratio of Sr⁹⁰ to stable strontium in the human body must be the same as this ratio is in our food. From this premise we can predict how much Sr⁹⁰ will reach the human body. From the total yield of fission energy released in all nuclear tests to date, one can calculate exactly how much Sr⁹⁰ has been produced. This amount turns out to be about 100 pounds. Approximately one half of this amount has been deposited in and near the test sites in the close-in fallout. (Most of the radioactivity comes from the big bombs, and most of these have been burst on the ground or over shallow water.) A small portion of the 100 pounds has disintegrated in the cloud. The remainder, roughly 50 pounds, is partially still in the stratosphere and partially has been disseminated around the world in the tropospheric and stratospheric fallout. At the present time measurements show that 25 or 30 pounds have actually been returned to the surface of the earth. Local values vary from about one third to more than twice the average world-wide value. In the northern part of the United States, in the regions of frequent rainfall, the measured values are about twice the world-wide average. In the latitudes between 10°S and 50°N the average value is about 50 per cent greater than the world-wide average. For the rest of the world one finds, with some variations, about one third the world-wide average. Most of the Sr⁹⁰ fallout is caught in the top two or three inches of the soil. It exists there in a water-soluble form that is readily assimilated by plants. Also in the soil, chemically inseparable from the Sr⁹⁰, is stable natural strontium. Plants, animals, and human beings have no way of distinguishing between the two. It is not easy to determine how much natural strontium is in a form which is available to the plants. Some of the natural strontium is insoluble; and some is below the root depth. Our best estimate is that there are about 60 pounds per acre actually available for uptake by the plants. This is, of course, an average.