At the same time we have seen how many factors influence the eventual uptake into the human body: geographical latitude, frequency of rainfall, the chemical form in which strontium is found, the calcium content of the soil, the method of agriculture. Even though the United States has pushed this investigation vigorously since 1952 the bulk of the work is still ahead of us. For instance, in the United States, dairy products provide most of the calcium and strontium in our diets. In Japan, however, the situation is somewhat different. There the main source of calcium and strontium is rice. As a result, the ratio of strontium to calcium may be passing differently from the soil to man. Also the fallout strontium might be washed deeper into the soil and the soluble to non-soluble ratio might be different. Considering the complex nature of the Sr⁹⁰ uptake into man, it is important to keep close track of the actual Sr⁹⁰ levels in the soil, in our food, and in our own bodies. The following graphs show how these levels have risen in the last several years due to the bomb tests: [Illustration: Sr⁹⁰ in the soil—measured in thousandths of a gram per square mile.] [Illustration: Average Sr⁹⁰ in U.S. milk—measured in trillionths of a gram per quart.] [Illustration: Average radiation doses from Sr⁹⁰ in bones of young children (U.S.)—measured in roentgens per year.] The actual amounts of Sr⁹⁰ in the soil, in the milk, and in the bones of young children are only approximately known. But the main point that we are trying to illustrate, is that since 1954 the buildup of Sr⁹⁰ has gone on at a rather steady rate. How far will this buildup continue? More radioactivity was released in tests in the year 1954 than in all other years put together. Probably more than one-half of that activity has already been deposited. Since that time the fission energy produced in U.S. tests has steadily decreased. Furthermore, we have learned how to minimize the world-wide fallout by employing ground bursts which deposit most of their activity in the close-in fallout near the test site. It is also possible to place chemical additives near the bomb in order to convert the strontium into a more insoluble form or else into a form which will more readily fall out in the immediate neighborhood of the explosion. And what is most important—we are developing clean nuclear weapons, which produce blast and heat but greatly reduced radioactivity. In the future these clean weapons may eliminate the additional radioactivity altogether.