The reader will recall that the spacing of the ionization depends only on the charge and the speed of the ionizing particle. The ionizing particle from the Sr⁹⁰ is an energetic beta ray, which has a charge of one and a speed close to that of light. A large part of the background radiation which reaches our bones comes from the cosmic rays. The main portion of the cosmic rays is due to the mesons. The meson, like the beta ray, has a unit charge and a speed close to that of light. The two particles may therefore be expected to produce identical biological effects. The only difference between their effects is that the beta ray does not have enough energy to leave the bones, while the meson is so energetic that it deposits its energy both in our bones and throughout our whole body. Thus if we compare a Sr⁹⁰ dose with the same dose of cosmic rays the same effect to the bones must be expected. But the cosmic rays give rise to additional effects in our bodies. The total background dose to the bones is about 0.15 roentgens per year for the average person living at sea level in the United States. Of this amount, about 0.035 roentgens is due to cosmic rays. At higher altitudes the cosmic ray dosage increases. In Denver, at an altitude of 5000 feet, the cosmic rays contribute 0.05 roentgens per year. The above numbers should be compared with the present level of world-wide fallout radiation to the bones: about 0.003 roentgens per year (from Sr⁹⁰ and other sources). The fallout radiation is thus only a few per cent of the natural cosmic radiation. It is small even when compared to the variation of cosmic ray intensity between sea level and 5000 feet. A correlation between the frequency of leukemia and bone cancer, and the intensity of natural radiation has been looked for. Some statistics for the year 1947, before weapons testing began, are available. They show the number of cases of these diseases occurring in that year per 100,000 population. _Bone Cancer_ _Leukemia_ Denver 2.4 6.4 New Orleans 2.8 6.9 San Francisco 2.9 10.3 The extra radiation that one gets in Denver from cosmic rays is many times greater than the fallout radiation. But the table shows no increased incidence of bone cancer or leukemia. On the contrary—the incidence of these diseases is actually lower in Denver. Not all of the natural background radiation is due to cosmic rays. Part of the background comes from natural radioactive elements in the soil and in the drinking water. These include uranium, potassium⁴⁰, thorium and radium. Radium behaves like calcium and strontium, and gets deposited in our bones. All these effects are, to the best of our knowledge, at least as intensive in the Denver area as in San Francisco or New Orleans.