important. The most serious of these is the high-yield fission product, radioactive strontium (Sr90), which because of its own radiation and those of its short- lived daughter, yttrium-90 (Y90), and because of its chemical similarity to the bone-building element, calcium, finds itself jsepcsitad in bone structure. Other radioactivities produced would be as bad if they spent as long a time in the body or if their radioactive lifetimes were long enough, or if they were produced in high-yield. Strontium has all of these characteristics. So, for the fission products which have survived the first weeks, the most important fallout constituent and the one most seriously to be considered is Sr90. Neither radiostrontium nor its yttrium daughter emits gamma radiation, but only_beta radiation. After the first year Cesium-137 (Csl137), with a 33-year half-life, is the principal source of the residual gamma radiation, and any gamma radiation exposures due to fission products which are more than one year old are due very largely to radioactive cesium. In fact, old fallout can be thought of as a mixture of roughly equal radiation intensities in millicuries of radioactive Sr90 and radioactive Csl37. The other isotopes either constitute no ingestive hazard or fail to emit gamma radiation in appreciable intensity. So, the hazards of world-wide fallout reduce themselves largely to the ingestive hazard of radio, achaye Sr90 and the external exposure from radioactive Ss e as The mechanism by which atomic weapon debris is disseminated leads to three kinds of fallout. First, the strictly local fallout, which is due to the return to earth of the larger particles in the fireball. These may have their origin in the dirt, the soil, or tower structures which are taken into the fireball and either wholly or partially vaporized. The fraction of the total wnich falls out locally depends very much upon the conditions of firing. The most serious factor is the degree of contact of the fireball with the surface; another is the nature of the surface. For example, soil appears to be much more effective than water in producing local fallout. (more) BS (22