population involved makes optimum use of available shelter. Quantita- tive statements regarding lethality carry the inherent uncertainties of the estimates relating percentage of deaths to levels of radiation. The area involving a potential early hazard lies in a local pattern with its origin at ground zero, and arises primarily from the gemma radiation of the deposited fission products. The contribution vy bomb formed tritiunand other artificial radioactivity formed by neutron bombardment of soil and bomb debris does not appear to increase the hazard to any marked degree. For a given burst condition, the size of the local hazard area is roughly proportional to the fission yield of the weapon. Accurate pre-shot delineation of the local hazard areas likely to be involved following a nuclear detonation cannot be accomplished because of the sensitive wind-dependence of the deposition mechanisan. Changes in the wind structure after shot time can radically alter the predicted deposition patterns based on wind soundings taken at or before shot time. Approximate local hazard areas likely to be involved for a given nuclear weapon and burst condition can be determined in advance by currently available means. Approximate simplified elliptical con- towrs can be drawn in a few minutes by wtrained personnel and more exact contours can be calculated by hand or machine with somewhat greater effort and more complex inputs. As much as 90% of the potentially lethal fall-out area from a land-surface burst nuclear weapon will lie outside the lethal area for blast and thermal effects, largely in a roughly elliptical pattern extending downwind from the burst point. The radiation intensity in a gross fission product field decays in such a manner that shelter during the first few hours or days efter fall-out begins is considerably more important than shelter at later times. Experience at field tests indicates that the t71°@ decay rate of mixed fission products holds sufficiently well to calculate 129