The scope and nature of several blast hazards are delineated. Tentative criteria are set forth for threshold damage to humans. These criteria are related 10 nuclear weapons in terms of ground ranges and areas involved for one MT and ten MT surface detonations. To allow appreciation of the relative importance of blast with other effects, appropriate values are noted for ionizing and thermal radiation. Four categories of blast hazards are defined, and the character of each is described. The occurrence of combined injuries from pressure, missiles, and displacement is discussed. Experiences in the Texas City disaster of 1947 are reviewed. Selected data relate environmental conditions to gross biologic damage from overpressures, missiles, and impact loading. 86 references. (C.H.)