There are keyed by letter to the same By contrast, the primary radionuclides contributing to the external exposure levels in the Harshail Istlends are fission and activation products, particularly cesiun-137 and cobait-60, fre the weapons tests; these radionuciiaes plus strentium-S0 readily enter the food chain and are consumed by ran, thereby contributing significant ty to an iternal radiation exposure. radte tion exposure through the food chain, It must be noted, however, that the primary radionuclides contributing to these external exnosure levels in the United States are from the naturally occuring docay series of uranium, thorium and actiniua (including, for example, radius, polonium, thorium and radon); generally these radicnuclides do not contribute sicnificantly to any Soy CL Jevels of annual dose rate as were used previously on maps of atolls and islands in the Marshall Islands. Tevels at various locations. This map of the continental United States shows external radiation mat ay