might call for emergency measures against radiation exposures even in the face of other possible hazards. Table 3 shows the approximate areas encompassed by the three isodose lines. For areas where the fallout occurs a few hours or more following detonation, many days or weeks will be required to accumulate the major portion of effective biological doses, so that spot decisions involving additional hazards a) might not be necessary. The question is frequently asked as to the time one must spend within a shelter or remain outside of a contaminated area. The answer depends upon a number of parameters, such as the criteria established for maximum permissible dose, as well as length of stay within the area of contamination. With knowledge of the magnitude of the radiation levels present and an assumed rate of desay, (t)71°2, it is possible to plan and execute a short stay even in a highly contaminated area. tensive analysis. Planning for continuous occupancy requires more exThe following data may aid in such evaluation. The fallout map (Idealized Fallout Diagram on page ____) and Table 3 suggest the degree of radiation exposure received in continuous occupancy under normal Living conditions beginring with the time of initial fallout. For those entering the contaminated zone four months after the first fallout, however, and then living there indefinitely, the area encompassed by the 50 r effective biological isodose line will have shrunk from about 25,000 to 2,500 square miles. At such time (four months after fallout), an area of about 1,000 square miles within the 50 r isodose line might have the highest residual contamination, amounting to about three times the dose rates at the periphery. ~15-