The goal of radiation surveys is to determine what the radiation levels are and what the expected radiation dose to a person will be. On average a person will receive about 300 mrem of radiation per year from natural sources. 50 rem in a short period of time (like hours) can cause radiation sickness, and >100 rem is the baseline for increasing the probability of fatalities. Cancer due to radiation exposure is less well known (though it is better characterized for high levels of radiation than low levels). For this reason, radiation workers are limited to 5 rem/yr (and only 3 rem/qtr) and the general public is limited to 100 mrem/yr. This means that if they want to technically call the area safe they must ensure that nobody would be expected receive greater than 100 mrem/yr in this area. They also have to comply with radioactive material contamination limits. This typically means that you will have less than about 500 pCi per an area of 100 cm^2 (which registers 100 counts above background on a frisker—a portable radioactive contamination detector).
The Reader David reminded us on the NY dbomb scare about what polonium can do - remember tracking that stuff all over the place after Litvenenko.
More than I needed to know but many thanks.
seems to be a total non story here anyway .