[en] In the event of major radiation accident, a monitoring will be carried out for exposure of evacuees from the affected areas, and, by the result, the individuals who need medical checkup and body decontamination will be selected, that is often called 'screening'. A screening level of 10 rems in thyroid dose is assumed in this paper for the convenience to application, and relationship between measured value and the screening level is discussed. From dose of 10 rems to thyroid due to inhaled radioiodine, a surface contamination level of 10^-3 μCi/cm² is derived. The measurement can be readily carried out by ordinary βγ-survey meters. Deposition of 0.08 μCi of ¹³¹I in thyroid, also, corresponds to 10 rems of thyroid dose. External measurement of the site of thyroid will be adequately carried out by γ-ray survey meters with higher sensitivity such as NaI(Tl) type instruments, which yield about 40 μR/h in net indication. A correction factor is estimated to the measured value by γ-ray survey meters for the assessment of thyroid dose to the probable isotopic composition of radio-iodine released after an accident of light water reactors. The correction factor of 0.3 may be proposed for the early stage of the accident not later than 1.5 days after reactor shutdown. (author)