Hi Roger, We use 100 days as the biological half-life for iodine. If you fold in the physical half-life you get an effective half-life of about 40 days. It is somewhat variable. We use 20nCi as an investigational level for I-125 and 50nCi as an action level. It may be conservative, but there is no reason at a university fot anyone to get more. Just our opinion. Dale