Cumulative effects?

Given the continuing presence of detectable levels of Iodine 131 and Cesium 137 I was hoping to revisit the question of Excess Cancer Risk. I have really appreciated the opinions of the BRAWM team members about the low levels we have seen not being worth taking precautionary action against. But now I am wondering about what time-frame these levels are considered insignificant by you all, especially for infants and young children. In other words if these levels persist for years would this elevated level of radiation still be insignificant in your view? Or had you been deeming the elevations insignificant on the assumption that they would return to below MDA in short order?
Thanks so much