ABSTRACT
Intervention levels for emergency response are for national authorities to decide, but the latest information suggests that stable iodine prophylaxis for children up to the age of 18 years be considered at 10 mGy, that is 1/10th of the generic intervention level expressed in the International basic safety standards for protection against ionizing radiation and for the safety of radiation sources.  For adults over 40, the scientific evidence suggests that stable iodine prophylaxis not be recommended unless doses to the thyroid from inhalation are expected to exceed levels that would threaten thyroid function. This is because the risk of radiation induced thyroid carcinoma in this group is very low while, on the other hand, the risk of side effects increases with age.  The latest information on the balance of risks and benefits will also need to be properly considered in the plans for any distribution and storage of stable iodine. It suggests that stockpiling is warranted, when feasible, over much wider areas than normally encompassed by emergency planning zones, and that the opportunity for voluntary purchase be part of national plans.

http://www.who.int/ionizing_radiation/pub_meet/Iodine_Prophylaxis_guide.pdf