Guidelines for Iodine Prophylaxis following Nuclear Accidents

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Guidelines for iodine prophylaxis following nuclear accidents1999 updateAbsorption through the skin is a possible route, but negligible incomparison with inhalation.2.2 Deterministic and stochastic effectsDeterministic effects from thyroid exposure are hypothyroidism andacute thyroiditis. Stochastic effects from thyroid exposure are thyroidcancer and benign thyroid nodules.The selective and rapid concentration and storage of radioactiveiodine in the thyroid gland results in internal radiation exposure of thethyroid, which may lead to an increased risk of thyroid cancer andbenign nodules and, at high doses, hypothyroidism. These risks can bereduced or even prevented by proper implementation of stable iodineprophylaxis.Hypothyroidism is caused by a radiation dose of the order of morethan several Gy to the thyroid. A dose that large could, in practice, beincurred through inhalation only near the point of the accidentalrelease. Because exposures from other radionuclides are also likely tobe large in such cases, plans will usually include options to evacuateand/or shelter the population, and stable iodine can be a useful adjunctto these actions.In regions where only the likelihood of stochastic effects is a cause forconcern, stable iodine prophylaxis should be considered for sensitivepopulation groups if potential exposure to radioactive iodine byinhalation or exposure by ingestion is expected to approach thereference levels given in Table 1, and cannot be prevented bysheltering or food and milk control. In severe accidents such situationsmay occur in areas quite far from the accident site.Intake through ingestion of contaminated food, particularly milk,begins after deposition and transfer to the food chain. In the absenceof any countermeasures, ingestion is likely to be the main route ofinternal radiation exposure to radioactive iodine. The exposure islikely to continue for a longer period, cover a wider area and affect alarger population than exposure by inhalation.3
Guidelines for iodine prophylaxis following nuclear accidents1999 update2.3 Experience from the Chernobyl accidentEvidence of a marked excess of thyroid cancer in children exposed tothe fallout from the Chernobyl accident has been established (1–10). Inthe most affected area in Belarus, the yearly incidence has risen close to100 per million children, which is more than 100-fold compared to thesituation before the accident. It is now generally accepted that thisexcess has resulted from exposure to the radioactive iodine released inthe accident. The largest part of the dose to the thyroid was caused by131 I although the shorter lived isotopes of iodine and 132 Te may havecontributed significantly to the inhalation dose in some instances.Table 1. Reference levels for different population groups forconsideration in planning stable iodine prophylaxis aPopulation groupNeonates, infants,children, adolescents to18 years and pregnantand lactating womenExposure pathways to beconsideredInhalation (and ingestion b )Reference levels10 mGy c avertabledose to the thyroidAdults under 40 Inhalation 100 mGy c avertabledose to the thyroidAdults over 40 years Inhalation 5 Gy d projected doseto the thyroidNotesa These idealized levels do not take into account the practicalities involved in planning torespond to an accident involving many radionuclides in unknown quantities in real time.For this reason, a generic intervention level of 100 mGy has been specified in the BasicSafety Standards. Nevertheless, this does not preclude the need to consider thepracticality of planning to implement iodine prophylaxis for specific age groups.b Ingestion of milk by infants where alternative supplies cannot be made available.c Adherence to these values would ensure that doses for all age groups would be wellbelow the threshold for deterministic effects.d Intervention for this group is undertaken to ensure prevention of deterministic effects inthe thyroid. 5Gy is the recommended limit for deterministic effects given in the BasicSafety Standards.Following the Chernobyl accident there were several thousands ofchildren who accumulated a dose to the thyroid of several Gy.4
Page 1 and 2: WHO/SDE/PHE/99.6English onlyDist.:
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Guidelines for Iodine Prophylaxis following Nuclear Accidents

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