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Absolute risk is defined as the product of assumed relative risks times the total population at risk. Relative risk is defined as the ratio of the risk in those exposed to the risk to those not exposed. The difference between the two risk models leading to major differences in the projected number of cancer deaths lies in the calculated excess of cancers arising from the 0 - 9 years age group at the time of irradiation. Because data on relative risks are sparse and inconclusive, and more data exist supporting the absolute model, the absolute model was used to calculate the latent health effects. Also, because the effectiveness of low exposure rates and/or low radiation exposure doses for producing late health effects remains unresolved, projected cancer deaths were calculated with dose effectiveness factors (DEF) of 1.0 and 0.2 for low exposure rates and doses.* (see below) Also, because there is insufficient data to warrant limiting the risk plateau period to 30 years, a 40 year risk period was used. 9)(see Shapley page 9) Estimates of radiation genetic risks are also uncertain. Reference 8(see Shapley page 9) estimates that the doubling dose for genetic risks to be between 20 and 200 rems although the possibility of it being lower than 20 rems or higher than 200 rems is not dismissed. Since a doubling dose of 100 rems was suggested by Reference 6 (see Shapley page 9) and it is within the estimated range of Reference 8, (see Shapley page 9) it was used to project the genetic risks. It follows that if the doubling dose is 20 rems then the projected number of genetic disorders (spontaneous abortions and "other genetic effects") should be multiplied by 5, and if the doubling dose is 200 rems then the projected number of genetic disorders should be halved.

* A DEF=0.2 implies that the radiation received is only one-fifth as effective per unit of dose for producing latent effects when compared to a high dose received over a short period of time.

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Also, for thyroid exposures from ingested I-131, the effectiveness of the exposure is estimated to be one-tenth that of an external (gamma) exposure. 6)(see Shapley page 9)

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