Estimating Individual and Collective Doses to Groups with ‘Less Than Detectable’ Doses

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Abstract
Distributions of annual external doses to worker populations are often found to be distributed lognormally below 15 mGy (1.5 rad). Using the properties of the lognormal distribution, and starting from individual dosimeter results, a method is presented whereby estimates can be made of collective and individual doses “missed” due to the fact that dosimeters have a threshold of detection or minimum detectable dose (MDD). For the case where only annual dose totals are available for a population, if MDD results were recorded as zero and if monitoring was done on a quarterly basis, the method developed is shown to yield reasonably good estimates of “missed” collective dose. For the other cases of only annual totals being available (i.e. monitoring was done more frequently than quarterly, or MDD results were recorded as equal to the MDD), it is shown that the method does not yield useful results. The estimates developed here may be useful in radiation epidemiology, employee relations, and in probability-of causation calculations.