The impact of recent ICRP recommendations on dose coefficients, annual limits on intake, and monitoring programmes for thorium

Much of the radiological protection legislation throughout Europe is based on ICRP Publication 30 and its companion, ICRP Publication 54. Since 1990 ICRP has revised many of the models and parameters used in internal dosimetry: tissue weighting factors and the annual dose limit were revised in Publication 60; a new model for the respiratory tract was given in Publication 66; and a new biokinetic model for thorium, including recommendations on the treatment of decay products, was given in Publication 69. This paper investigates the effects of these changes on the committed effective dose per unit intake and the annual limit on intake (ALI) for inhalation of selected compounds of ²²⁸Th and ²³²Th. It is found that the combined effect is a very substantial rise (by up to a factor of 20) in the ALI. The new models can also be used to calculate bioassay quantities such as urinary and faecal excretion, and lung and whole-body content. Here too, there are substantial changes in the predictions of the new models. On a practical note, it is shown that intakes which would lead to doses in the region of the annual dose limit could result in values of bioassay quantities which are well above the typical detection limits of a competent laboratory. Significant intakes were far less easy to detect under the previous models and the changes could have significant implications for the design of personal monitoring programmes in industries which use thorium.