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The update of predictions of mortality from pleural mesothelioma in the Netherlands1 provides welcome news that the peak number and the total during 2000–28 are now predicted to be only a little more than half of the figures predicted only four years earlier.2 This marked change in prediction has occurred because the known decrease in asbestos use after 1984 and a ban in 1993 were taken into account in the modelling, and there were five extra years of data (1994–98).
Since most mesotheliomas are caused by asbestos the pattern of use during different periods of time has a marked influence on the risk in cohorts whose working lives covered different periods. The marked effect of the discontinuation of crocidolite importation by 1970 into the United Kingdom on the amount of crocidolite found in the lungs of men born in 1943 or later, who developed a mesothelioma between 1990 and 1996 when aged 36–52 years,3 was clear when compared with the lung contents of mesotheliomas in 1976 and 1977.4 The average amount of crocidolite in the lungs of the men with mesothelioma in 1990–96 was about a tenth of that in the lungs of the 1976–77 cases. This difference was a consequence of the shorter period of exposure and the longer time since exposure during which elimination of fibres occurred in the absence of new exposure.
The importance of the effect of elimination is illustrated by results from former workers at the Wittenoom crocidolite mine and mill in Western Australia. Exposure of this group ceased in 1966 when the mine and mill were closed. Based on an analysis of the number of deaths with mesothelioma in men to the end of 1986, predictions were made of the number occurring up to 2020.5,6 The model of the mesothelioma death rate used was that the rate increased with a power of time since exposure, moderated by a factor representing elimination of crocidolite fibres over time since exposure. Rates of elimination from zero to 15% per year were considered. Assuming no elimination, which was the usual model used at that time, predicted more than twice as many mesotheliomas by 2020 than an elimination rate of 15% per year. Preliminary results to 1999 were given by Musk and colleagues,7 and it is now known that the number of mesothelioma deaths in men in the period 1987 to 2000 was similar to the lowest predictions made based on the number up to 1986. This result is evidence that models of mesothelioma incidence that take account of a gradual elimination of crocidolite from the lungs after exposure are more realistic. There is strong evidence from other sources that such elimination does occur and that for crocidolite the rate of elimination is in the range of 10–15% a year.8
There is a high continuing toll from the use of asbestos in Europe and from the mining and milling of crocidolite at Wittenoom in Australia, but fortunately recent evidence strongly suggests that the number of mesotheliomas will not be as high as earlier predictions.
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