McDonald et al (Occup Environ Med 2006;63:8525) present an updated analysis of a cohort of mostly women who made gas masks incorporating crocidolite asbestos between 1940 and 1944. They found a high risk of death from pleural and peritoneal mesothelioma, confirming the carcinogenicity of crocidolite, but during the last eight years of follow-up there were no further deaths from mesothelioma (their fig 1 and table 3).

They noted that the mesothelioma rate (number of mesothelioma per 100000 person years at risk) did not continually increase with time since exposure, as is often believed, but levelled off after about 30 years. They attributed these observations on the mesothelioma rate to clearance of the fibres from the lung, although they recognised that the absence of deaths in the final years of follow-up could not be explained by clearance alone.

We agree with the authors that clearance of fibres from the lungs is not a viable explanation for all of their observations. To illustrate this we have modelled the risk with a few simplifying assumptions, …