The incidence of pleural mesothelioma in chrysotile asbestos textile workers.

Analysis of the incidence of pleural mesothelioma in asbestos textile workers in relation to age, time since first exposure, intensity of exposure and period of observation indicates that age per se is largely irrelevant; that is, the incidence 30 years after first exposure appears to be much the same irrespective of age at first exposure. This has been shown to be true in animal experiments and has been suggested on theoretical grounds for certain human cancers; it explains the evolution of lung cancer rates in relation to changing smoking habits but has not been demonstrated directly by human data. These results also suggest that mesothelioma incidence may not have been much underestimated in the past, which would imply that the recent marked increase in recorded rates is largely real and not merely a diagnostic artefact. The risk does not appear to have been much higher among men who were initially very heavily exposed. One possible explanation of this surprising observation is that chrysotile is eliminated, or ceases to be biologically active, more quickly than other types of asbestos.