Effect of age at inoculation of asbestos on occurrence of mesotheliomas in rats.

An experiment in which rats were injected intrapleurally with asbestos, the injection taking place at an age of either 2 or 10 months, is reported. In the former group 19 out of 48 rats developed a mesothelioma compared with 17 out of 90 in the latter group. However, by considering the two groups at equal times after injection, eliminating the effects of natural mortality and also taking into account that not all of the mesotheliomas were the cause of death, it was shown that the incidence rate of mesotheliomas was significantly higher (p less than 0.01) in the rats injected at age 10 months and the relative rate was estimated as about 4, with 95% confidence limits of 1.5 and 12. An alternative explanation that the mesotheliomas occurred in the two groups at similar rates, except that in the group injected at age 10 months they occurred on average 4 months sooner after injection, is discussed. Two other groups were also injected, one at age 17 months and the other at 22 months. In the former group two early mesotheliomas were observed, 10 and 74 days after injection, but the next was not observed until over a year later. No explanation of this finding could be suggested other than the possibility that these two tumours were not associated with the asbestos treatment.