LETTER TO THE EDITOR
Reply to Dr. Dunnigan
Dr. Dunnigan's commentary (1988) raises some very important points, both about the issue of chrysotile as a mesothelial carcinogen in man, and about the interpretation of studies of lung mineral content. As Dr. Dunnigan is undoubtedly aware from his long association with the Quebec chrysotile industry, the distinction between disease caused by chrysotile and that caused by amphiboles (amosite and crocidolite) has often been ignored, both in the scientific literature and in the regulatory process. I agree with him entirely that this distinction is crucial, because it is clear that chrysotile and the amphiboles do have different potential for inducing disease. I have recently reviewed, in detail, elsewhere [Churg, 19881 the topic of chrysotile, tremolite, and their relation to mesothelioma in man, and, rather than repeat all of the detail and numerous references here, I shall just summarize the main conclusions as they relate to Dr. Dunnigan's presentation.
A basic question of immediate importance in this regard is: "Does chrysotile cause mesothelioma in man at all, and, if so, what dose is required?" Dr. Dunnigan's review seems to imply that because various epidemiologic studies have concluded that amphibole is mainly responsible for mesothelioma in man, this implies that amphibole is entirely responsible for mesothelioma in man. However, there is a crucial difference between saying that a certain type of asbestos is a weak mesothelial carcinogen and saying that it is not a mesothelial carcinogen (between "little or no" to use Dr. Dunnigan's last sentence), and Dr. Dunnigan appears to be equating these terms.
One of the major problems in addressing this issue, of course, is the frequent existence of coexposures to both chrysotile and amphibole. However, in my review [Churg, 19881, I was able to find 53 cases in which a reasonable argument (based in part on analysis of mineral content, see below) could be made that a mesothelioma was caused by chrysotile. In saying this, it is worth noting that, of those 53 cases, 41 were found in individuals exposed to chrysotile ore in some form, usually from mining and milling. Ten cases occurred in chrysotile factory workers, and in two cases there was evidence of chrysotile exposure in other settings (albeit these two cases were included more for the sake of argument than as proven examples). Given the interest in the question of chrysotile and mesothelioma in man, the fact that I was only able to find 53 possible cases of chrysotile induced mesothelioma reinforces the idea that chrysotile by itself is not a very powerful carcinogen.