Asbestos Fibers - Fatal Mesothelioma, Pleural and Peritoneal
Carcinogenic Asbestos Fibers
Table 5. Number of mesotheliomas in groups of 50 hamsters following an intrapleural injection.
| Dose mg | Chrysotile | Amosite | Anthophyllite | Crocidolite |
| 25 | 9 | -- | -- | __ |
| 10 | 4 | 3 | 3 | 10 |
| 1 | 0 | 0 | -- | 2 |
| From: Smith, W. E.: "Experimental studies on biological effects of tremolite talc on hamsters" (43-48). Proceedings of Symposium on Talc, Washington (DC), May 8 1978 (Washington, DC, Bureau of Mines, 1978). _. | ||||
Do the very short fibres have no carcinogenic effect then? For testing in animals the hypothesis of size and shape of the fibres critical for carcinogenesis it proved difficult to obtain adequate thin and short fibres. Objections were raised that by ball-milling not only had the length been diminished but the crystal structure of the fibres had also been lost or changed by the heat or otherwise. In other experiments the samples of short fibres contained an admixture of long fibres and the effect appeared to be proportionally related to the percentage of the long fibres in the sample. It proved technically very difficult to produce a reasonable quantity of a uniform sample of thin asbestos fibres a few microns long with all crystal and chemical properties preserved, a sample which could be used in a reasonable number of animals for injection, still less for inhalation experiments. At least in one series of experiments in which good samples were available, the short fibres produced no cancers while the long ones did. The practical importance of this kind of testing is based on the fact that, except in the vicinity of industrial sources, the public at large is exposed almost exclusively to very short asbestos fibres. Such airborne fibres come from natural outcroppings, from the various city activities such as construction, demolition and insulation. Further, very short fibres are ingested with water and beverages, and even some injection drugs were shown to contain very short asbestos fibres. Such exposures are a matter of legitimate questioning by the public. However, continuing evidence of no-effect has been reassuring.
The fact that most cases of mesothelioma are caused by asbestos, and the very malignant course of mesothelioma, for which there is so far no cure, leads to a high level of public attention to this disease. However, it is the asbestos-linked bronchogenic cancer of the lungs which must call for our attention still more urgently. No matter how tragic every single case of mesothelioma is, the absolute number of cases is fortunately small.
Statistical data
Altogether 668 cases of mesothelioma were identified in the entire population of Canada during 15 years (1960-75) and of the United States during one year (1972) after an active search among all pathologists of both countries, where there are several major mining areas of chrysotile, a large manufacturing industry using chrysotile, amosite and crocidolite, and many large shipyard operations. In contrast, in the United States alone there were 50 481 deaths from lung cancer among the male population in one year (1969). In the Federal Republic of Germany an annual average (1976-78) of approximately 150 cases of mesothelioma were reported by all the pathologists out of 63 000 autopsies from 750 000 deaths a year. Among workers exposed to asbestos a sum of 15 studies done in various countries showed 7 885 deaths from all causes over various periods up to 30 years. Of these, 882 (11.2%) were due to lung cancer and 198 (2.5%) to mesothelioma, both pleural and peritoneal.
Medical monitoring
In some countries an effort is made to identify all persons working with asbestos now or having worked with asbestos in the past and to follow up their state of health through their life. While very little can be done to prevent fatal mesothelioma by such medical monitoring, there is some hope for early detection of a curable bronchogenic cancer, and it is possible to persuade the workers examined to stop smoking, and so to reduce the risk of cancer substantially. With the above figures of the incidence of the two diseases in mind, the practical relevance of an attempt to decrease the frequency of lung cancer is obvious.
Some other cancers
Most inhaled asbestos is removed from the respiratory passages by the mucocilliary escalator and leaves the body via the gastrointestinal tract. Consequently cancers of the oesophagus, the stomach and the intestines were to be suspected in substantial industrial exposures. Some occupational epidemiological studies do indicate excess incidence of gastrointestinal cancers but the excess is generally less than that of the pleuropulmonary cancer. Other epidemiological studies do not show any excess, however. In steady populations with exposure to asbestos in drinking water in the vicinity of the asbestos mines in Quebec and in the city of Duluth in the United States, no excess of digestive cancers has been detected, while one study done in California indicated excess linked to asbestos in water. The population of an area with drinking water carried by asbestos-cement pipes over 30 years was studied and no excess of cancers was detected. The possible link to asbestos exposure has not yet been sufficiently clear. The main reason is that cancers of the digestive tract are very common among the general population. Their origin is very strongly influenced by nutritional circumstances and habits, and these habits in turn are influenced by climate, geographical location, and mainly by the ethnic origin of the population. Several animal experiments involving food or drinking water containing asbestos have been undertaken, one of them an unusually extensive experiment with drinking water containing amosite, but none so far has produced any significant excess of digestive cancers.
Carcinoma of the larynx has been found in excess in several groups of asbestos-exposed workers. However, the causal link with inhalation of asbestos is as yet far from clear. A much stronger link between carcinoma of the larynx and cigarette smoking, and even with consumption of alcohol, has been demonstrated, and both of these habits also appear to be high among the causes of carcinoma of the oesophagus.
PELNAR, P. V.