Document XRXogr8my66Gg03mZgJmj0xXw

tup and pleural membranes at well at cancers. Whereas identilication.of asbestos dust as a cause of fibrosing in flammation of lung tissue occurred as early as 1907/J* it was not until 1930 lhara more definitive study by Merewether and Price01 resulted in the regulations which greatly im proved hygienic conditions in asbestos factories in the I United Kingdom. The development of lung cancer in as bestos workers, first reported by Wood and Cloyne1'1 in the U.K. in 1934 and by lynch and Smith*1' in the U.S. in 1935, was not firmly established until 1955 by the publication of Doll1*' of a study of workers in an English asbestos textile factory, and in the United States by the paper of Selikoff et af> in 1964 concerned with cancers in insulation workers. In 1960 the relationship between the inhalation of asbestos dust and mesothelioma was demonstrated by Wagner ef al.m Asbestosis is a diffuse but nonuniform fibrosis of ihe lungs that is generally most severe in the basilar portions As a result of the fibrosis some of the airspaces (alveoli) are not perfused with blood and alveoli that are perfused with blood may not be adequately ventilated because of stiff, thickened alveolar walls. The fibrosis makes the lungs less compliant, thereby increasing the energy requirement of breathing. There is increasing impairment in diffusion of gases leading to increasing breathlessness. It is not uncommon to find thickening of the visceral pleura, sometimes very severe, by extension of the paren chymal inflammation. This causes an additional increase in the effort of breathing. The parietal pleura may show patches of severe thicken ing, particularly over the diaphragm and the lower portions of the chest wall -- resulting in the so-called pleural hya line olaoues. These may become visible in X-ray films of the chest -- particularly, if they become impregnated with calcium salts. Such pleural plaques may develop from as bestos exposure in the absence of asbestosis. They cause no symptoms. A study of Ihe members of an asbestos insulators union revealed that deaths from lung cancer in this population was much greater than expected.*7' A later investigation by Hammond and Selikoff of a much larger number of these workers (17,600) showed that nearly all cancers .occurred in cigarette smokers.w The conclusion of these authors was as follows: , _ "fr seems clear, then, that lung cancer is uncommon among asbestos insulation workers who have no hislory of cigarette smoking, and that if the risk is increased such an increase is not great." ) f ) The total lung cancer rate in this cohort of workers was 4.8 times the expected. The asbestos insulators who had a his tory of cigarette smoking had a lung cancer rate 5.4 times the expected rate; but compared to the lung cancer rate of the nonsmoking workers, the smoking insulators' lung can cer rate was 14 times greater. All types of asbestos are known to cause Ihe inflamma tory changes in the lungs and pleurae described above and lung cancer. However, there is experimental and epidemi ologic evidence that there may be differences in the polAmial of the different asbestos types to produce disease. Thus, it has been suggested that crocidolite has the great v est potential to produce disease; chrysolite. the smallest; with amosiie occupying an intermediate position.*'0' In a 2a study i>( 114(1 retirees from the astx-stos industry by Enter line and Henderson.'"' the respiratory cancer rate of men exposed only to chrysotil- was 3.4 times the expected, whereas this rate was 5.3 times the expected for men who had been exposed to a combination of chrysotile and cro cidolite. In the asbestos cement industry a similar differ ence was observed. Workers exposed only to chrysotile and cement (shingles and sheets) had a respiratory cancer rale of 1.4 limes the expected, whereas workers exposed to both, chrysotile and crocidolite and cement (asbestos ce ment pipes), had a respiratory cancer rale 6.1 times the expected. Mesotheliomas are rare, usually ranidlv fatal cancers that originate from the surface lining the chest or abdomi nal cavity. From 1960 through 1975, 4539 mesotheliomas have been reported worldwide.*'7' The vast majority of these cancers were in people exposed to crocidolite alone or in combination with other types of asbestos. McDonald and McDonald*'7' tabulated those reports of mesothelioma where the type of asbestos exposure was known. Although the number of such cases is small, where the exposure was to crocidolite alone or in combination with other types of asbestos, death from mesothelioma constituted 6.1% of the deaths from all causes, with a range of 2.42% to 16,07%. In contrast, deaths from mesothelioma in workers exposed only to chrysotile constituted only 0.3% of the deaths from all causes, with a range of 0.24% to 0.87%. An even greater contrast is found in Ihe Finnish statistics of workers ex posed to anthophyllite. Meurman et a*"1 investigated 216 deaths that occurred among approximately 900 miners and millers of anthophyllite during the 32-ycar period 19361967 and found not one case of mesothelioma. Not all mesotheliomas result from asbestos exposure. There is a background of "naturally" occurring mesothelio mas that has been estimated to be about ten for males and four (or females per million persons aged 45 years and older.t'n Furthermore, it is not uncommon in the various epi demiologic studies reported that 15% or more of the me sothelioma cases have no history of ever having been exposed to asbestos. Perhaps the most important indica tion that mesotheliomas may result from causes other than asbestos exposures: in this instance, also environmental, comes from a report by Baris et afu> who described a me sothelioma incidence of 2.3% in 1974 in the village of Karain in Turkey (population, 604). This population has been exposed for many generations to dust from the soil that contains kaolin, mica, and vulcanic glass particles, but no asbestos. A small excess of deaths from f>aslro-in|pnstinal cancers have been noted in several epidemiologic studies of asbes tos workers.'*"' An association of laryngeal cancer with as bestos exposure has been claimed. P^ncrpatic cancprs and lymphomas have also been mentioned in this connection. However, conversion of the association to a causal rela tionship rests, as yet, on an insecure basis. Whether or not there is a dose-effect relationship asso ciated with asbestos dust has been answered affirmatively by a number of epidemiological surveys.'"'"1 Whereas this relationship is dear-cut with regard to asbeslosis and lung cancer, it is less well-marked with regard to mesothelioma; bui it is, nevertheless positive. McDonald'70' points to a case-control analysis based on seven cases of mesothelio ma at Theiford Mines that includes no case with less than 8002 0452 w HFM - 009714 t>orrsTTr*rr%