Is asbestos or asbestosis the cause of the increased risk of lung cancer in asbestos workers?

The molecular epidemiology of asbestos and tobacco in lung cancer

Asbestos is a well-known toxin and lung carcinogen. Epidemiologic studies have established tobacco smoke and asbestos exposures synergistically interact to enhance lung cancer risk. The biologic mechanism responsible for this interaction has been the subject of considerable debate. Studies have suggested that asbestos may act as a carcinogen by generating free radical and reactive oxygen species, by inducing tissue injury and subsequent cellular growth, via large-scale chromosome loss and by enhancing delivery of tobacco carcinogens to the respiratory epithelium. Recent molecular epidemiologic approaches further suggest that asbestos enhances the mutagenicity of tobacco carcinogens and that it acts, at least in part, independent of the tissue damage responsible for fibrosis.

Retention of asbestos fibres in lungs of workers with asbestosis, asbestosis and lung cancer, and mesothelioma in Asbestos township

OBJECTIVE

To conduct a mineralogical study on the particles retained in the necropsied lungs of a homogenous group of asbestos miners and millers from Asbestos township (and a local reference population) and to consider the hypothesis that there is a difference in size between fibres retained in the lungs of patients with asbestosis with and without lung cancer.

METHODS

Samples of lung tissue were obtained from 38 patients with asbestosis without lung cancer, 25 with asbestosis and lung cancer, and 12 with mesothelioma, from necropsied Quebec chrysotile miners and millers from Asbestos township. Fibre concentrations in the lungs of these patients were compared with those in tissue from necropsies carried out on a local reference population: men who had died of either accidental death or acute myocardial infarction between 1990 and 1992. 23 were born before 1940 and 26 after 1940.

RESULTS

Geometric mean (GM) concentrations were higher in cases than in the controls for chrysotile fibres 5 to 10 microns long in patients with asbestosis with or without lung cancer; for tremolite fibres 5 to 10 microns long in all patients; for crocidolite, talc, or anthophyllite fibres 5 to 10 microns long in patients with mesothelioma; for chrysotile and tremolite fibres > or = 10 microns long in patients with asbestosis; and crocidolite, talc, or anthophyllite fibres > or = 10 microns long in patients with mesothelioma. However, median concentrations of each type of fibre in the lungs did not show any significant differences between the three disease groups. Average length to diameter ratios of the fibres were calculated to be larger in patients with asbestosis and lung cancer than in those without lung cancer for crocidolite fibres > or = 10 microns long, for chrysotile, amosite, and tremolite fibres 5 to 10 microns long, and for chrysotile and crocidolite fibres < 5 microns long. However, there was no statistical difference in the median length to diameter ratios for any type of fibres across the disease groups when they were calculated in each patient. Cumulative smoking index (pack-years) was higher in the group with asbestosis and lung cancer but was not statistically different from the two other disease groups.

CONCLUSION

Lung cancers occurred in workers with asbestosis from Asbestos township who had an equal concentration of retained fibres but a tendency to a higher length to diameter ratio of amphiboles. These workers had a 29% higher average cumulative smoking index.

Exposure to crocidolite and the incidence of different histological types of lung cancer

OBJECTIVES

To estimate the relations between exposure to both tobacco smoke and crocidolite and the incidence of various histological types of lung cancer.

METHODS

In 1979 all former workers from the Wittenoom asbestos industry who could be traced were sent a questionnaire on smoking history. Of 2928 questionnaires sent, satisfactory replies were received from 2400 men and 149 women. Of the men, 80% had smoked at some time and 50% still smoked. Occupational exposure to crocidolite was known from employment records and follow up was maintained through death and cancer registries in Australia with histological diagnoses obtained from the relevant State Cancer Registry. Conditional logistic regression was used to estimate the effects of tobacco and asbestos exposure on incidence of different cell types of lung cancer in a nested case-control design.

RESULTS

Between 1979 and 1990, 71 cases of lung cancer occurred among men in this cohort: 27% squamous cell carcinoma, 31% adenocarcinoma, 18% small cell carcinoma, 11% large cell carcinoma, and 13% unclassified or indeterminate. Two of the classified cases and one unclassified case had never smoked. The incidence of both squamous and adenocarcinoma types of lung cancer were greatest in ex-smokers and in those subjects with the highest levels of exposure to crocidolite. After adjustment for smoking habit, the increase in incidence of lung cancer with increasing exposure to crocidolite was greater for squamous cell carcinoma than for adenocarcinoma.

CONCLUSIONS

The results from this study have shown significant exposure-response effects for exposure to crocidolite, and both adenocarcinoma and squamous cell carcinoma of the lung. They also provide some further evidence against the theory that parenchymal fibrosis induced by asbestos is a necessary precursor to asbestos induced lung cancer.

Study of occupational lung cancer in asbestos factories in China

A retrospective cohort study (1972-81) of occupational cancers in asbestos (chrysotile) factories has been previously published. In this paper the results of continued tracing and interviewing of members of this cohort from 1982 to 1986 is reported. The cohort included 5893 persons (45,974 person-years for men and 39,445 person-years for women). Malignant tumours played a large part in causes of death (36.9%). There were 183 cancers and 67 lung cancers among 496 deaths. The mortality due to lung cancer had a tendency to increase. By comparison with a control group, the RR of lung cancer was 5.32 (p < 0.01), and the SRR of lung cancer was 4.2 (p < 0.01), significantly higher than those of a control group. Among 148 cases of death from asbestosis there were 33 cases complicated with lung cancer (22.3%). The dose-response relations between exposure to asbestos and incidence of asbestosis and lung cancer were also studied in one asbestos factory. There was a positive correlation. A synergistic effect was found between cigarette smoking and lung cancer. Preventive and control measures and exposure limits for asbestos dust in the air of workplaces were recommended.

Lung cancer in the lower lobe is associated with pulmonary asbestos fiber count and fiber size

We studied exposure to asbestos, pulmonary fibrosis, fiber count, and fiber size in relation to the lobar origin of lung cancer in 90 consecutive patients. Among the 32 patients with a history of occupational exposure to asbestos, 22 were construction workers. The proportion of lower-lobe tumors increased with the duration of exposure from 45% in those working less than 15 years to 82% in those working 15 years or more in the construction trade, as compared with 25% in patients who were probably not exposed. The location of the tumor in the lower lobe was explained by the high number of total fibers [odds ratio (OR) = 9.0, CI = 2.3-34.6), of fibers 3 microns and longer (OR = 22.1, CI = 3.9-125), and fibers of anthophyllite (OR = 14.6, CI = 2.4-83.4) and crocidolite (OR = 7.0, CI = 1.2-41.2) when the effect of smoking and fibrosis was adjusted in the logistic regression analysis. The location of the tumor did not correlate with fibrosis, pack-years smoked, or the number of short (< 3 microns) fibers. Our findings suggest that asbestos causes an excess of lower-lobe tumors at a relatively low exposure level, independently of pulmonary fibrosis.

Absence of amosite asbestos in airway mucosa of non-smoking long term workers with occupational exposure to asbestos

There is considerable experimental evidence that asbestos fibres are taken up by epithelial cells, and that uptake of fibres is associated with various deleterious, particularly mutagenic, effects. It is not known, however, if asbestos fibres are taken up by human bronchial epithelial cells in vivo. To investigate this question, the amosite asbestos content of the mucosa of seven different airways and four parenchymal sites supplied by these airways in six necropsy lungs from heavily exposed never-smoking long term shipyard and insulation workers without asbestosis was examined. Amosite asbestos was readily found in moderately high concentration in all parenchymal samples, but 33 of 40 airway samples that could be evaluated showed no amosite fibres. The seven positive airways had fibre concentrations that were always much lower than the parenchymal concentrations, and these very few fibres may have been contaminants from the parenchyma. These data suggest that, at least in non-smokers, amosite asbestos either does not penetrate into or does not accumulate in human airway mucosa. These findings also call into question the idea that asbestos acts as a direct airway carcinogen in humans.

Histological type of lung carcinoma in asbestos cement workers and matched controls

Histological types of lung carcinoma were examined in a case series of workers exposed to asbestos cement dust (n = 29) and matched controls (n = 87). The proportion of adenocarcinomas was 31% among the exposed subjects and 15% among the controls (mid-p = 0.05). Among workers with high exposure the proportion of adenocarcinoma was even higher (45%, 5/11; mid-p = 0.03). The proportion of peripheral tumours tended to be higher among exposed cases than controls (24 v 12%, mid-p = 0.12). Lobe of origin did not differ, however, between exposed cases and controls. Thus the study indicates an association between the degree of exposure to asbestos and adenocarcinoma of the lung, and a peripheral rather than central localisation of the tumours, but with virtually the same distribution of lobe of origin as in the general population.

Experimental studies in rats on the effects of asbestos inhalation coupled with the inhalation of titanium dioxide or quartz

Rats were exposed for 1 year, with a 2-year follow-up, to dust clouds consisting of a mixture of amosite or chrysotile asbestos with either titanium dioxide or quartz. The addition of titanium dioxide to asbestos did not increase levels of pulmonary fibrosis above the amounts produced by chrysotile or amosite alone. Quartz, however, greatly increased fibrosis above that produced by the asbestos types alone. Both particulate dusts caused an increase in the numbers of pulmonary tumours and mesotheliomas compared to asbestos alone but while tumours in animals treated with asbestos and quartz tended to occur earlier than tumours with asbestos alone, in animals treated with dusts containing titanium dioxide, tumour production occurred later than with asbestos alone. In animals treated with mixtures of asbestos and quartz, there was evidence of increased transport of fibres across the visceral pleural surface and this may be associated with the finding of a higher proportion of pleural mesotheliomas than previously reported in experimental inhalation studies from any laboratory using the main asbestos varieties. The presence of particulate dusts made little difference to the amounts of amosite fibre retained in the lung tissue but, with chrysotile, titanium dioxide appeared to increase retention while quartz reduced it.

Mesothelioma dose response following intraperitoneal injection of mineral fibres

The relationship between injected dose and the development of peritoneal mesotheliomas has been examined in rats using the UTCC standard reference samples of chrysotile, crocidolite and amosite as well as a sample of fibrous erionite from Oregon. Doses injected into the peritoneal cavity ranged from 0.005 to 25 mg and with each dust a clear dose response was found. The proportion of animals developing tumours increased with the amount of dust injected while the tumour induction period was reduced. When times to death from mesothelioma were analysed using standard hazard models, erionite was the most carcinogenic dust by mass followed by chrysotile, amosite and crocidolite. The hazard slopes for erionite, chrysotile and crocidolite, over the range of doses examined, were parallel while the slope for amosite was shallower. The relative hazards for the various dust types were also examined with dose expressed as the number of injected fibres in a range of sizes as measured by SEM. No combination of fibre dimensions was found at which the hazard for the four dust types was equal although when dose was expressed as the number of long fibres injected (> 8 microns in length) the hazard slopes for chrysotile, crocidolite and amosite were relatively close. The hazard level of erionite remained well above the other dust types regardless of how the dose was expressed.

Human disease consequences of fiber exposures: a review of human lung pathology and fiber burden data

Inhalation of asbestos fibers results in a variety of neoplastic and nonneoplastic diseases of the respiratory tract. Some of these diseases, such as asbestosis, generally occur after prolonged and intensive exposure to asbestos, whereas others, such as pleural mesothelioma, may occur following brief exposures. Inhalation of nonasbestiform mineral fibers can occur as well, and these fibers can be recovered from human lung tissue. Thus, there has been considerable interest in the relationship between mineral fiber content of the lung and various pathologic changes. Techniques for fiber analysis of human tissues have not been standardized, and consequently results may differ appreciably from one laboratory to another. In all reported series, extremely high fiber burdens are found in the lungs of individuals with asbestosis. Although there is a correlation between the tissue concentration of asbestos fibers and the severity of pulmonary fibrosis, further studies of the mineralogic correlates of fiber-induced pulmonary fibrosis are needed. Mesothelioma may occur with fiber burdens considerably less than those necessary to produce asbestosis. More information is needed regarding the migration of fibers to the pleura and the numbers, types, and dimensions of fibers that accumulate at that site. Patients with asbestosis have a markedly increased risk for lung cancer, but the risk of lung cancer attributable to asbestos in exposed workers without asbestosis who also smoke is controversial. Combined epidemiologic-mineralogic studies of a well-defined cohort are needed to resolve this issue. In addition, more information is needed regarding the potential role of nonasbestos mineral fibers in the pathogenesis of lung cancer.

Lobe of origin in the attribution of lung cancer to asbestos

Lung cancer originates most commonly in the upper lobes in the general population but among workers with asbestosis it is most common in the lower lobes. Published data on lobar distribution were used to estimate the probabilities that lung cancer among asbestos workers is attributable to exposure to asbestos. This attribution varies directly with the relative risk. Critical values of the relative risk at which attribution of lung cancer to asbestos equalled its attribution to other causes, mainly smoking, were calculated. At a relative risk above 2.81 upper lobe cancers were more likely to be due to asbestos than not. For middle and lower lobe cancers, the critical relative risk was 1.55. These critical values were compared with published standardised mortality ratios reported for cohorts of workers with asbestosis. Since the ratios ranged from 6.3 to 9.1, the probability that lung cancer in such cases is due to asbestos is high regardless of lobe of origin. In many cohorts unstratified by the presence or absence of asbestosis the risk ratios are below one or both of these critical values. Since risk ratios are so high among workers with asbestosis, the ratios must be lower for workers without asbestosis than the overall ratios for unstratified cohorts. Therefore, the critical values may be useful in workers without asbestosis among such cohorts to estimate the upper limit of the probability that lung cancer in a given lobe is due to exposure to asbestos.