The distribution of amosite asbestos in the periphery of the normal human lung

Although theoretical models and experiments on animals exist that predict the distribution of asbestos fibres in the lung, there are few studies in man that relate to this question and they have generated contradictory results. To examine this distribution analytical electron microscopy was employed to determine the amosite fibre concentration, size, surface area, and mass in 29 circumferential sites around the periphery of a mid-sagittal slice from nine morphologically normal left lungs of heavily exposed shipyard workers and insulators. Fibre concentrations were heaviest in the apical segment of the upper lobe, and low concentrations were seen in the posterior basal portion of the lower lobe. Overall, the upper lung zones had significantly greater concentrations than the lower lung zones. Fibre length was shortest in the anterior portion of the upper lobe, greater in the lingula, and greatest in the posterior basal portion of the lower lobe; fibre length overall was significantly greater in the lower compared with the upper zones. Aspect ratio followed a similar pattern. Distinct geographic runs of high or low concentrations and long or short lengths and aspect ratios were present. No consistent distribution patterns for fibre width, surface area, or mass were found. It is concluded that: (1) in the periphery of the normal lung, concentration of amosite fibres is greatest in the apex and least in the peripheral lower lobe. This distribution is the opposite of what would be expected from the known distribution of asbestosis (peripheral lower zone); nor does it correlate with bronchial pathlength or branch number, contrary to predictions from studies on animals and theoretical models; (2) fibre length and related parameters show a distribution opposite to that of fibre concentration and again do not correlate with theoretical predictions.     

Distribution and characteristics of amphibole asbestos fibres, measured with the light microscope, in the left lung of an insulation worker.

Concentrations of uncoated and coated amphibole fibres were measured postmortem in samples taken from the periphery of both upper and lower lobes of the left lung of an insulation worker. Similar measurements were made on a more limited range of samples from elsewhere in the lung and on a hilar lymph node. The mean concentration of uncoated fibres in the upper lobe was twice that in the lower. Significant differences were observed in the mean concentration of uncoated fibres in different regions of the periphery, the diaphragmatic region having the lowest value. Variations in the concentrations of both uncoated and coated fibres in the costal region of the lower lobe may have been related to the effect of rib structures on ventilation. Concentrations of fibrous and non-fibrous dust were well correlated in the peripheral samples. The length distributions of uncoated and coated fibres were also measured and differences detected in fibres from the various regions of the lung. This work emphasises the care required in sampling lung tissue in order to obtain representative material for the determination of fibre concentration.     

Comparison of fibre types and size distributions in lung tissues of paraoccupational and occupational cases of malignant mesothelioma

The results of analysis of mineral fibres in lung tissues from 10 paraoccupational cases of malignant mesothelioma were compared with analysis obtained from seven cases of malignant mesotheliomas that had developed in gas mask workers. Nine of the paraoccupational cases were considered to have developed their tumours because of exposure to asbestos on their husbands' working clothes and one cancer developed in the daughter of a man who had died of asbestosis. The gas mask workers had direct exposure to asbestos while working in a factory that produced military gas masks. The results of mineral fibre analysis in the paraoccupational cases were variable; six showed high crocidolite concentrations, seven raised amosite concentrations and two normal concentrations of all types of asbestos fibre measured. Chrysotile was raised in one case but crocidolite and amosite were also increased. The gas mask workers showed a consistent pattern with high crocidolite concentrations and normal or low concentrations of chrysotile and amosite. Fibre lengths for chrysotile were similar in both groups and predominantly less than 5 microns. Crocidolite fibres tended to be longer in the gas mask workers than in the paraoccupational group and longer than chrysotile in both groups. Amosite fibres tended to be more variable in width than those of chrysotile or crocidolite.     

Case-referent survey of young adults with mesothelioma: I. Lung fibre analyses

OBJECTIVES: Our study aimed to determine the lung tissue concentration of asbestos and other mineral fibres by type and length in persons with mesothelioma aged 50 yr or less at time of diagnosis, compared to controls of similar age and geographical region. In this age group it was thought that most, but not all, work-related exposures would have been since 1970, when the importation of crocidolite, but not amosite, was virtually eliminated. METHODS: Eligible cases were sought from recent reports by chest physicians to the SWORD occupational disease surveillance scheme. Lung tissue samples were obtained at autopsy from 69 male and four female cases, and mineral fibres identified, sized and counted by electron microscopy. Fibre concentrations per microg dry tissue were compared with similar estimates from a control series of autopsies of sudden or accidental deaths. Unadjusted, and adjusted odds ratios calculated by logistic regression, assessed relative risk in relation to fibre type, length and concentration. RESULTS: Unadjusted and adjusted odds ratios increased steadily with concentration of crocidolite, amosite, tremolite and all amphiboles combined. There was also some increase with chrysotile, but well short of statistical significance. Incremental risk examined in a linear model was as highly significant for all amphiboles together as individually. Short, medium and long amphibole fibres were all associated with increased risk in relation to length. Mullite and iron fibres were significant predictors of mesothelioma when considered without adjustment for confounding by amphiboles, but, after adjustment, were weak and far from statistically significant. CONCLUSIONS: In this young age group, amosite and crocidolite fibres could account for about 80% of cases of mesothelioma, and tremolite for some 7%. The contribution of chrysotile, because of low biopersistence, cannot be reliably assessed at autopsy, but to the extent that tremolite is a valid marker, our results suggest that it was small. The steep linear trend in odds ratio shown by amphiboles combined indicates that their effects may be additive, with increased risk from the lowest detectable fibre level. Non-asbestos mineral fibres probably made no contribution to this disease. Contrary to expectation, however, some 90% of cases were in men who had started work before 1970; this was so whether or not amosite or crocidolite was found in lung tissue.     

Comparison of fibre types and size distributions in lung tissues of paraoccupational and occupational cases of malignant mesothelioma.

The results of analysis of mineral fibres in lung tissues from 10 paraoccupational cases of malignant mesothelioma were compared with analysis obtained from seven cases of malignant mesotheliomas that had developed in gas mask workers. Nine of the paraoccupational cases were considered to have developed their tumours because of exposure to asbestos on their husbands' working clothes and one cancer developed in the daughter of a man who had died of asbestosis. The gas mask workers had direct exposure to asbestos while working in a factory that produced military gas masks. The results of mineral fibre analysis in the paraoccupational cases were variable; six showed high crocidolite concentrations, seven raised amosite concentrations and two normal concentrations of all types of asbestos fibre measured. Chrysotile was raised in one case but crocidolite and amosite were also increased. The gas mask workers showed a consistent pattern with high crocidolite concentrations and normal or low concentrations of chrysotile and amosite. Fibre lengths for chrysotile were similar in both groups and predominantly less than 5 microns. Crocidolite fibres tended to be longer in the gas mask workers than in the paraoccupational group and longer than chrysotile in both groups. Amosite fibres tended to be more variable in width than those of chrysotile or crocidolite.     

Airborne fibre concentrations and lung burden compared to the tumour response in rats and humans exposed to asbestos

The excess risk of tumours exposed to asbestos were previously compared with the results of rat inhalation experiments. It could be demonstrated that humans at the workplace suffer from a tumour risk at fibre concentrations which are 300 times lower than those needed in the rat inhalation model to produce the same risk. However, the estimation of human risk was based on the study of workers at a chrysotile textile factory, whereas animal experimental results were related to exposure to amphiboles. Since for this comparison the risk of cancer due to exposure to amosite or crocidolite fibres at the workplace is of interest, quantitative exposure-response relationships for lung cancer and mesothelioma for the white workforce of South African amosite and crocidolite mines were discussed. On comparing the risk of lung cancer in this study with the risk of lung cancer for chrysotile textile workers, it can be concluded, that the risk of lung cancer and mesothelioma from crocidolite and amosite was higher than in the chrysotile textile factory.It could be also demonstrated, on the basis of a study of the lung burden of mesothelioma cases and of controls, that a significantly increased odds ratio of about 5 was established at amphibole concentrations of between 0.1 and 0.2 f μg⁻¹ dry lung (WHO fibres longer than 5 μm from TEM analysis). On the other hand, carcinogenic response was observed at a fibre concentration 6000 times higher in animal inhalation experiments with crocidolite asbestos (SEM analysis of WHO fibres). As a result of these findings, it has been concluded that inhalation studies in rats are not sufficiently sensitive for the detection of hazards and risks to humans exposed to man-made fibres.     

Evaluation of the oncogenic potential of man-made vitreous fibres: The inhalation model

A rodent inhalation model has been developed for the evaluation of the eoncogenic potential of man-made vitreous fibres. It is successful in delivering a quantified dose of wellchacterized fibres to the lungs of rodents, and with it sufficiently high fibre aerosol concentrations were lofted to enable a maximum tolerated dose to be achieved.Fischer 344 male rats were exposed to a well-defined rat-respirable aerosol at concentrations for MMVF of 30,16 or 3 mg m ⁻³, 6 h per day, 5 days per week for 104 weeks with final sacrifice at 20% survival. A control group was exposed to filtered air. The high dose was chosen based upon a 28-day maximum tolerated dose study with refractory ceramic fibres (RCF). The fibre aerosol generation system lofted fibres without breaking, grinding or contaminating the bulk material. Exposure was by flow-past nose-only systems which provided fresh fibre in a laminar stream to each animal individually. The study was performed according to the Good Laboratory Practice regulations.Fibre count, fibre diameter and length distribution, aerosol mass and chemical composition were determined throughout the study. Interim sacrifices were performed at 3 or 6 month intervals for 24 months. At each sacrifice, full necropsy was performed, the accessory lobe removed for subsequent digestion to determine the fibre lung burden and the remaining lobes inflated with fixative for histopathological evaluation. The lungs were evaluated by a pathologist and graded for the degree of macrophage infiltration, bronchiolization, fibrosis and pleural thickening, and were also scored according to the Wagner scale. Lesions were evaluated according to the number of adenomas, carcinomas and mesotheliomas. The accessory lobe was digested by low-temperature plasma ashing and the number, size distribution and chemical composition of the fibres determined.This model provides a sensitive and reproducible method for evaluating existing and new fibres. A variety of different of ceramic, glass, rockwool and slagwool fibres have been evaluated with this model.     

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.     

Differential release of superoxide anions by macrophages treated with long and short fibre amosite asbestos is a consequence of differential affinity for opsonin.

OBJECTIVE--To investigate the ability of short and long fibre samples of amosite asbestos to stimulate superoxide production in isolated rat alveolar macrophages, and to determine how opsonisation with rat immunoglobulin might modify this response. METHODS--Macrophages were isolated from rat lung by bronchoalveolar lavage and challenged with both opsonised and non-opsonised long and short fibres of amosite asbestos. Release of superoxide anions was measured by the spectrophotometric reduction of cytochrome c, in the presence and absence of superoxide dismutase. RESULTS--Both long and short fibre samples of amosite asbestos without opsonisation were ineffective in stimulating isolated rat alveolar macrophages to release superoxide anions in vitro. After opsonisation with immunoglobulin, however, a dramatic enhancement of release of superoxide anion was seen with long fibres, but not short, which confirms the importance of fibre length in mediating biological effects. The increased biological activity of the long fibre sample is explained by increased binding of the opsonin to the fibre surface as, at equal mass, the long fibres bound threefold more immunoglobulin than the short fibres. CONCLUSION--Opsonisation is an important factor in modulation of the biological activity of fibres at the cellular level. Differences in binding of opsonin to samples of fibre previously considered to be identical apart from length, suggest that surface reactivity needs to be taken into account when fibres are compared. Binding of biological molecules, in vivo, may thus be an important modifying factor in the pathological processes initiated by fibres.     

Physical parameters of airborne asbestos fibres in various work environments-preliminary findings.

The results of a pilot investigation to describe the physical parameters, length, aspect ratio, mass and shape of airborne fibres in a variety of industries producing processing and handling chrysotile, amosite and crocidolite are described. Samples of airborne dust were collected on nucleopore membrane filters and examined by scanning electron microscopy. The diameters and lengths of airborne fibres collected during the dumping of raw amosite at an asbestos products plant were greater than those of fibres collected during the application of amosite insulation. Chrysotile fibres collected in the carding area of an asbestos textile plant also tended to have smaller diameters than fibres collected in the dryer and bagging areas of an asbestos mill. The measurements of fibre dimensions indicate that the degree of protection afforded a worker by optical counts using the membrane filter technique is likely to depend on variety of asbestos and stage of processing. Preliminary results are not in conflict with experimental data suggesting that asbestosis might be related to the mass of airborne dust and primary malignant mesothelial tumors to exposure to fibres in a specific range of fibre diameter and length.     

Concentrations and dimensions of coated and uncoated asbestos fibres in the human lung.

Concentrations and length distributions of uncoated and coated amphibole-type fibres in samples of human lung taken at necropsy were measured by optical microscopy using the membrane filter technique that enables fibres with diameters down to about 0.2 micron to be detected. The subjects included 10 who died with mesothelial tumours, three with lung cancer, and eight of other causes. Measurements of fibre concentrations are compared with those of other workers. It can be deduced from the length distributions that fibres less than 5 microns long are cleared from the lung more efficiently than longer ones. The clearance of short fibres appears to be inhibited in subjects with asbestosis, however. The length distributions of uncoated and coated fibres were dissimilar. In general, few fibres less than 10 microns in length were coated and few greater than 40 microns in length were uncoated. The probability of a fibre of given length, however, becoming coated varied considerably from subject to subject. Possible reasons for this are discussed.     

Influence of fibre length, dissolution and biopersistence on the production of mesothelioma in the rat peritoneal cavity.

A range of respirable man-made mineral fibres were tested for evidence of carcinogenicity by injection into the peritoneal cavity of male SPF Wistar rats; and differences in carcinogenicity were related to the dimensions and biopersistence of the injected fibres. The fibres tested included an amosite asbestos, a silicon carbide whisker, a special purpose glass microfibre, and a range of other man-made vitreous fibres (MMVFs) and refractory ceramic fibres (RCFs) from the TIMA fibre repository. The injected dose of each was designed as the estimated mass required to contain 10(9) fibres > 5 microns in length, as determined by optical microscopy. The numbers of long fibres (> 15 microns) contained in these doses ranged across fibres from 0.1 x 10(9) to 0.8 x 10(9) fibres; the number of long fibres thinner than 0.95 micron ranged from 0.015 x 10(9) to 0.4 x 10(9). The treatment groups contained between 18 and 24 animals. Animals were killed when they showed signs of debilitation. At autopsy, the diagnosis of mesothelioma was usually obvious macroscopically. Otherwise, histological examination of peritoneal organs was used to search for early tumour development. Judged by median survival time, four of the fibre types, in the doses administered, presented higher mesothelioma activity than amosite asbestos. The other fibres tested were less carcinogenic than the amosite. Only a ceramic material derived by extreme heating to simulate the effect of furnace or oven conditions, produced no mesotheliomas. Attempts were made, using regression models, to relate these differences to fibre dimensions and to measures of durability from separate experiments. The results pointed principally to a link with the injected numbers of fibres > 20 microns in length and with biopersistence in the rat lung of fibres longer than 5 microns. Improved quantification of the relative importance of fibre dimensions and biopersistence indices requires experimentation with a range of doses.     

Mineralogical and exposure determinants of pulmonary fibrosis among Québec chrysotile miners and millers

Objectives: Lung fibre content was determined for 86 former chrysotile miners and millers in two Québec mining regions: Thetford mines (TM) and the Asbestos region (AR). Methods: Fibres were assessed using transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDS). Asbestos body (AB) concentrations were assessed by microscopy of tissue digests. Corresponding histological lung tissue sections were quantitatively graded for the severity of interstitial fibrosis on a 12-point scale. Fibrosis score and its associations with (1) fibre concentrations and fibre dimensions within three fibre length intervals (less than 5 μm, 5–10 μm, and over 10 μm), and (2) several exposure variables were evaluated using correlation coefficients and regression techniques. Results: Concentration of short (<5 μm) tremolite fibres was the best predictor of fibrosis grade in both mining groups (r=0.44, P<0.01 and r=0.39, P<0.01 for TM and AR, respectively). Chrysotile fibre concentration showed a lower correlation with the fibrosis grade for subjects from TM only. Long (>10 μm) amosite fibre concentration showed a linear relationship with the fibrosis score in miners and millers from AR. Exposure variables, including smoking, had no predictive value for fibrosis grade. Within fibre length categories, fibre dimension was not related to the fibrosis score. Conclusion: Lung fibre concentration as measured by TEM/EDS, especially that of short (<5 μm) tremolite fibres, is a better predictor of fibrosis grade in these two groups of chrysotile miners than either the concentration of ABs or the duration of exposure. Due to the limitation of our counting method, almost all fibres longer than 10 μm observed in this study were shorter than 14 μm. Thus, if length plays a role in fibrogenesis, it may be related to fibres of greater length than those covered in this study.     

Lung fibre burden in lung cancer cases employed in the rock and slag wool industry

OBJECTIVES: To evaluate the relationship between estimated exposure to man-made vitreous fibres (MMVF) and to asbestos fibres and their concentration in the lung tissue of lung cancer cases amongst MMVF production workers. METHODS: Retrospective retrieval of available lung tissue specimens was conducted following a case-control study that assessed estimated occupational exposures of MMVF workers. Fibre recovery and analysis by transmission electron microscopy (TEM) were conducted to determine fibre type, fibre dimension and numbers per gram of dry lung tissue. For cases with detailed exposure data, geometric mean (GM) concentrations were compared across the exposure categories, and regression models were used to investigate the relationship between the lung fibres and the variables of estimated exposure, with and without additional variables that may affect fibre retention. RESULTS: A total of 24 samples from 17 cases of lung cancer were available for analysis: MMVF were detected in all cases. Asbestos fibres were detected in 16. No difference or trend in GM MMVF concentration was observed across the estimated exposure categories. Odds ratio (OR) for MMVF g(-1) dry lung was 0.5 (95% confidence interval: 0.1-2.4) for the second, and 3.5 (0.6-18.9) for the third quartile of index of average exposure to MMVF in industry, compared with the first (lowest exposed) quartile (no cases in the highest quartile). CONCLUSIONS: No observable relationship existed between estimated exposure and directly-measured lung fibres among this sample of cases. Retrospective specimen collection, intra-individual variability in fibre concentration, effect of unknown factors on fibre retention and small sample size militated against this study providing evidence for or against a relationship between estimated exposure and lung fibre concentrations.     

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.     

Experimental asbestosis: an investigation of functional and pathological disturbances. I. Methods, control animals and exposure conditions.

The measurement of respiratory function and quantitative assessment of changes in lung morphology are described in a long-term study of asbestos-exposed guinea pigs. Resistance and dynamic compliance of the guinea pig respiratory system were measured non-destructively, and the tidal volume and rate of spontaneous breathing were also determined. A point counting technique was used with histological preparations to quantify morphological changes. In one group of control animals, investigated between 3 and 28 months of age, dynamic compliance and tidal volume tended to increase, but resistance and breathing rate tended to decrease, as the animals matured. In contrast the percentage by volume of various lung components remained constant throughout this period. The animals were exposed to chrysotile and amosite aerosols, both aerosols containing high concentrations of short fibres (less than 5 micrometer) and at least 1000 fibres/ml longer than 5 micrometer. The total fibre concentration during chrysotile exposure was probably at least double that during amosite exposure. A gradient of fibre concentration (less than 5 micrometer) was detected along the exposure chamber, but this gradient was not reflected in the respiratory function of animals exposed at different positions along the chamber. These findings are discussed in relation to possible dose-dependence of the response.     

Experimental asbestosis: an investigation of functional and pathological disturbances. I. Methods, control animals and exposure conditions

The measurement of respiratory function and quantitative assessment of changes in lung morphology are described in a long-term study of asbestos-exposed guinea pigs. Resistance and dynamic compliance of the guinea pig respiratory system were measured non-destructively, and the tidal volume and rate of spontaneous breathing were also determined. A point counting technique was used with histological preparations to quantify morphological changes. In one group of control animals, investigated between 3 and 28 months of age, dynamic compliance and tidal volume tended to increase, but resistance and breathing rate tended to decrease, as the animals matured. In contrast the percentage by volume of various lung components remained constant throughout this period. The animals were exposed to chrysotile and amosite aerosols, both aerosols containing high concentrations of short fibres (less than 5 micrometer) and at least 1000 fibres/ml longer than 5 micrometer. The total fibre concentration during chrysotile exposure was probably at least double that during amosite exposure. A gradient of fibre concentration (less than 5 micrometer) was detected along the exposure chamber, but this gradient was not reflected in the respiratory function of animals exposed at different positions along the chamber. These findings are discussed in relation to possible dose-dependence of the response.     

Inflammation generating potential of long and short fibre amosite asbestos samples

Previous studies have shown that long thin asbestos fibres are more pathogenic in in vivo and more active in in vitro assays than short fibre samples. In the present study a long fibre amosite asbestos sample and a short fibre sample prepared from it were tested for ability to cause inflammation in the peritoneal cavity of the mouse; a UICC sample intermediate in fibre size and an inert compact dust, TiO2, were also tested. The ability of the dust samples to cause inflammation, as judged by macrophage and neutrophil recruitment, was ranked in the order long fibre greater than UICC greater than short fibre greater than TiO2. Ability of amosite samples to cause inflammation was therefore related to the proportion of long fibres. The enhanced ability of long fibres to cause inflammation and cause macrophage activation is probably a key factor in the ability of long fibres to cause pulmonary fibrosis and may also be important in fibre carcinogenesis.     

Inflammation generating potential of long and short fibre amosite asbestos samples.

Previous studies have shown that long thin asbestos fibres are more pathogenic in in vivo and more active in in vitro assays than short fibre samples. In the present study a long fibre amosite asbestos sample and a short fibre sample prepared from it were tested for ability to cause inflammation in the peritoneal cavity of the mouse; a UICC sample intermediate in fibre size and an inert compact dust, TiO2, were also tested. The ability of the dust samples to cause inflammation, as judged by macrophage and neutrophil recruitment, was ranked in the order long fibre greater than UICC greater than short fibre greater than TiO2. Ability of amosite samples to cause inflammation was therefore related to the proportion of long fibres. The enhanced ability of long fibres to cause inflammation and cause macrophage activation is probably a key factor in the ability of long fibres to cause pulmonary fibrosis and may also be important in fibre carcinogenesis.     

Iron associated with asbestos bodies is responsible for the formation of single strand breaks in phi X174 RFI DNA.

The ability of amosite cored asbestos bodies isolated from human lungs to catalyse damage to phi X174 RFI DNA in vitro was measured and compared with that of uncoated amosite fibres with a similar distribution of length. Asbestos bodies (5000 bodies) suspended for 30 minutes in 50 mM NaCl containing 0.5 micrograms phi X174 RFI DNA, pH 7.5, did not catalyse detectable amounts of DNA single strand breaks. Addition of the reducing agent ascorbate (1 mM), however, resulted in single strand breaks in 10% of the DNA. Asbestos bodies in the presence of a low molecular weight chelator (1 mM) and ascorbate catalysed the formation of single strand breaks in 21% of the DNA with citrate or 77% with ethylenediamine tetra-acetic acid (EDTA), suggesting that mobilisation of iron may increase damage to DNA. Preincubation for 24 hours with desferrioxamine B, which binds iron (Fe (III)) and renders it redox inactive, completely inhibited the reactivity of asbestos bodies with DNA, strongly suggesting that iron was responsible. Amosite fibres (5000 fibres/reaction), with a similar length distribution to that of the asbestos bodies, did not catalyse detectable amounts of single strand breaks in DNA under identical reaction conditions. The results of the present study strongly suggest that iron deposits on the amosite core asbestos bodies were responsible for the formation of DNA single strand breaks in vitro. Mobilisation of iron by chelators seemed to enhance the reactivity of asbestos bodies with DNA. It has been postulated that the in vivo deposition of the coat material on to fibres may be an attempt by the lung defenses to isolate the fibre from the lung surface and thus offer a protective mechanism from physical irritation. These results suggest, however, that the iron that is deposited on asbestos fibres in vivo may be reactive, potentially increasing the damage to biomolecules, such as DNA, above that of the uncoated fibres.