Survey of the biological effects of refractory ceramic fibres: overload and its possible consequences.

This paper summarizes the biological effects of refractory ceramic fibres (RCFs). RCFs are aluminosilicate glass insulation wools with similar chemical properties to other synthetic vitreous fibres (SVFs) or 'man-made vitreous fibres' (MMVFs). There is concern that RCFs could be significantly more pathogenic than other SVFs. This paper critically reviews the data on which this perception is based. Morbidity studies on workers in RCF manufacturing indicated that, in the United states, RCF exposure was associated with an increased incidence of pleural plaques and in both the united states and Europe with statistically significant changes in some measures of lung function (though not at present exposure levels). No interstitial fibrosis was found. An ongoing mortality study of limited statistical power has failed to indicate any increased incidence of lung cancer or mesothelioma. Findings in several early animal studies led to a large series of inhalation studies where rats exposed to high levels of RCF developed fibrosis and tumours but not those exposed to other SVFs. Similarly hamsters exposed to one sample (RCF1) developed mesothelioma. Subsequent analyses of the data indicated that the RCF used in these experiments had a significantly greater proportion of non-fibrous particles than those present in the other types of SVFs tested or in workplace air. Short-term studies indicated that pulmonary overload occurred at the same as RCF tissue burdens as those in the long-term animal bioassay. When RCFs were prepared in the same way as the other SVFs, a sample resulted with a more representative ratio of particles to fibres; this sample did not produce overload in short-term tests. SVFs have various abilities to persist in the lung tissue and thus accumulate to varying degrees. It is suggested that biopersistence is a key property. While RCFs are among the more persistent they are similar to many other fibre types. The scientific and regulatory implications of these findings are examined.

Subchronic inhalation study of stone wool fibres in rats.

Pathology results after subchronic inhalation in rats of three separate fibres representing the new biosoluble high-aluminium low-silica HT type stone wool are given, and the results were compared with the results from a similar study done with the traditional stone wool MMVF21. Male Wistar rats were exposed at one exposure level by nose-only inhalation to well-characterized fibre test atmospheres. The fibres had been size selected to be largely rat respirable. The target dose was an exposure to 150 long fibres/ml (length>20 microm) in each group, and this dose was achieved for all the fibres. The negative control groups were exposed to filtered air. The exposure duration was 6 h/day, 5 days/week for 3 months, with a subsequent non-exposure period lasting 3 months. The rats were killed 1 week after the last exposure and additional post-exposure kills were performed at 1.5 and 3 months to monitor the progression of pulmonary change and fibre numbers in the lung. The assessments included bronchoalveolar lavage fluid (BALF) for evaluation of inflammatory response (e.g. protein content, enzymes, increase in polymorphonuclear leucocytes) and measurement of cell proliferation, assessment of early fibrosis through histological examination and comparison of body weight and lung lobe weights. After exposure of rats to the new biosoluble fibres no biologically significant effects were observed except that a statistically significant increase in lung weight was observed up to 1.5 months post-exposure in all three treatment groups. At 3 months post-exposure, the small increase was no longer significant. The increase in lung weight was still present in the MMVF21 group at the 3 months post-exposure kill. After 3 months exposure, lung retention of long fibres (length>20 microm) varied from 0.4 to 5.2 x 10(6) per lung for the biosoluble fibres. At 3 months post-exposure, the long fibre concentration in the lungs had decreased to 1-7% of this figure. The fibre with the relatively highest biopersistence (RIF41001) showed the highest fibre retention. The retention of the more biopersistent traditional stone wool MMVF21 was 5.7 x 10(6) per rat lung after 3 months exposure and had decreased to 64% of this figure at 3 months post-exposure. There was no clear difference in the bronchoalveolar lavage fluid cell concentration and percentage of cells between MMVF21 and the HT groups. Fibre inhalation caused a significant increase after 3 months in all the biochemical parameters measured in the BALF. Cell proliferation was enhanced at the end of exposure for MMVF21 for all three labelling indices, but only for the bronchiolar epithelium in the RIF41001 group and for alveolar parenchymal cells in the RIF43006-1 group. At the termination of the 3 month exposure period, as well as after 1.5 and 3 month recovery periods, minimal morphological changes were diagnosed in the biosoluble fibre groups. These changes included alveolar macrophage aggregation and/or microgranulomas at the bronchiolar-alveolar junction in the few rats affected. No fibrogenic potential was noted for any of the three fibres. No clear-cut difference between the different biosoluble fibre types was noted. In the MMVF21 group, minimal interstitial fibrosis was observed that gradually decreased after the 1.5 and 3 month non-exposure periods. In this study, the pathological changes found in the lungs of exposed rats were in accordance with the pathology previously reported from full lifespan inhalation studies. This may indicate that for fibres belonging to the man-made vitreous fibres group a well conducted biopersistence study is sufficient to predict possible pathogenic effects for new fibre types. The biological parameters examined in a 90 day study may indicate little additional information to contribute to the prediction of the outcome of carcinogenicity studies.

Tumorigenicity of cellulose fibers injected into the rat peritoneal cavity.

Cellulose fibers, along with many other organic fibers, are durable. Therefore, if inhaled, they have the potential to persist within the lung, and may then cause disease. Here we report the effects of injecting high-purity cellulose fibers into the abdominal cavity of rats. A respirable fraction of cellulose fiber was collected from an aerosol of a thermo-mechanically-processed wood pulp. A sample of respirable crocidolite asbestos, known to produce mesotheliomas in rats, was used as a positive control. Total doses of 10(6), 10(7), 10(8), or 10(9) WHO fibers were injected intraperitoneally as 3 weekly aliquots. A negative control was provided by phosphate-buffered saline used to suspend the fibers for injection. There were 50 rats per treatment group except for the 10(8) and 10(9) fibers crocidolite groups which were reduced to 26 rats because of the expectation of high tumor incidence in these groups. The two higher doses of crocidolite asbestos caused greatly reduced survival compared to the saline controls. With cellulose there was a much less marked effect on survival. In the highest dose cellulose group, multiple large nodules (granulomas) and widespread adhesions (bands of new tissue connecting organs to each other and to the abdominal wall) were present in all animals. Granulomas were not observed in the 10(9) fibers crocidolite group. More than 80% of animals in the 10(8) and 10(9) crocidolite asbestos groups had mesotheliomas, a type of tumor sometimes observed in people exposed to asbestos. In contrast, there were only 2 animals in the cellulose groups with mesothelioma tumors, 1 in the 10(7) and 1 in the 10(8) groups. However, 9 (18%) of the 10(9) cellulose group had malignant tumors that, in contrast to the usual pattern of mesothelioma development following treatment with mineral fibers in rats, showed no obvious involvement of mesothelial tissues, were not associated with blood-stained ascites fluid, and were thus classified as sarcomas. This study has demonstrated that a high dose of cellulose fibers is capable of producing tumors when injected into the abdominal cavity of rats.

Carcinogenicity studies after intraperitoneal injection of two types of stone wool fibres in rats.

A summary is given of the pathology results after intraperitoneal (i.p.) injection in rats of insulation wool HT, representing the new biosoluble types. The pathology results are compared with a previously conducted i.p. study with traditional stone wool D6 (with similar chemical composition to MMVF21). The HT fibre is characterized by a relatively high content of aluminium and a relatively low content of silica compared to MMVF21. HT has a high in vitro dissolution rate at pH 4.5, a relatively low dissolution rate at pH 7.5 and is less biopersistent than the MMVF21 fibre. Female Wistar rats received a dose of 2 x 10(9) WHO HT fibres by i.p. injection. The fibres had been size-selected to be largely rat respirable. The negative control group was exposed to saline. Following exposure, the animals were maintained until survival in one group fell below 20%. At this time, all animals were killed. All animals were subjected to a necropsy examination; any gross abnormalities observed at necropsy were subjected to histopathological examination. In addition, histopathology was carried out on a predefined list of tissues. The incidences of lesions and survival in the control and fibre dosed animals were compared using appropriate statistical methods to determine whether the dosed animals showed adverse effects on survival or a positive carcinogenic response. The main protocol for the previously conducted study with D6 (MMVF21) was similar, but the animals were maintained as long as they survived, and the WHO fibre dose was lower. The results of the comparative study showed a marked difference in the i.p. pathogenicity of D6 (MMVF21) and HT in terms of their carcinogenic potential. D6 (MMVF21) caused a statistically significant increase of mesotheliomas in the peritoneal cavity compared to the negative control, but the HT fibre did not cause any mesotheliomas or any increase in other tumour types.

Quantification of fibrosis in the lungs of rats using a morphometric method.

The Wagner grading system is a qualitative histopathologic method designed to describe the severity of nonmalignant respiratory disease (NMRD) as it pertains to the pathology induced by fibrous particulates in humans and later in rats. However, once the method had been used in several rodent fiber studies it was found that it did not adequately differentiate the magnitude of early fibrosis. This article describes a modification of the Wagner scoring system that incorporates a semiquantitative yet simple approach to assuage this problem.