Silica-induced malignant histiocytic lymphoma: incidence linked with strain of rat and type of silica

Systemic sclerosis, silica exposure and cellular therapies: The sand in the gears?

Systemic sclerosis (SSc) is a chronic orphan autoimmune disease with the highest mortality rate among rheumatic diseases. SSc-related interstitial-lung disease (ILD) remains among the leading causes of SSc-related mortality with still few therapeutic effective strategies. In patients with crystallin silica exposure, SSc is recognized as an occupational disease according to the French social security system (Table 25A of the general insurance regimen). Lympho-ablative or myeloablative immunosuppression followed by autologous hematopoietic stem-cell transplantation (aHSCT) is the only therapeutic approach with demonstrated efficacy, improved survival with disease modifying effects on SSc-fibrotic manifestations (skin disease and ILD) and quality of life. A documented past and/or present occupational silica exposure, with extensive exposure and/or silica-related ILD and/or with persistent silica content in the broncho-alveolar lavage fluid are contra-indications to aHSCT in SSc patients, due to the risk of silica-related malignancy or of SSc relapse. This article aims to discuss alternative options in SSc patients with a history of silica exposure, and how innovative cellular therapies (mesenchymal stromal cells, CAR cells) could represent new therapeutic options for these patients.

Comparing the role of silica particle size with mineral fiber geometry in the release of superoxide from rat alveolar macrophages

Particulate air pollutants and mineral fibers activate inflammatory cells to release oxidants, which contribute to inflammation and injury in the lower respiratory tract. Our aim was to compare the role of silica particle size with mineral fiber length and width in the ability to induce superoxide release from rat alveolar macrophages. We estimated the ability of four types of silica particle samples, with different mode diameter, and three types of mineral fiber samples, with different geometric mean lengths and widths, to induce lucigenin-dependent chemiluminescence (CL) from the cells per number of dust particles (i.e., silica particles and mineral fibers). A close positive correlation was observed between dust size and the ability to induce CL in silica as well as mineral fiber samples. Moreover, the ability of silica samples to induce CL was weaker than that of long mineral fiber sample. This ability increased at a larger rate in small silica particle and thin mineral fiber samples than in large silica particle and thick mineral fiber samples at the initial stage of administration. These results suggest that the kinetics of the induction superoxide release from macrophages is similar between silica particles and mineral fibers; moreover, this depends on silica particle size and mineral fiber geometry. Finally, large silica particles were more active than small ones.

State-of-the-science review of the occupational health hazards of crystalline silica in abrasive blasting operations and related requirements for respiratory protection

Excessive exposures to airborne crystalline silica have been known for over 100 years to pose a serious health hazard. Work practices and regulatory standards advanced as the knowledge of the hazards of crystalline silica evolved. This article presents a comprehensive historical examination of the literature on exposure, health effects, and personal protective equipment related to silica and abrasive blasting operations over the last century. In the early 1900s, increased death rates and prevalence of pulmonary disease were observed in industries that involved dusty operations. Studies of these occupational cohorts served as the basis for the first occupational exposure limits in the 1930s. Early exposure studies in foundries revealed that abrasive blasting operations were particularly hazardous and provided the basis for many of the engineering control and respiratory protection requirements that are still in place today. Studies involving abrasive blasters over the years revealed that engineering controls were often not completely effective at reducing airborne silica concentrations to a safe level; consequently, respiratory protection has always been an important component of protecting workers. During the last 15-20 yr, quantitative exposure-response modeling, experimental animal studies, and in vitro methods were used to better understand the relationship between exposure to silica and disease in the workplace. In light of Occupational Safety and Health Administration efforts to reexamine the protectiveness of the current permissible exposure limit (PEL) for crystalline silica and its focus on protecting workers who are known to still be exposed to silica in the workplace (including abrasive blasters), this state-of-the-science review of one of the most hazardous operations involving crystalline silica should provide useful background to employers, researchers, and regulators interested in the historical evolution of the recognized occupational health hazards of crystalline silica and abrasive blasting operations and the related requirements for respiratory protection.

Silica, silicosis, and lung cancer: a response to a recent working group report

The relationship between crystalline silica and lung cancer has been the subject of many recent publications, conferences, and regulatory considerations. An influential, international body has determined that there was sufficient evidence to conclude that quartz and cristobalite are carcinogenic in humans. The present authors believe that the results of these studies are inconsistent and, when positive, only weakly positive. Other, methodologically strong, negative studies have not been considered, and several studies viewed as providing evidence supporting the carcinogenicity of silica have significant methodological weaknesses. Silica is not directly genotoxic and is a pulmonary carcinogen only in the rat, a species that seems to be inappropriate for assessing particulate carcinogenesis in humans. Data on humans demonstrate a lack of association between lung cancer and exposure to crystalline silica. Exposure-response relationships have generally not been found. Studies in which silicotic patients were not identified from compensation registries and in which enumeration was complete did not support a causal association between silicosis and lung cancer, which further argues against the carcinogenicity of crystalline silica.

The sizes, shapes, and mineralogy of asbestos structures that induce lung tumors or mesothelioma in AF/HAN rats following inhalation

Data from inhalation studies in which AF/HAN rats were exposed to nine different types of asbestos dusts (in 13 separate experiments) are employed in a statistical analysis to determine if a measure of asbestos exposure (expressed as concentrations of structures with defined sizes, shapes and mineralogy) can be identified that satisfactorily predicts the observed lung tumor or mesothelioma incidence in the experiments. Due to limitations in the characterization of asbestos structures in the original studies, new exposure measures were developed from samples of the original dusts that were re-generated and analyzed by transmission electron microscopy using a direct transfer technique. This analysis provided detailed information on the mineralogy (i.e., chrysotile, amosite, crocidolite or tremolite), type (i.e., fiber, bundle, cluster, or matrix), size (length and width) and complexity (i.e., number of identifiable components of a cluster or matrix) of each individual structure. No univariate measure of exposure was found to provide an adequate description of the lung tumor responses observed among the inhalation studies, although the measure most highly correlated with tumor incidence is the concentration of structures > or = 20 microns in length. Multivariate measures of exposure were identified that do adequately describe the lung tumor responses. Structures contributing to lung tumor risk appear to be long (> or = 5 microns) thin (0.4 microns) fibers and bundles, with a possible contribution by long and very thick (> or = 5 microns) complex clusters and matrices. Potency appears to increase with increasing length, with structures longer than 40 microns being about 500 times more potent than structures between 5 and 40 microns in length. Structures < 5 microns in length do not appear to make any contribution to lung tumor risk. This analysis did not find a difference in the potency of chrysotile and amphibole toward the induction of lung tumors. However, mineralogy appears to be important in the induction of mesothelioma with chrysotile being less potent than amphibole.

Immunohistochemical localization of transforming growth factor-beta 1 in rats with experimental silicosis, alveolar type II hyperplasia, and lung cancer

Immunohistochemical localization of transforming growth factor-beta 1 (TGF-beta 1) was studied in the lungs of rats given crystalline silica or ferric oxide by single intratracheal instillation. Ferric oxide elicited no progressive granulomatous reaction, no epithelial hyperplasia, and no lung tumors; no demonstrable reactivity to TGF-beta 1 was observed. Silica induced a granulomatous reaction with progressive fibrosis, adjacent alveolar type II hyperplasia, and alveolar carcinomas. Rabbit polyclonal antibodies to synthetic peptides corresponding to the first 30 amino acids of mature TGF-beta 1, anti-LC (1-30), and anti-CC (1-30) were used for the localization of intracellular and extracellular TGF-beta 1. An antibody to a peptide corresponding to amino acids 266-278 of the TGF-beta 1 precursor sequence, anti-Pre (266-278), was used to detect the TGF-beta precursor and the latency-associated peptide. Intracellular mature TGF-beta (anti-LC) was demonstrated in fibroblasts and macrophages located at the periphery of silicotic granulomas and in fibroblasts adjacent to hyperplastic type II cells. Extracellular mature TGF-beta 1 was localized in the connective tissue matrix of the granulomas and in the stroma of both hyperplastic type II cells and well-differentiated adenocarcinomas. Immunoreactivity to anti-Pre was localized, intracellularly, in hyperplastic alveolar type II cells and their proliferative lesions adjacent to granulomas, in adenomas, but not in adenocarcinomas. The hyperplastic type II cells appear to be the sites of production and secretion of TGF-beta 1, which may regulate their own growth and differentiation and mediate the production of extracellular TGF-beta 1-associated matrix. The lack of reactivity to TGF-beta 1 precursor in the adenocarcinomas is consistent with the loss of normal cellular differentiation and function. TGF-beta 1 appears to have a pathogenetic role in silica-induced mesenchymal and epithelial lesions. The role of TGF-beta 1 and other cytokines in silica-induced carcinogenesis requires further investigation.

Alterations in the pulmonary microsomal cytochrome P-450 system after exposure of rats to silica

Because some evidence indicates that there is an increased incidence of lung cancer in silicosis, we studied the effects of exposing rats to silica on the pulmonary microsomal cytochrome P-450 system. Rats were exposed to silica by intratracheal administration, lung microsomes were obtained from untreated and silica-treated animals, and the amount of microsomal tissue, the level of total cytochromes P-450 (all isozymes), the activity of NADPH cytochrome P-450 reductase, the metabolism of two xenobiotics, and the relative amounts of cytochrome P-4502B1 and P-4501A1 were measured. Lungs from silica-treated rats were almost 2-fold heavier and contained more than 10 times more alveolar phospholipids than lungs from untreated animals, indicating that acute silicosis had been produced. In lungs from silica-treated animals, the concentration of microsomal tissue, expressed as milligrams of microsomal protein per gram of lung, was increased by more than 2-fold, and total microsomal protein content was increased by almost 5-fold relative to untreated animals. When expressed as activity or amount per milligram of protein, the microsomal concentrations of NADPH cytochrome P-450 reductase, total cytochromes P-450, 7-ethoxycoumarin (EC)-0-deethylase, and cytochrome P-4502B1 are reduced by approximately 50% in silica-treated rats. However, when expressed as total activity or amount in the lungs, all are increased by approximately 1.5- to 2.5-fold in silica-treated lungs. On the other hand, total lung 7-ethoxyresorufin (ER)-0-deethylase activity and cytochrome P-4501A1 are increased by 4- to 5-fold in silica-treated lungs.(ABSTRACT TRUNCATED AT 250 WORDS)

Side-effects of dental ceramics

Evaluation of side-effects to low-dose exposure of any agent is difficult, especially if the agent exhibits a low toxicity. The most common way to approach such evaluation is to define special groups which are exposed more than others. Studies of such risk groups may facilitate interpretation of information related to those exposed to a low dose. For dental materials, dentists, dental assistants, and laboratory technicians represent typical risk groups. In addition to receiving dental treatments and having restorations like anyone else, they handle the materials in their daily work. The exposure to the materials occurs more frequently and at a higher dose for these groups than for the patient receiving dental treatment. Thus, the possibilities for side-effects are greater. Some materials are handled more closely by laboratory technicians than by other members of the dental team, e.g., dental ceramics.

Crystalline silica and lung cancer: a review of recent experimental evidence

The International Agency for Research in Cancer (IARC) has recently stated that crystalline silica should be regarded as a potential carcinogen. The IARC bases this statement on a finding that there is limited evidence of carcinogenicity in humans and sufficient evidence for carcinogenicity in animals. Recent laboratory animal experiments demonstrating a carcinogenic response to silica exposure have intensified scientific and regulatory concern for crystalline silica as a respiratory carcinogen. Studies of human populations have been contradictory in demonstrating a causal relationship between crystalline silica exposure and lung cancer. This paper reviews recent experimental evidence and attempts to identify the gaps and inconsistencies in our understanding of the relationship between exposure to crystalline silica and the two diseases of concern: silicosis and pulmonary neoplasia. Given our current level of understanding and the need for more scientific data it seems premature to initiate changes in exposure regulations at this time.

Sister chromatid exchanges in human lymphocytes treated with silica

The abilities of Min U Sil quartz or tridymite particles to induce sister chromatid exchanges (SCEs) in cultures of human lymphocytes plus monocytes or of human purified lymphocytes were investigated. With cultures of lymphocytes plus monocytes the level of SCEs was significantly enhanced after treatment with tridymite at the highest dose tested (50 micrograms/cm2). No effect was observed with purified lymphocytes. Quartz did not give clear cut results. Complementary experiments with tridymite filtrates suggested that phagocytosis of tridymite particles by monocytes was a necessary step for the induction of SCEs in human lymphocytes.

Mortality pattern of silicotic subjects in the Latium region, Italy

A mortality study was carried out on 595 workers who were compensated for silicosis in the Latium region, Italy, during the period 1946-84 who died between 1 January 1969 and 31 December 1984. Respiratory disorders, tuberculosis, lung cancer, bone cancer, and cirrhosis of the liver showed significantly increased risk ratios (4.1, 3.7, 1.5, 4.1, and 1.9 respectively); excesses of brain cancer and leukaemia did not reach statistical significance. Lung cancer mortality was further analysed by age, period of compensation, final degree of disability, and occupational activity. The possible confounding role of smoking was assessed by comparing the lifetime smoking habits of a sample of silicotic subjects with those of the general male population as estimated by a national health survey; the prevalence of ever smokers among silicotic subjects (70.7%) was similar to that estimated for the general population (68.5%). The present study indicates that silicosis is associated with lung cancer even though it does not clarify the respective roles of exposure to silica and silicosis.

Secretion of fibronectin by mineral dust-derived alveolar macrophages and activated peritoneal macrophages

The in vitro secretion of fibronectin by rat alveolar macrophages recovered following the intratracheal instillation of various mineral dusts was examined using a competitive enzyme-linked immunoassay (CELIA) method. Cells derived with the fibrogenic dusts DQ12 quartz and UICC crocidolite asbestos had elevated rates of fibronectin secretion when compared ith those derived from titanium dioxide or saline. The in vitro culture of alveolar macrophages with dusts did not lead to elevated rates of fibronectin secretion, suggesting that mechanisms other than the direct interaction between dusts and macrophages may be responsible for elevated rates of fibronectin secretion by cells exposed to fibrogenic dusts. This suggests that fibronectin deposition seen in pneumoconiotic lesions in immunohistochemical studies may in part have been derived from macrophages. Thioglycollate-induced activated mouse peritoneal macrophages secreted significantly less fibronectin than resident peritoneal macrophages, a finding contrasting with those of Tsukamoto et al. [7].

Cancer risks associated with 10 inorganic dusts: results from a case-control study in Montreal

A multicancer site, multifactor case-control study was undertaken to generate hypotheses about possible occupational carcinogens. Probing interviews were carried out with eligible cases, comprising all incident cases of 20 types of cancer who were male, aged 35-70 years, and a resident in Montreal. The interview was designed to obtain detailed lifetime job histories and information on potential confounders. Each job history was reviewed by a team of chemists and industrial hygienists who translated it into a history of occupational exposures. These occupational exposures were then analyzed as potential risk factors in relation to the sites of cancer included; 3,726 cases were interviewed. For each site of cancer analyzed, controls were selected from among the other sites in the study. This report concerns the associations between the 12 main types of cancer in our series and 10 inorganic dusts that are found mainly in construction and metal industries. All site-exposure combinations were investigated. After intensive control for confounding, nonadenocarcinoma (NAC) of the lung was associated with long duration-high level exposure to silica (odds ratio [OR] = 1.4), excavation dust (OR = 1.9), concrete dust (OR = 2.5), abrasives dust (OR = 1.4), and alumina (OR = 1.5). It was difficult to disentangle the relative effects of those substances, and confounding among them was a distinct possibility. Although residual confounding by some uncontrolled factors may explain the elevated ORs, the results were compatible with the hypothesis of a nonspecific relation between NAC of the lung and respirable inorganic dusts as a class. Other associations that remained suggestive after in-depth analysis were silica and stomach cancer (OR = 1.2) and concrete dust and lymphoma (OR = 2.9).

Lung cancer among Dutch coal miners: a case-control study

A case-control study was conducted in the southern part of The Netherlands to investigate the risk of lung cancer in coal miners; 381 age-matched pairs of primary lung cancer cases and controls, diagnosed between 1972 and 1988, were selected from the pathology department of the University Hospital in the region. Information about past employment in coal mines was obtained through the registers of the collaborative pension fund for Dutch miners. 20% of the cases were (at some time) employed in coal mining, compared with 21% of the controls (odds ratio 0.95; 95% confidence interval: 0.65-1.38). 9% of both cases and controls had an underground work history (odds ratio 0.96; 95% confidence interval: 0.56-1.65). The duration of underground coal mining did not differ substantially between cases and referents (average duration: respectively 117 and 108 months). No relation between specific histologic tumor cell types and coal mining could be demonstrated. The study gives no indication that workers in Dutch coal mines have an increased risk of developing lung malignancies.

Regulation of tumor development: the biphasic effects of silica and of lipopolysaccharide on natural resistance

The impact on tumor development of the BRM silica and LPS was assessed through analysis of changes in NR parameters in vivo and in vitro. Although injection of the fumed silica Cab-o-sil 3 days before a threshold s.c. inoculum of L5178Y-F9 cells increased the tumor frequency in syngeneic DBA/2 mice, tumors recovered from silica-treated animals exhibited an augmented resistance to NAb and to in vivo NR. Cab-o-sil increased in vivo NR and induced a biphasic modulation of anti-tumor NAb and NK activities. The appearance of more autonomous tumors in Cab-o-sil-treated mice corresponding with a stimulation of NR parameters, suggests that the adjuvant activity of silica also contributes to its co-carcinogenic effect by accelerating tumor development. While injection of LPS 2-3 days before a threshold tumor inoculum lowered the tumor incidence, the survival of tumor cells injected within 1 day of LPS was increased. A corresponding early decrease in NAb activity occurred, in contrast with increases in NK cell and NAb levels previously observed after 5 days. This biphasic effect of LPS on NR effectors assayed in vitro was also seen on in vivo NR. Although their frequency was higher, tumors initiated during the period of LPS-induced NR abrogation exhibited greater reductions in NAb binding and sensitivity to NR than tumors from control mice. These data extend the support for NAb acting against tumor cells in vivo and reveal the dual nature of NR in tumor development, defending against small tumor foci and driving the progression of the surviving neoplasm.

Silica-induced alveolar cell tumors in rats

Min-U-Sil5, a form of alpha quartz, has been shown to induce peripheral lung tumors in rats exposed to the dust by inhalation. The animals were exposed to a nominal particle concentration of 12.4 mg/m3 for 8 hr/day, 4 days/week, for 2 years. The induced tumors were large and peripheral, and, when examined by electron microscopy, were found to be composed predominantly of alveolar type II cells. These cells were found in papillary, acinar, and solid forms of the tumors and were characterized by lamellar inclusion bodies. This is in contrast to the mouse, in which the papillary form was associated with Clara cells and the acinar form was linked with the type II cell. In this study, the Clara cell was a minor component of the tumor mass. No clear risk is established in man linking silica exposure to increased lung tumor rates.

A proportionate mortality study of granite cutters

Several recent studies (animal and human) have suggested an association between lung cancer and silica exposure. To test the hypothesis, we have studied death benefit records of 1,905 members of the Granite Cutters Union. A proportionate mortality analysis (PMR) was conducted, using U.S. deaths as a comparison population. Statistically (PMR) was conducted, using U.S. deaths as a comparison population. Statistically significant excesses were observed for death from nonmalignant respiratory significant excesses were observed for death from nonmalignant respiratory disease (largely silicosis) (183 obs, 43.7 exp) and for tuberculosis (largely silicotuberculosis) (262 obs, 19.3 exp). Other significant excesses were observed for bone cancer (6 obs, 1.9 exp) and arthritis (5 obs, 1.5 exp). A significant decrease was observed for leukemia (5 obs, 13.0 exp). For lung cancer a slight but nonsignificant excess was observed (97 obs, 81.1 exp, PMR = 1.19, 95% CI 0.97-1.46). A proportionate cancer mortality analysis (PCMR) showed similar results for lung cancer (PCMR = 1.09, 95% CI 0.89-1.33). Lung cancer mortality also failed to show any trend with either calendar time or duration of exposure. Although no significant excess of lung cancer was observed for the entire silica-exposed cohort, there was an indication that those who were silicotic had an excess risk of lung cancer, based on a review of contributing causes on the death certificate.

Peritoneal mesothelioma and malignant lymphoma in mice caused by fibrous zeolite

Dust from the village of Karain containing the fibrous zeolite erionite, talc, and physiological saline were tested by intraperitoneal injection in 486 Swiss albino mice. Malignant tumours were found in 84 (41 mesotheliomas, 31 lymphomas, 1 peripheral epidermoid carcinoma, and 11 lymphomas and mesotheliomas together) of the 321 animals which died spontaneously within nine to 32 months after injection of Karain dust (26.1%). Three mesotheliomas and no lymphomas were found among 24 animals injected in the same way with talc during the same time (12.5%). In 46 control animals injected with physiological saline three mesotheliomas and one lymphoma were seen (8.7%). Thus Karain dust appears to be a potent carcinogen, causing both mesotheliomas and malignant lymphomas.

Cell population and histochemistry of asbestos related lesions of rat pleural cavity after injection of various inorganic dusts

Rats injected intrapleurally with either crocidolite or chrysotile asbestos or silica or saline, were killed at intervals up to 2 years of age. The pleural cavities were washed out immediately after death and the washing used for enumerating cells. In addition tissue from granulomas and mesotheliomas was sectioned and stained for lysosomal enzymes. The total cellular response to silica found in the washout showed a pronounced increase when compared with either asbestos dust or controls; crocidolite gave a decreased response in an early group of the individual cells studied. The most important finding was a decrease in the number of mast cells found to be associated with the injection of both types of fibres. Crocidolite induced granulomas showed the presence of lysosomal enzymes and non-specific esterase in mononuclear cells and giant cells, even two years after injection. With chrysotile, giant cells were only present up to three to four months, and few positively staining cells were noted after 18 months. While the response of cells in the pleural cavity does not differ greatly between the two types of fibres, that in the granulomas highlights the longer lasting action of crocidolite.

The cytotoxicity of kaolin towards macrophages in vitro

The inhalation of china clay dust by man can cause pneumoconiosis. In an attempt to identify the factors responsible the cytotoxicity in vitro of china clay dust towards mouse peritoneal macrophages was examined. Respirable dusts collected at china clay drying plants were cytotoxic towards the cells. This activity was caused by kaolinite (the major mineral in china clay) and was not due to the presence of ancillary minerals. The cytotoxicity of kaolinite was not due to particle morphology and the positively charged edges of the mineral contributed only slightly to cytotoxicity. An electron microscope study showed that macrophages phagocytosed PVPNO-coated kaolinite particles indicating that the low cytotoxicity of these particles was not due to poor phagocytosis. Residence of china clay in rat lungs appeared to reduce its cytotoxicity. It was concluded that the cytotoxicity of kaolinite was probably related to the proposed amorphous silica-rich gel coating on the particles. The relevance of the findings in vitro to the effects in vivo of china clay is discussed.

In vitro cytotoxicity and quantitative silica analysis of diatomaceous earth products

Mouse peritoneal macrophages were used to evaluate the relative cytotoxicity of a series of diatomaceous earth products in vitro. The amorphous and crystalline silica content of the products was determined by a combination of infrared spectroscopy and x ray powder diffraction techniques. The cytotoxicities of the high cristobalite content flux calcined materials were similar to that of the standard cristobalite ; both the natural and straight calcined materials had significantly greater activities than the flux calcined materials. Thus within the limitations of the macrophage cytotoxicity test the hypothesis that crystalline content is the only determinant of fibrogenicity of diatomaceous earth is not supported.

Effect of mineral dusts on metabolic cooperation between Chinese hamster V79 cells in vitro

The mode of action of inorganic fibers as carcinogens is unknown. There is conflicting evidence whether they act by inducing lesions in DNA, such as initiating carcinogens, or alternatively act as promoters of carcinogenesis. Recent evidence has suggested that promoters inhibit the process of cell-to-cell chemical communication known as metabolic cooperation. Chinese hamster V79-4 cells in culture exhibit metabolic cooperation which can be blocked by the promoter tetraphorbol-13-acetate (TPA). Three kinds of mineral dust were tested to determine whether or not they could interfere with the intercellular communication. The three dusts studied were: amosite, a fibrous asbestos from the Union Internationale Contre le Cancer (UICC) standard reference series, which is a typical carcinogenic fibrous dust inducing pleural tumors following intrapleural injection into rats; ball-milled amosite, a nonfibrous dust derived from UICC amosite by ball-milling, which has the physical properties characteristic of a noncarcinogenic dust; and Min-U-Sil silica, a widely available sample of fibrogenic quartz which does not induce pleural tumors following intrapleural injection into rats. The results demonstrated that metabolic cooperation between V79-4 cells cannot be reduced by carcinogenic asbestos fibers or by relatively large numbers of nontoxic dust particles. It is concluded that if inorganic fibers promote carcinogenesis, then they do not operate by blocking metabolic cooperation.

Cytotoxicity of talc for macrophages in vitro

The cytotoxicity of seven specimens of respirable talc dust for mouse peritoneal macrophages in vitro was studied. All talcs showed modest but consistent macrophage cytotoxicity and would be expected to be fibrogenic in vivo. Available data suggest that under certain circumstances respirable talc can cause lung fibrosis in animal inhalation studies. As most of the talc specimens under investigation were of high purity it seemed unlikely that the cytotoxicity was due to the presence of small quantities of contaminating minerals.

Does occupational exposure to silica cause lung cancer?

Silica is not generally considered to be a carcinogen, however, occupations characterized by high exposure to crystalline silica have excessive rates of lung cancer mortality. Respiratory cancer excesses have been reported from North America and from Europe for the following dusty trades in which exposure to silica is a common factor: iron and steel foundry workers, steel casting workers, sand blasters, metal molders, non-uranium miners, and ceramic workers. These findings have been reinforced by two reports from the Swedish Pneumoconiosis Register and the Ontario Ministry of Labor indicating that silicotics have statistically significant risks of lung cancer mortality. Animal studies suggest that silica can be an initiating carcinogen or can act as a cocarcinogen or promoter when combined with benzo(a)pyrene. We propose three candidate hypotheses and two pathways for silicocarcinogenesis.