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The purpose of this study was to determine the potential toxicity of docosahexaenoic acid (DHA)-rich microalgae (DRM) from Schizochytrium sp., administered in the diet of growing swine. DRM was administered in the diet to groups of castrated male growing pigs (mixed commercial breeds, Landrace & Large White) reared from early weaned (weighing approximately 20 lbs) to approximately 250–270 lbs. Over the course of the 120 day study, animals were fed ad libitum four DRM treatment diets, each designed to optimize weight gain over the growing cycle, and a control diet. DRM was incorporated into the diet of the first treatment group at a level delivering 2.680 kg DRM per pig over the course of 120 days (a constant, whole-life exposure) equating to 598 g DHA per pig. DRM was incorporated into finisher diets only (administered over the last 42 days of the growing cycle) to treatment groups 2, 3, and 4 delivering 1.169, 3.391, and 5.746 kg DRM per pig (261, 756, and 1281 g DHA per pig). These levels represent approximately 1, 3, and 5 times the anticipated commercial dose and were delivered in a feeding strategy designed to mimic commercial use. Vitamin E was added to all diet groups to provide supplementary dietary antioxidant given the high content of polyunsaturated fat in DRM. There were no statistically significant treatment-related effects in clinical observations, body weights, food consumption, mortality, hematologic values, gross necropsy findings, organ weights or histopathology. The only DRM treatment-related changes were higher weight gain and feed conversion efficiency, anticipated results based on the increased fat content in the experimental DRM treatments. This study demonstrates that administration of DRM (at up to five times the anticipated commercial dose) did not produce any treatment-related adverse effects in commercial strains of swine.

Schizochytrium sp. dried microalgae (DRM) contains oil rich in highly unsaturated fatty acids (PUFAs). Docosahexaenoic acid (DHA n-3) is the most abundant PUFA component of the oil. DHA-rich oil extracted from Schizochytrium sp. is intended for use as a nutritional ingredient in foods. As part of a comprehensive safety assessment program, the reproductive toxicity of DRM was examined in Sprague-Dawley-derived rats Crl:CD(SD)BR (30/sex/group) provided DRM in the diet at concentrations of 0, 0.6, 6.0, and 30%. These dietary levels corresponded to overall average dosages of approximately 400, 3900, and 17,800 mg/kg/day for F0 males (premating) and 480, 4600, and 20,700 mg/kg/day for F0 females, respectively. Prior to mating, males and females of the F0 generation were treated for 10 and 2 weeks, respectively. Treatment of males continued throughout mating and until termination (approximately 3 weeks after mating). Treatment of the females was continued throughout gestation and through lactation day 21. The females were killed after raising their young to weaning at 21 days of age. Food consumption was measured weekly throughout the study (except during mating) and body weights were recorded at least weekly during premating, gestation, and lactation. Reproductive parameters including estrus cycle duration, mating performance, fertility, gestation length, parturition, and gestation index were evaluated. Litter size and offspring body weights were recorded, offspring viability indices were calculated, and physical development (vaginal opening and preputial separation) was assessed for the F1 generation. All adult F0 and F1 animals were subjected to a detailed necropsy. DRM treatment had no effect on estrus cycles or reproductive performance including mating performance, fertility, gestation length, parturition, or gestation index. Litter size, sex ratio, and offspring viability indices were similarly unaffected and there were no effects of DRM treatment on the physical development of F1 animals.

The purpose of this study was to determine the potential toxicity of docosahexaenoic acid-rich microalgae from Schizochytrium sp. (DRM), administered in the diet to rats for at least 13 weeks. DRM was administered in the diet to groups of 20 male and 20 female Sprague-Dawley derived rats (Crl:CD(SD)BR) to provide dosages of 0, 400, 1500, and 4000 mg/kg/day for at least 13 weeks. DRM contained high levels of fat (approximately 41% w/w) of which long-chain highly unsaturated fatty acids (PUFAs) were a major component. Vitamin E acetate was added to DRM at manufacture to provide supplementary dietary antioxidant given the highly unsaturated fat content of DRM. Untreated controls received the basal diet only. An additional group of 20 males and 20 females received basal diet mixed with fish oil (Arista) to provide a target dosage of 1628 mg/kg/day, an amount of fat comparable to that received by rats administered the highest dose of DRM. Vitamin E acetate was also added to the fish oil to provide a comparable level of dietary antioxidant provided to high-dose DRM rats. There were no treatment-related effects in clinical observations, body weights or weight gains, food consumption, hematologic or urinalysis values, gross necropsy findings, or organ weights and there were no deaths. The only treatment-related changes in clinical chemistry parameters were decreases in high-density lipoproteins and cholesterol in the DRM and fish oil groups when compared to the untreated controls. These changes were expected based on the high PUFA content of DRM and fish oil. There were no microscopic findings suggestive of toxicity. Periportal hepatocellular fat vacuolation (accumulation of fat) was observed only in the livers of female rats in both the DRM (all dosages) and fish oil groups. This finding was expected given the higher fat content of both the DRM and the fish oil diets compared to the basal diet fed to the untreated controls. A slight increase in the incidence, but not severity, of cardiomyopathy was observed only in the 4000 mg/kg/day DRM males. This finding was not considered adverse because cardiomyopathy occurs spontaneously in rats and especially male rats of the Sprague-Dawley strain when fed high levels of fat. Since cardiomyopathy does not develop in other species including primates fed high-fat diets, its occurrence in rats is considered to have little relevance to human health. This study demonstrates that administration of DRM did not produce any treatment-related adverse effects in Sprague-Dawley rats of relevance to humans at dosages up to 4000 mg/kg/day for 13 weeks.

Humans are exposed to silicones in a number of commercial and consumer products. Some of these silicones, including octamethylcyclotetrasiloxane (D4), are volatile. Therefore, there is a potential for respiratory exposure. A pharmacokinetic analysis of respiratory exposure to D4 is presented in the accompanying paper (M. J. Utell et al., 1998, Toxicol. Sci. 44, 206-213). Possible immune effects of respiratory exposure to D4 are investigated in this paper. Normal volunteers were exposed to 10 ppm D4 or air for 1 h via a mouthpiece using a double-blind, crossover study design. Assays were chosen to screen for immunotoxicity or a systemic inflammatory response. Assessment of immunotoxicity included enumeration of peripheral lymphocyte subsets and functional assays using peripheral blood mononuclear cells. Because in humans there is no direct test for adjuvant effect of respiratory exposure, we analyzed proinflammatory cytokines and acute-phase reactants in peripheral blood, markers for a systemic inflammatory response, as surrogate markers for adjuvancy. These tests were repeated when the volunteers were reexposed to D4 approximately 3 months after this initial exposure. Blood was obtained prior to exposure, immediately postexposure, and 6 and 24 h postexposure. In these short-term, controlled human exposures, no immunotoxic or proinflammatory effects of respiratory exposure to D4 were found.

A new model has been developed to describe the clearance and retention of kaolin refractory ceramic fibers (RCF) in the rat lung. Fiber dissolution, breakage and differential clearance were considered in this model and their respective rates were determined from the available experimental data. It was shown that the new model offered a better interpretation of the observed fiber size distribution in the lung than our previous model (Yu et al. (1994a) Environ. Res. 65 , 243–253) in which none of these effects were considered.

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 well-characterized 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 slagwood fibres have been evaluated with this model.

A mathematical model has been developed to calculate the deposition of refractory ceramic fibers (RCF) in the human respiratory tract. For a typical size distribution of airborne RCF found in the work place, the fractional deposition and size distribution of deposited fibers in various regions of the respiratory tract are determined. These results are compared with those obtained earlier in rats and hamsters. The deposition fraction in the human lung is higher consisting of more coarse and long deposited fibers. This is caused by less efficient filtration of RCF in the human extrathoracic airways. Over the same exposure period however, total number of fibers deposited in the pulmonary region of the humans lung is about 5–10 times smaller per gram of lung or 1–2.5 times smaller per unit lung surface area than the rodents, depending upon the mode of breathing.

Kaolin-based refractory ceramic fiber (RCF) is a man-made vitreous fiber used primarily in industrial high-temperature applications, especially for insulation of furnaces and kilns. Because of its increasing use and potential for human exposure, a chronic toxicity/ carcinogenicity inhalation study was conducted in Syrian golden hamsters. Two groups of 140 weanling male hamsters were exposed via nose-only inhalation to either HEPA-filtered air (chamber controls) or 30 mg/m3 (-220 fibers/cm3) of “size-selected” RCF fibers (1 µm in diameter and -25 /jm in length) for 6 h/day, 5 dayslwk for 18 mo. They were then held unexposed until -20% survival (20 mo). A positive control group of 80 hamsters was exposed to 10 mg/m3 chrysotile asbestos (0.09 µm average diameter and 2.2 µm average length). Groups of 3 hamsters were sacrificed at 3, 6, 9, 12, 15, and 18 mo to follow the progression of lesions. Additional groups of 3 hamsters were removed from exposure at 3, 6, 9, and 12 mo and were held until 18 mo (r...

A dosimetry model of refractory ceramic fibers in the hamster lung has been developed based upon the data from recent exposure and recovery experiments conducted by the Research and Consulting Company in Geneva, Switzerland. The modeling results of hamsters showed significant differences from those of an earlier study in rats regarding deposition and clearance of these fibers in the lung. Despite smaller airway size of hamsters, alveolar deposition per breathing cycle in the hamster was found to be higher than that in the rat due to the higher upper airway deposition in rats. The calculated mean deposited fiber size was found to be larger in the hamster, and there were more thin and long fibers deposited in the hamster lung. It was also found that alveolar clearance of refractory ceramic fibers was faster in the hamster. The clearance rate in the hamster did not appear to be fiber size dependent but it varied with lung burden. The higher clearance rate in the hamster resulted in a lower fiber accu...

Refractory ceramic fibers (RCF) are man-made vitreous fibers used primarily in industrial high-temperature applications, especially for insulation of furnaces and kilns. Because of their increasing use and potential for human exposure, a chronic toxicity/carcinogenicity inhalation study was conducted in Fischer 344 (F344) rats. Five groups of 140 weanling male F344 rats were exposed via noseonly inhalation to either HEPA-filtered air (chamber controls) or 30 mg/m(3) (approximately 220 fibers/cm(3)) of three types [kaolin-based, high-purity, and aluminum zirconia silica (AZS)] of "size-selected" RCF fibers (approximately 1µ in diameter and approximately 20 um in length) and an "after-service" heat-treated (2400°F for 24 h) kaolin-based fiber for 6 h/day, 5 days/wk for 24 mo. They were then held unexposed until approximately 20% survival and then sacrificed at 30 mo. A positive control group of 80 F344 rats was exposed to 10 mg/m(3) chrysotile asbestos. Croups of 3-6 animals were sacrificed at 3, 6, 9, 12, 15, 18, and 24 mo to follow the progression of lesions and to determine fiber lung burdens. Additional groups of 3 rats were removed from exposure at 3, 6, 9, 12, and 18 mo and were held until sacrificed at 24 mo (recovery groups) for similar determinations. Lung burdens increased rapidly for all RCFs, appearing to plateau by about 12 mo. By 24 mo, lung burdens ranged from 2.6 to 9.6 × 10(5) fiberslmg of dry lung tissue for the RCFs tested. Treatment-related lesions were restricted to the lungs. To some extent all types of RCF resulted in macrophage infiltration, bronchiolization of proximal alveoli, and microgranuloma formation by 3 mo of exposure. Interstitial fibrosis was observed at 6 mo for all types of RCF, except the "after-service" fiber where fibrosis was not seen until 12 mo. The lesions progressed in severity until 12-15 mo, after which they plateaued. A minimal amount of focal pleural fibrosis was first observed at 9 mo and progressed to a mild severity by the end of the study. Fxposure-related pulmonary neoplasms (bronchoalveolar adenomas and carcinomas combined) were observed with all 4 types of RCF [kaolin, 16 of 123 (13%); AZS, 9 of 121 (7.4%); high-purity, 19 of 121 (15.7%); and "after-service,"4 of 118 (3.4%)], compared to 2 of 120 (1.5%) in the untreated air controls. Pleural mesotheliomas were observed in two kaolin, three AZS, two high-purity, and one "after-service" exposed rats. A comparable but slightly greater amount of fibrosis was observed in the lungs of the positive (chrysotile asbestos) controls. The incidence of bronchoalveolar neoplasms in the chrysotile exposed rats was 13 of 69 (18.8%), and a mesothelioma occurred in 1 (1.4%) animal. The results of this study showed that the four types of RCF studied had carcinogenic activity in rats at the maximum tolerated dose.

This article describes the relationship between fiber biopersistence and the chronic toxicity of different chemical compositions of man-made vitre? ous fibers (MMVF) in the lung. Rats were exposed in "nose-only" inhalation chambers, 6 hr/day, 5 days/week, for 24 months to aerosol concentra? tions of 30 mg/m3 containing comparable fiber numbers and similar dimensions of fibrous glass (FG) or refractory ceramic fiber (RCF). Interim sacrifices were performed periodically to monitor fiber number and dimensions in the lung and the progression of pulmonary alterations. At each interim sacrifice, three to six recovery animals were removed from each exposure group and held until two years to determine the biopersistence of fibers after different exposure times. Fibers were recovered from the ashed lungs, counted, and measured using optical and scanning electron microscopy (SEM). Fiber chemistry was assessed in 91-week recovery lungs using energy dispersive spectroscopy (EDS) analysis. RCF induced lung fibrosis and an elevation in lung tumors and pleural mesotheliomas. FG exposure resulted in no lung fibrosis, no statistically significant increase in the lung tumor incidence, and no mesotheliomas. After two years of continuous exposure, the number of World Health Organization fibers per milligram dry lung recovered from RCF and FG exposed lungs was comparable. EDS analysis of recovery lungs showed that most of the alkalis and alkaline earths had leached from the FG fibers over time. A slight change in RCF chemistry was observed. These findings indicate that the change in the chemical composition of fibers may be an important determinant of the chronic toxicity of MMVFs. ? Environ Health Perspect 102(Suppl 5): 133-137 (1994).

Chronic exposure and postexposure experiments have been recently performed in rats to evaluate the biological responses of inhaled refractory ceramic fibers (RCF) at different concentration levels. The lung burden data in the accessory lobe of the rat lung were collected during and after different exposure and postexposure periods. The size distribution of retained fibers in the lung at different time points was also measured. We used these data to develop a mathematical model of fiber clearance from the rat lung. It was found that the clearance rate did not depend significantly upon fiber size but there was a clear dependence on lung burden. As lung burden increased, the clearance rate was found to decrease. An empirical equation was derived for the clearance rate as a function of lung burden. At low burdens, rats had a retention half-time of about 126 days for RCF compared to a typical half-time of about 60 days for insoluble nonfibrous particles.

An earlier mathematical model (Asgharian and Yu, J. Aerosol Sci. 20, 355–366, 1989) has been modified to calculate the deposition of refractory ceramic fibers (RCF) in the rat lung. Using the anatomical whole lung model and lobar lung model proposed by Schum and Yeh ( Bull. Math. Biol. 42, 1–15, 1980), fractional deposition efficiency and size distribution of deposited fibers in various regions of the lung were determined. It was found that a large fraction of long and thick fibers deposited in the nasopharynx and the tracheobronchial tree by the mechanisms of impaction and interception, while the geometric mean diameter and length of the fibers deposited in the pulmonary region decreased considerably from their respective values in the aerosol state. The calculated size distribution in the pulmonary region of the accessory lobe was found in good agreement with the data obtained in recent exposure experiments.

Refractory ceramic fibers (RCF) are man-made vitreous fibers used primarily in industrial high-temperature applications, especially for insulation of furnaces and kilns. Because of their increasing use and potential for human exposure an in an effort to define the dose-response, as a follow up to a maximum tolerated dose [30 mg/m(3)] study in rats (Mast et al., 1995), a multiple dose chronic toxicity/carcinogenicity inhalation study was conducted in Fischer 344 (F344) rats. Four groups of 140 weanling male F344 rats were exposed via noseonly inhalation to either HEPA-filtered air (chamber controls) or 3, 9, or 16 mg/m(3)(approximately 36, 91, and 162 fibers/cm(3)) of kaolin-based "size-selected" RCF fibers (approximately 1 µm in diameter and approximately 20 µm in length) for 6 h/day, 5 days/wk for 24 mo. They were then held unexposed until approximately 20% survival and sacrificed (30 mo). Croups of 3-6 animals were sacrificed at 3, 6, 12, 18, and 24 mo to follow the progression of pulmonary lesions and to determine fiber lung burdens. Additional groups of 3-6 rats were removed from exposure at 3, 6, 12, and 18 mo and were held until sacrificed at 24 mo (recovery groups) for similar determinations. A dose-related increase in fiber lung burden was observed. Lung burdens at 24 mo ranged from 5.6 × 10(4) to 27.8 × 10(4) fibers/mg dry lung tissue. Significant increases in lung weights and lung to body weight ratios were seen in the high-dose group. Treatment-related lesions were restricted to the lungs. To some extent, all doses of RCF resulted in minimal to mild macrophage infiltration, bronchiolization of proximal alveoli, and microgranuloma formation by 12 mo of exposure. Interstitial fibrosis was observed at 12 mo in the 9 and 16 mg/m(3) groups but not in the low-dose group at any time point. A minimal amount of focal pleural fibrosis was first observed at 12 mo in the 9 and 76 mg/m(3) dose groups and progressed to a mild severity in the high-dose group by the end of the study. The incidence of pulmonary neoplasm's was well within the range typically reported in the male F344 rat. Neoplasm's (bronchoalveolar adenomas and carcinomas) were observed in all groups 10 mg/m(3) (air control), 1 of 129 (0.8%); 3 mg/m(3), 2 of 123 (1.6%); 9 mg/m(3), 5 of 127 (3.9%); 16 mg/m(3), 2 of 124 (1.6%)]. A single pleural mesothelioma was observed in an animal exposed to 9 mg/m(3) of kaolin RCF. The results of this study suggest that the dose response for primary lung neoplasms is steep, while that for mesothelioma may not be.

Octamethylcyclotetrasiloxane, D4, is a low viscosity, silicone fluid consisting of four dimethyl-siloxy units ((CH3)2SiO)4 in a cyclic structure. It is primarily used as a building block in the industrial synthesis of long chain silicone polymers. The combination of D4 with decamethylcyclopentasiloxane (D5) is commonly referred to as cyclomethicone which has a wide range of applications as a formulation aid in personal care products. To extend the existing database regarding the biological activities of D4, a 28 day whole body vapor inhalation study was conducted using Fischer 344 rats at 0 (room air), 7, 20, 60, 180 and 540 ppm for 6 hours/day, 5 days/week. Parameters measured included body weights, organ weights, gross pathology, histopathology, serum chemistries, and urinalysis. In addition to these standard toxicological endpoints, the ability of D4 exposed animals to mount an IgM antibody response was evaluated by a splenic antibody forming cell (AFC) assay and a serum enzyme-linked immunosorbant assay (ELISA). The results of this 28-day inhalation study indicate that D4 exposure caused no adverse effects on body weight, food consumption, or urinalysis parameters. In addition, there were no exposure related histopathological alterations at any site for any exposure group. A statistically significant increase in liver weight and the liver to body weight ratio was observed in both male (180-540 ppm) and female (20-540 ppm) rats, which was not observed in the 14-day recovery group animals. There were no other significant organ weight changes. Although statistically significant changes were observed in several hematological and serum chemistry parameters in both the terminal and 14-day recovery animals, the changes were marginal and within the normal range of values for the rat. Under these experimental conditions, there were no alterations noted in immune system function at any of the D4 exposure levels.

Isofrax and Insulfrax are two new synthetic vitreous fibers (SVFs) developed for high-temperature insulation (1800-2300 degrees F) applications. In an attempt to significantly reduce or eliminate the potential of adverse health effects, these two fibers were specifically designed to have high solubility and, thus, low in vivo biodurability. In this paper, we review the effects of chemical composition on biodurability, in vitro fiber dissolution rates (K(dis)), and the relevance and relationship of K(dis) to pulmonary fibrosis and lung tumors in chronic rat inhalation studies. We also examine the correlations between K(dis) and weighted in vivo half-life (t(0.5)) of long fibers (>20 microm) and their relation to pulmonary effects in chronic rat inhalation bioassays. Predictions for outcomes of inhalation bioassays and development of nonsignificant risk levels of exposure are provided. Additionally, justification for the use of inhalation versus noninhalation animal data is provided as is a brief review of human health effects of SVFs. We conclude, inter alia, that Isofrax and Insulfrax have low biodurability, would not be expected to produce either pulmonary fibrosis or lung tumors in a well-designed animal inhalation bioassay, have weighted half-lives beneath the threshold established by the European Union for classification as a carcinogen, and based on epidemiological data for SVFs would not be expected to result in incremental cancer in human cohorts. Finally, it is estimated that approximately 90% of workplace exposure concentrations of these materials would be beneath 1 f/cc. At a concentration of 1 f/cc, neither fiber would be expected to result in an incremental working lifetime cancer risk greater than 10(-5).

A mathematical retention model has been developed to predict the lung burden and size distribution of kaolin refractory ceramic fibers (RCF) in the pulmonary region of the human lung during exposure. Fiber dissolution, breakage, and differential clearance are considered in this model; rates for these processes are obtained by extrapolation from available data on laboratory rats. The lung burden predicted by this model is in general agreement with fiber counts from three factory workers. An important prediction from this study is that clearance of RCF is not significantly impaired at a fiber concentration beneath 10 f/cm 3 during occupational exposure.

This study was initiated to assess the potential for kaolin based refractory ceramic fibers (RCF) to induce toxicity and tumors in Syrian golden hamsters after chronic exposure. To simulate the fiber dimensions found in workplace air and to increase fiber respirability in the rodent, RCF fibers were size-selected before aerosolization to be approximately 1µm in diameter and ~20 µm in length. Three groups of animals were exposed in nose-only inhalation chambers, 6 hrs/day, 5 days/week, for 18 months to 30 mg/m3 of RCF, 10 mg/m3 of chrysotile asbestos (positive control; average diameter = 0.09 µm, average length = 2.2 µm), or to filtered air (negative control). Animals from each group were sacrificed at three month intervals for evaluation of the sequential changes in lung pathology over the course of the study. Microscopic examination of the RCF exposed lungs revealed pulmonary interstitial fibrosis (grade 4 on the Wagner Pathology Grading Scale) starting at 6 months and showed little progression of the lesion through the remainder of the exposure. Examination of the asbestos exposed hamster lungs revealed an average pathology grade of 4.3 at 3 months, which progressed to an average score of 5.0 at 6 months and plateaued at that level. The latter change was characterized by interlobular linking of the lesions observed in the grade 4 lungs. In addition, a time-dependent increase in collagen deposition in the pleura was observed in the RCF exposed animals and to a lesser extent in the positive control animals. RCF exposure resulted in no pulmonary neoplasms. However, pleural mesotheliomas were found in 43 out of 102 RCF exposed hamsters (42%). No lung neoplasms or mesotheliomas were observed in either the chrysotile exposed or negative control hamsters.

Lifetime "nose-only" inhalation studies were conducted in rats using four types of refractory ceramic fibers (FCF), 1 micron in diameter x 22 to 26 microns length: High Purity, Kaolin, Zirconia, and After-Service; and on hamsters using Kaolin RCF. For comparison, animals also were exposed to chrysotile fibers. Rats were exposed 6 hr/day, 5 days/week for 24 months to concentrations ranging between 3 and 30 mg/m3. Time- and dose-dependent lesions in the rat included the development of interstitial fibrosis, pleural fibrosis, pulmonary tumors, and mesothelioma. Exposure to 3, 9 or 16 mg/m3 produced no excess lung tumors; no fibrosis was seen at 3 mg/m3. A significant increase in lung tumors and interstitial fibrosis was observed at 30 mg/m3. A single mesothelioma was observed in rats exposed to 9 mg/m3, while two occurred at 30 mg/m3. Hamsters were similarly exposed to 30 mg/m3 Kaolin RCF for 18 months; no lung tumors were induced, but pulmonary and pleural fibrosis were observed and there was a 42% incidence of mesothelioma. Multiple interim sacrifices together with recovery animals allowed detailed assessment of the lung burden of RCF, which was found to be dose related and, at the high doses, exceeded 10(5) fibers/mg of dry lung. During the various recovery periods there was a clear reduction in fiber burden. Mathematical modeling of these data for deposition, clearance, and retention and for species is currently underway.