Chronic toxicity/oncogenicity study of styrene in CD-1 mice by inhalation exposure for 104 weeks

Groups of 70 male and 70 female Charles River CD-1 mice were exposed whole body to styrene vapor at 0, 20, 40, 80 or 160 ppm 6 h per day 5 days per week for 98 weeks (females) or 104 weeks (males). The mice were observed daily; body weights, food and water consumption were measured periodically, a battery of hematological and clinical pathology examinations were conducted at weeks 13, 26, 52, 78 and 98 (females)/104 (males). Ten mice of each gender per group were pre-selected for necropsy after 52 and 78 weeks of exposure and the survivors of the remaining 50 of each gender per group were necropsied after 98 or 104 weeks. An extensive set of organs from the control and high-exposure mice were examined histopathologically, whereas target organs, gross lesions and all masses were examined in all other groups. Styrene had no effect on survival in males. Two high-dose females died (acute liver toxicity) during the first 2 weeks; the remaining exposed females had a slightly higher survival than control mice. Levels of styrene and styrene oxide (SO) in the blood at the end of a 6 h exposure during week 74 were proportional to exposure concentration, except that at 20 ppm the SO level was below the limit of detection. There were no changes of toxicological significance in hematology, clinical chemistry, urinalysis or organ weights. Mice exposed to 80 or 160 ppm gained slightly less weight than the controls. Styrene-related non-neoplastic histopathological changes were found only in the nasal passages and lungs. In the nasal passages of males and females at all exposure concentrations, the changes included respiratory metaplasia of the olfactory epithelium with changes in the underlying Bowman's gland; the severity increased with styrene concentration and duration of exposure. Loss of olfactory nerve fibers was seen in mice exposed to 40, 80 or 160 ppm. In the lungs, there was decreased eosinophilia of Clara cells in the terminal bronchioles and bronchiolar epithelial hyperplasia extending into alveolar ducts. Increased tumor incidence occurred only in the lung. The incidence of bronchioloalveolar adenomas was significantly increased in males exposed to 40, 80 or 160 ppm and in females exposed to 20, 40 and 160 ppm. The increase was seen only after 24 months. In females exposed to 160 ppm, the incidence of bronchiolo-alveolar carcinomas after 24 months was significantly greater than in the controls. No difference in lung tumors between control and styrene-exposed mice was seen in the intensity or degree of immunostaining, the location of tumors relative to bronchioles or histological type (papillary, solid or mixed). It appears that styrene induces an increase in the number of lung tumors seen spontaneously in CD-1 mice.

Chronic toxicity/oncogenicity study of styrene in CD rats by inhalation exposure for 104 weeks

Groups of 70 male and 70 female Charles River CD (Sprague-Dawley-derived) rats were exposed whole body to styrene vapor at 0, 50, 200, 500, or 1000 ppm 6 h/day 5 days/week for 104 weeks. The rats were observed daily, body weights and food and water consumption were measured periodically, and a battery of hematologic and clinical pathology examinations was conducted at weeks 13, 26, 52, 78, and 104. Nine or 10 rats per sex per group were necropsied after 52 weeks of exposure and the remaining survivors were necropsied after 104 weeks. Control and high-exposure rats received a complete histopathologic examination, while target organs, gross lesions, and all masses were examined in the lower exposure groups. Styrene had no effect on survival in males, but females exposed to 500 or 1000 ppm had a dose-related increase in survival. Levels of styrene in the blood at the end of a 6-h exposure during week 95 were proportional to exposure concentration. Levels of styrene oxide in the blood of rats exposed to 200 ppm or greater styrene were proportional to styrene exposure concentration. There were no changes of toxicologic significance in hematology, clinical chemistry, urinalysis, or organ weights. Males exposed to 500 or 1000 ppm gained less weight than the controls during the first year and maintained the difference during the second year. Females exposed to 200, 500, or 1000 ppm gained less weight during the first year; those exposed to 500 or 1000 ppm continued to gain less during months 13-18. Styrene-related non-neoplastic histopathologic changes were confined to the olfactory epithelium of the nasal mucosa. There was no evidence that styrene exposure caused treatment-related increases of any tumor type in males or females or in the number of tumor-bearing rats in the exposed groups compared to controls. In females, there were treatment-related decreases in pituitary adenomas and mammary adenocarcinomas. Based on an overall evaluation of eight oncogenicity studies, there is clear evidence that styrene does not induce cancer in rats.

Subchronic inhalation studies of styrene in CD rats and CD-1 mice

Groups of 10 male and 10 female Charles River (CRL) CD (Sprague-Dawley-derived) rats were exposed to styrene vapor at 0, 200, 500, 1000, or 1500 ppm 6 hr per day 5 days per week for 13 weeks. Styrene had no effect on survival, hematology, or clinical chemistry. Males at 1500 ppm weighed 10% less after 13 weeks and males and females at 1000 and 1500 ppm consumed more water than controls. Histopathologic changes were confined to the olfactory epithelium of the nasal mucosa. Groups of 20 male and 20 female CRL CD-1 and B6C3F1 mice were exposed to styrene vapor at 0, 15, 60, 250, or 500 ppm 6 hr per day 5 days per week for 2 weeks. Mortality was observed in both CD-1 and B6C3F1 mice exposed to 250 or 500 ppm; more female mice, but not males, died from exposure to 250 ppm than from 500 ppm. Groups of 10 male and 10 female CRL CD-1 mice were exposed to styrene vapors at 0, 50, 100, 150, or 200 ppm 6 hr per day 5 days per week for 13 weeks. Two females exposed to 200 ppm died during the first week. Liver toxicity was evident in the decedents and in some female survivors at 200 ppm. Changes were observed in the lungs of mice exposed to 100, 150, or 200 ppm and in the nasal passages of all treatment groups, those exposed to 50 ppm being less affected. Satellite groups of 15 male rats and 30 male mice were exposed as described above for 2, 5, or 13 weeks for measurement of cell proliferation (BrdU labeling). No increase in cell proliferation was found in liver of rats or mice or in cells of the bronchiolar or alveolar region of the lung of rats. No increase in labeling index of type II pneumocytes was seen in mouse lungs, while at 150 and 200 ppm, an increased labeling index of Clara cells was seen after 2 weeks and in occasional mice after 5 weeks. Large variations in the labeling index among animals emphasize the need for large group sizes. For nasal tract effects, a NOAEL was not found in CD-1 mice, but in CD rats, the NOAEL was 200 ppm. For other effects, the NOAEL was 500 ppm in rats and 50 ppm in mice.