Widespread exposure to cobalt metal dust occurs occupationally through the production of alloys, in the manufacture of cobalt salts, and in nuclear technology. It is an effective catalyst for many organic reactions, particularly in hydrotreating catalysts, which have molybdenum and cobalt sulfides as active components. Concerns have been raised about the occurrence of occupational disease, i.e. hard metal disease, associated with exposure to cobalt and its compounds, including cobalt metal-tungsten carbide. Cobalt metal is also widely dispersed in low concentrations in the environment and the general population may be exposed by breathing air, drinking water, or skin contact with soil, water, cobalt alloys, or other substances that contain cobalt. In addition, cobalt metal is an essential trace element as a component of cyanocobalamin (vitamin B12). Cobalt metal dust was nominated for toxicology and carcinogenesis studies by the United Auto Workers and the Cobalt Development Institute based on the widespread occupational exposure and limited availability of data on chronic toxicity and carcinogenic potential of inhaled insoluble cobalt compounds, particularly cobalt metal dust. Inhalation was selected as the route of exposure because this is the most common route of exposure to cobalt metal dust in occupational settings in humans. Male and female F344/N or F344/NTac rats and B6C3F1/N mice were exposed to cobalt metal by inhalation for 2 weeks, 3 months, or 2 years (F344/NTac rats). In addition, genetic toxicology studies were conducted in Salmonella typhimurium, Escherichia coli, and mouse peripheral blood erythrocytes.
Two-week study in rats
Groups of five male and five female rats were exposed to cobalt metal particulate aerosol by inhalation at concentrations of 0, 2.5, 5, 10, 20, or 40 mg/m3, 6 hours plus T90 (12 minutes) per day, 5 days per week for 16 days. Additional groups of five female rats were exposed to the same concentrations for 16 days for tissue burden studies. All rats exposed to 40 mg/m3 and all male and three female rats exposed to 20 mg/m3 died before the end of the study. Mean body weights of males exposed to 10 mg/m3 and of females exposed to 10 or 20 mg/m3 were significantly decreased. Females exposed to 20 mg/m3 lost weight during the study. Exposure-related clinical findings included abnormal breathing, lethargy, and thinness in male rats exposed to 20 or 40 mg/m3 and in females exposed to 40 mg/m3. Dark lungs were observed at necropsy in all rats exposed to 40 mg/m3 and most rats exposed to 20 mg/m3 that died early. Absolute lung weights of females exposed to 10 or 20 mg/m3 and the relative lung weights of both sexes exposed to 10 mg/m3 and females exposed to 20 mg/m3 were significantly greater than those of the chamber controls. Absolute and relative liver weights of males exposed to 2.5 mg/m3 or greater and absolute liver weights of females exposed to 5 mg/m3 or greater were significantly less than those of the chamber controls.
The relative liver weight of 20 mg/m3 females was significantly greater than that of the chamber controls. Absolute kidney weights of males exposed to 10 mg/m3 and females exposed to 20 mg/m3 were significantly less than those of the chamber controls. The absolute testis weight of the 10 mg/m3 group was significantly less than that of the chamber controls. Increased incidences of nonneoplastic lesions of the lung occurred in exposed male and female rats and included hemorrhage, acute inflammation, alveolar epithelium hyperplasia, histiocytic cellular infiltration of the alveolus, cytoplasmic vacuolization of bronchiolar epithelium, necrosis of the bronchiolar epithelium, and interstitial fibrosis of the alveolar epithelium. Increased incidences of nonneoplastic lesions of the nose occurred in exposed male and female rats and included olfactory epithelium necrosis, olfactory epithelium atrophy, respiratory epithelium necrosis, and respiratory epithelium squamous metaplasia. Tissue concentrations of cobalt increased with increasing exposure concentration in all tissues examined.
Two-week study in mice
Groups of five male and five female mice were exposed to cobalt metal particulate aerosol by inhalation at concentrations of 0, 2.5, 5, 10, 20, or 40 mg/m3, 6 hours plus T90 (12 minutes) per day, 5 days per week for 17 days. Additional groups of five female mice were exposed to the same concentrations for 17 days for tissue burden studies. Three male and three female mice exposed to 40 mg/m3 died before the end of the study. Final mean body weights were significantly decreased in male and female mice exposed to 20 or 40 mg/m3, and mean body weight gains of 20 and 40 mg/m3 males and all exposed groups of females were significantly less than those of the chamber controls. Females exposed to 20 mg/m3 and males and females exposed to 40 mg/m3 lost weight during the study. Exposure-related clinical findings included abnormal breathing, lethargy, and thinness in male mice exposed to 20 or 40 mg/m3 and females exposed to 10 mg/m3 or greater. At necropsy, tan lungs were observed in most males and females exposed to 20 or 40 mg/m3. Lung weights of both sexes exposed to 10 mg/m3 or greater were significantly greater than those of the chamber controls. Liver weights of exposed male and female mice were significantly less than those of the chamber controls (except relative weight at 40 mg/m3). Increased incidences of nonneoplastic lesions of the lung occurred in exposed male and female mice and included alveolar histiocytic cellular infiltration, cytoplasmic vacuolization of the bronchiolar epithelium, alveolar/bronchiolar epithelium karyomegaly, interstitial fibrosis, and acute inflammation. Increased incidences of nonneoplastic lesions of the nose occurred in exposed groups of male and female mice and included acute inflammation, olfactory epithelium atrophy, olfactory epithelium necrosis, cytoplasmic vacuolization of the respiratory epithelium, and squamous metaplasia of the respiratory epithelium. Tissue concentrations of cobalt increased with increasing exposure concentration in all tissues examined.
Three-month study in rats
Groups of 10 male and 10 female rats were exposed to particulate aerosols of cobalt metal by inhalation at concentrations of 0, 0.625, 1.25, 2.5, or 5 mg/m3, 6 hours plus T90 (12 minutes) per day, 5 days per week for 14 weeks. Additional groups of 10 male rats (clinical pathology study) and 32 to 36 female rats (special study) were exposed to the same concentrations for 14 weeks. All male and female rats survived to the end of the study. Final mean body weights of males and females exposed to 5 mg/m3 were significantly less than those of the chamber controls, and the mean body weight gain of 5 mg/m3 males was significantly less than that of the chamber controls. At necropsy, pale foci were noted in the lungs of most exposed male and female rats. In male rats, exposure concentration-related increases in the hemoglobin concentration, erythrocyte count, hematocrit value, and manual packed cell volume occurred in the 2.5 and 5 mg/m3 groups on days 3 and 23 and in all exposed groups by week 14; at week 14, female rats also had increases in these parameters. Exposure concentration-related decreases in cholesterol concentrations were observed at all three time points in male and female rats. While this change was not always observed in the lower exposure groups, decreases were consistently observed in the 2.5 and 5 mg/m3 groups of both sexes on day 23 and at week 14. In addition, glucose concentration was decreased in males exposed to 1.25 mg/m3 or greater at week 14. Lung weights of all exposed groups of males and females were significantly greater than those of the chamber controls. Sperm motility was significantly decreased in male rats exposed to cobalt, suggesting a potential for cobalt metal to be a reproductive toxicant in male rats.
In the lung, chronic active inflammation and alveolar proteinosis occurred in all exposed males and females, and bronchiole epithelium hyperplasia occurred in all males and females exposed to 1.25 mg/m3 or greater. In the nose, incidences of olfactory epithelium degeneration and respiratory epithelium hyperplasia were significantly increased in males and females exposed to 2.5 or 5 mg/m3. The incidences of olfactory epithelium hyperplasia were significantly increased in 2.5 and 5 mg/m3 males and in 5 mg/m3 females. Significantly increased incidences of turbinate atrophy occurred in 2.5 mg/m3 females and 5 mg/m3 males and females. Tissue concentrations of cobalt increased with increasing exposure concentration in all tissues examined.
Three-month study in mice
Groups of 10 male and 10 female mice were exposed to particulate aerosols of cobalt metal by inhalation at concentrations of 0, 0.625, 1.25, 2.5, 5, or 10 mg/m3, 6 hours plus T90 (12 minutes) per day, 5 days per week for 14 weeks. Additional groups of 32 to 36 female mice (special study) were exposed to the same concentrations for 14 weeks. One 2.5 mg/m3 female mouse was accidentally killed during the first week of the study; all other mice survived to the end of the study. The mean body weights of males and females exposed to 10 mg/m3 were significantly less than those of the chamber controls. Abnormal breathing was noted in approximately 50% of males and females exposed to 10 mg/m3. At necropsy, tan lungs were noted in mice exposed to 5 or 10 mg/m3. Lung weights of males exposed to 2.5 mg/m3 or greater and females exposed to 5 or 10 mg/m3 were significantly greater than those of the chamber controls. Liver weights of males exposed to 10 mg/m3 and females exposed to 2.5 mg/m3 or greater were significantly less than those of the chamber controls. Kidney weights of males and females exposed to 5 or 10 mg/m3 were significantly less than those of the chamber controls. Testes weights of males exposed to 5 or 10 mg/m3 were significantly less than those of the chamber controls. Exposure concentration-related decreases in reproductive tissue weights, spermatid and epididymal spermatozoa counts, and sperm motility in combination with histopathologic findings in both the testis and epididymis indicate that cobalt metal is likely to be a reproductive toxicant in male mice.
In the lung, alveolar histiocytic cellular infiltration and bronchiole epithelium cytoplasmic vacuolization occurred in the lung of all exposed male and female mice. Bronchiole epithelium hyperplasia occurred in all mice exposed to 2.5 mg/m3 or greater. Alveolar proteinosis and alveolar/bronchiolar epithelium karyomegaly occurred in all males and females exposed to 5 or 10 mg/m3. The incidences of hemorrhage were significantly increased in 5 mg/m3 females and in 5 and 10 mg/m3 males. In the nose, the incidences of olfactory epithelium degeneration were significantly increased in males and females exposed to 1.25 mg/m3 or greater. Incidences of respiratory epithelium degeneration were significantly increased in males exposed to 1.25 mg/m3 or greater and females exposed to 2.5 mg/m3 or greater. Incidences of respiratory epithelium squamous metaplasia were significantly increased in males and females exposed to 2.5 mg/m3 or greater, and incidences of turbinate atrophy and chronic active inflammation were significantly increased in the 5 and 10 mg/m3 groups of males and females. The incidences of squamous metaplasia were significantly increased in the larynx of all exposed groups of males and females. Tissue concentrations of cobalt increased with increasing exposure concentration in all tissues examined.
Two-year study in rats
Groups of 50 male and 50 female rats were exposed to cobalt metal particulate aerosol by inhalation at concentrations of 0, 1.25, 2.5, or 5 mg/m3, 6 hours plus T90 (12 minutes) per day, 5 days per week for up to 105 weeks. Additional groups of 35 lung burden study female rats were exposed to the same concentrations of cobalt metal for up to 105 weeks. Survival of female rats exposed to 2.5 mg/m3 was significantly less than that of the chamber control group. Mean body weights of 2.5 and 5 mg/m3 males were at least 10% less than those of the chamber control group after weeks 99 and 12, respectively, and those of 2.5 and 5 mg/m3 females were at least 10% less after weeks 57 and 21, respectively. Exposure-related clinical findings included abnormal breathing and thinness in male and female rats.
In the lung, the incidences of alveolar/bronchiolar adenoma, alveolar/bronchiolar carcinoma, and alveolar/bronchiolar adenoma or carcinoma (combined) occurred with positive trends in male and female rats and with the exception of the incidence of alveolar/bronchiolar adenoma in 1.25 mg/m3 females, the incidences were significantly greater than those in the chamber controls. The incidences of multiple alveolar/bronchiolar adenoma and carcinoma generally increased with increasing exposure concentration, and the incidences of multiple carcinoma were significantly increased in all exposed groups of males and in 5 mg/m3 females. The incidences of cystic keratinizing epithelioma were increased in exposed groups of female rats; cystic keratinizing epithelioma also occurred in two exposed males. One female rat exposed to 5 mg/m3 had a squamous cell carcinoma. The incidences of alveolar epithelium hyperplasia, alveolar proteinosis, chronic active inflammation, and bronchiole epithelium hyperplasia in all exposed groups were significantly greater than those in the chamber control groups.
There was a higher frequency and different spectrum of point mutations within hot spot regions of Kras, Egfr, and Tp53 genes within alveolar/bronchiolar carcinomas from cobalt metal-exposed male and female rats compared to spontaneous alveolar/bronchiolar carcinomas. Kras mutations and G→T transversions were most frequent in rats chronically exposed to cobalt metal.
A spectrum of nonneoplastic lesions occurred in the nose of exposed male and female rats including chronic active and suppurative inflammation, respiratory metaplasia, atrophy, hyperplasia, basal cell hyperplasia, and necrosis of the olfactory epithelium; hyperplasia, squamous metaplasia, and necrosis of the respiratory epithelium; and atrophy of the turbinate.
In the adrenal medulla, incidences of benign pheochromocytoma, malignant pheochromocytoma, and benign or malignant pheochromocytoma (combined) occurred with positive trends in male and female rats, and with the exception of the incidence of malignant pheochromocytoma in 2.5 mg/m3 females, the incidences in rats exposed to 2.5 or 5 mg/m3 were significantly greater than those in the chamber controls. The incidences of bilateral benign and malignant pheochromocytoma were significantly increased in the 5 mg/m3 groups. Incidences of hyperplasia were significantly increased in female rats exposed to 1.25 or 2.5 mg/m3.
The incidences of carcinoma and adenoma or carcinoma (combined) of the pancreatic islets occurred with positive trends in male rats. The incidences of adenoma in 2.5 mg/m3 males and of adenoma or carcinoma (combined) in males exposed to 2.5 or 5 mg/m3 were significantly greater than those in the chamber controls. Incidences of neoplasms of the pancreatic islets in 5 mg/m3 females were slightly increased.
The incidences of mononuclear cell leukemia were significantly increased in all exposed groups of female rats.
In the combined standard and extended (step-section) evaluations of the kidney, the incidence of renal tubule adenoma or carcinoma (combined) was increased in male rats exposed to 5 mg/m3.
The incidence of infarct in the testes was significantly increased in male rats exposed to 5 mg/m3.
Cobalt concentrations in the lung increased with increasing exposure concentration.
Two-year study in mice
Groups of 50 male and 50 female mice were exposed to cobalt metal particulate aerosol by inhalation at concentrations of 0, 1.25, 2.5, or 5 mg/m3, 6 hours plus T90 (12 minutes) per day, 5 days per week for up to 105 weeks. Additional groups of 35 lung burden study female mice were exposed to the same concentrations of cobalt metal for up to 105 weeks. Survival of males exposed to 2.5 or 5 mg/m3 was significantly less than that of the chamber control group. Mean body weights of 5 mg/m3 males and females were at least 10% less than those of the chamber control groups after weeks 85 and 21, respectively. Abnormal breathing and thinness were noted in exposed male and female mice.
In the lung, incidences of alveolar/bronchiolar carcinoma and alveolar/bronchiolar adenoma or carcinoma (combined) occurred with positive trends in male and female mice, and the incidences were all significantly greater than those in the chamber controls. The incidences of alveolar/bronchiolar adenoma were significantly increased in 2.5 mg/m3 males and in 5 mg/m3 females. The incidences of multiple alveolar/bronchiolar carcinoma were significantly increased in all exposed groups of males and females.
The incidences of alveolar/bronchiolar epithelium hyperplasia and cytoplasmic vacuolization, alveolar epithelium hyperplasia, proteinosis, and alveolus infiltration cellular histiocyte were significantly increased in all exposed groups of males and females. The incidences of bronchiole epithelium hyperplasia were significantly increased in males exposed to 5 mg/m3 and females exposed to 2.5 or 5 mg/m3. The incidence of bronchiole epithelium erosion was significantly increased in males exposed to 2.5 mg/m3. The incidences of suppurative inflammation were significantly increased in males exposed to 2.5 or 5 mg/m3 and females exposed to 5 mg/m3.
There was a higher frequency and different spectrum of point mutations within hot spot regions of Kras, Egfr, and Tp53 genes within alveolar/bronchiolar carcinomas from cobalt metal-exposed male and female mice compared to spontaneous alveolar/bronchiolar carcinomas. Kras mutations and G→T transversions were most frequent in mice chronically exposed to cobalt metal.
In all groups of exposed male and female mice, significant increases occurred in nasal lesions including suppurative inflammation; olfactory epithelium atrophy, hyperplasia, and respiratory metaplasia; cytoplasmic vacuolization and squamous metaplasia of the respiratory epithelium; and atrophy of the turbinate. The incidences of atypical respiratory metaplasia of the olfactory epithelium and hyaline droplet accumulation of the respiratory epithelium were significantly increased in 1.25 and 2.5 mg/m3 males and females.
The incidences of respiratory epithelium squamous metaplasia and cytoplasmic vacuolization of the larynx in all exposed groups of males and females were significantly greater than those in the chamber control groups. The incidences of squamous epithelium hyperplasia were significantly increased in all exposed groups of females and in males exposed to 5 mg/m3. In the trachea, the incidences of epithelium cytoplasmic vacuolization were significantly increased in all exposed groups of males and females.
The incidence of germinal epithelium degeneration in the testes was significantly increased in male mice exposed to 5 mg/m3.
Cobalt concentrations in the lung increased with increasing exposure concentration.
Genetic toxicology
Cobalt metal was mutagenic in S. typhimurium strain TA98 in the absence of exogenous metabolic activation (S9); no activity was seen in the presence of S9. Cobalt metal induced a small increase in mutant colonies in strain TA100 in the absence of S9, and no mutagenic activity was seen with S9. No mutagenic activity was detected in E. coli strain WP2 uvrA/pKM101 with or without S9. Results of peripheral blood erythrocyte micronucleus tests in male and female mice in the 3 month study were negative.
Conclusions
Under the conditions of these 2-year inhalation studies, there was clear evidence of carcinogenic activity of cobalt metal in male F344/NTac rats based on increased incidences of alveolar/bronchiolar adenoma and carcinoma in the lung, including multiples, and on increased incidences of benign and malignant pheochromocytoma of the adrenal medulla, including bilateral neoplasms (a summary of the Peer Review Panel comments and the public discussion on this Technical Report appears in Appendix O). The increased incidences of pancreatic islet adenoma or carcinoma (combined) were considered related to exposure. The occurrences of cystic keratinizing epithelioma of the lung and of renal tubule adenoma or carcinoma (combined) may have been related to exposure. There was clear evidence of carcinogenic activity of cobalt metal in female F344/NTac rats based on increased incidences of alveolar/bronchiolar adenoma and carcinoma in the lung, including multiples, and on increased incidences of benign and malignant pheochromocytoma of the adrenal medulla, including bilateral neoplasms. The occurrences of squamous cell neoplasms of the lung (predominantly cystic keratinizing epithelioma), and of mononuclear cell leukemia were considered related to exposure. The occurrences of pancreatic islet carcinoma may have been related to exposure. There was clear evidence of carcinogenic activity of cobalt metal in male and female B6C3F1/N mice based on increased incidences of alveolar/bronchiolar neoplasms of the lung (predominantly carcinoma), including multiple carcinoma.
Exposure to cobalt metal resulted in increased incidences of nonneoplastic lesions of the lung and nose in male and female rats, the testes in the male rats and mice, the adrenal medulla in female rats, and the lung, nose, larynx, and trachea in male and female mice.
National Toxicology Program (NTP). 2014. NTP technical report on the toxicology studies of cobalt metal (CASRN 7440-48-4) in F344/N rats and B6C3F1/N mice and toxicology and carcinogenesis studies of cobalt metal in F344/NTac rats and B6C3F1/N mice (inhalation studies). Research Triangle Park, NC: National Toxicology Program. Technical Report 581. https://doi.org/10.22427/NTP-TR-581