Nickel oxide (NiO) "sinters" are used in stainless steel and alloy steel production. Nickel oxide was nominated by the National Cancer Institute to the NTP for testing because exposure to this form of nickel is prevalent in the nickel industry. Increased incidences of lung and nasal sinus cancers have occurred among workers in certain nickel refining facilities, and nickel oxide was studied as part of a class study of nickel compounds. Male and female F344/N rats and B6C3F1 mice were exposed to nickel oxide (high temperature, green nickel oxide; mass median diameter 2.2 ± 2.6 mm; at least 99% pure) by inhalation for 16 days, 13 weeks, or 2 years. Genetic toxicology studies were conducted in peripheral blood of B6C3F1 mice exposed to nickel oxide for 13 weeks.
Sixteen-day study in rats
Groups of five male and five female F344/N rats were exposed to 0, 1.2, 2.5, 5, 10, or 30 mg nickel oxide/m3 (equivalent to 0, 0.9, 2.0, 3.9, 7.9, or 23.6 mg nickel/m3) by inhalation for 6 hours per day, 5 days per week for a total of 12 exposure days during a 16-day period. Additional groups of five male and five female rats were exposed to 0, 1.2, 5, or 10 mg/m3 for tissue burden studies. All core study rats survived until the end of the study, final mean body weights of exposed male and female rats were similar to those of the controls, and there were no clinical findings related to nickel oxide exposure. Absolute and relative lung weights of male and female rats exposed to 10 or 30 mg/m3 were significantly greater than those of the controls. Pigment particles in alveolar macrophages or within the alveolar spaces were observed in the lungs of exposed groups of males and females. Chronic-active inflammation and accumulation of macrophages in alveolar spaces of the lungs and hyperplasia in the respiratory tract lymph nodes were most severe in 10 and 30 mg/m3 males and females. Hyperplasia of bronchial lymph nodes occurred in 30 mg/m3 rats. Atrophy of the olfactory epithelium was observed in one male and one female exposed to 30 mg/m3. The concentrations of nickel oxide in the lungs of exposed groups of rats were greater than those in the lungs of control groups (males, 42 to 267 mg nickel/g lung; females, 54 to 340 mg/g lung).
Sixteen-day study in mice
Groups of five male and five female B6C3F1 mice were exposed to 0, 1.2, 2.5, 5, 10, or 30 mg nickel oxide/m3 by inhalation for 6 hours per day, 5 days per week for a total of 12 exposure days during a 16-day period. Additional groups of five male and five female mice were exposed to 0, 1.2, 2.5, or 5 mg/m3 for tissue burden studies. No exposure-related deaths occurred among core study mice, and final mean body weights of exposed male and female mice were similar to those of the controls. There were no chemical-related clinical findings. Pigment particles were present in the lungs of mice exposed to 2.5 mg/m3 or greater. Accumulation of macrophages in alveolar spaces was observed in the lungs of 10 and 30 mg/m3males and females. The concentrations of nickel oxide in the lungs of exposed groups of mice were significantly greater than those in the lungs of control animals (males, 32 to 84 mg nickel/g lung; females, 31 to 71 mg/g lung).
Thirteen-week study in rats
Groups of 10 male and 10 female F344/N rats were exposed to 0, 0.6, 1.2, 2.5, 5, or 10 mg nickel oxide/m3 (equivalent to 0, 0.4, 0.9, 2.0, 3.9, or 7.9 mg nickel/m3) by inhalation for 6 hours per day, 5 days per week for 13 weeks. Additional groups of 18 male and 18 female rats were exposed to 0, 0.6, 2.5, or 10 mg/m3 for tissue burden studies. No exposure-related deaths occurred among core study rats, final mean body weights of exposed male and female rats were similar to those of the controls, and no clinical findings in any group were related to nickel oxide exposure. Lymphocyte, neutrophil, monocyte, and erythrocyte counts; hematocrit values; and hemoglobin and mean cell hemoglobin concentrations in exposed rats were minimally to mildly greater than those of the controls; these differences were most pronounced in females. Mean cell volumes in exposed rats were generally less than those in the controls. Absolute and relative lung weights of exposed groups of males and females were generally significantly greater than those of controls.
Chemical-related nonneoplastic lesions were observed in the lungs of male and female rats exposed to concentrations of 2.5 mg/m3 or higher, and the severity of these lesions generally increased with exposure concentration. Accumulation of alveolar macrophages, many of which contained black, granular pigment, was generally observed in all exposed groups of males and females, and increased incidences of inflammation occurred in males and females exposed to 2.5 mg/m3 or higher. In addition, lymphoid hyperplasia and pigment occurred in the bronchial and mediastinal lymph nodes of 2.5, 5, and 10 mg/m3 males and females.
The concentration of nickel oxide in the lungs of 0.6, 2.5, and 10 mg/m3males was greater than in the lungs of controls at 4, 9, and 13 weeks, and nickel continued to accumulate in the lung at the end of the 13-week exposures (4 weeks, 33 to 263 mg nickel/g lung; 9 weeks, 53 to 400 mg/g lung; 13 weeks, 80 to 524 mg/g lung).
Thirteen-week study in mice
Groups of 10 male and 10 female B6C3F1 mice were exposed to 0, 0.6, 1.2, 2.5, 5, or 10 mg nickel oxide/m3 by inhalation for 6 hours per day, 5 days per week for 13 weeks. Additional groups of six male and six female mice were exposed to 0, 0.6, 2.5, or 10 mg/m3 for tissue burden studies. No exposure-related deaths occurred among core study animals, final mean body weights of exposed male and female mice were similar to those of the controls, and no clinical findings in any group were related to nickel oxide exposure. Hematocrit values and erythrocyte counts in 5 and 10 mg/m3 females were minimally greater than those of the controls, as was the hemoglobin concentration in 5 mg/m3 females. Absolute and relative lung weights of 10 mg/m3 males and females were significantly greater than those of controls, and absolute and relative liver weights of 10 mg/m3 males were significantly less than those of controls.
Accumulation of alveolar macrophages, many of which contained pigment particles, occurred in all groups of mice exposed to nickel oxide. Inflammation (chronic active perivascular infiltrates or granulomatous) occurred in 2.5, 5, and 10 mg/m3 males and females. In addition, lymphoid hyperplasia and pigment occurred in the bronchial lymph nodes of males and females exposed to 2.5 mg/m3 or higher.
The concentration of nickel in the lung was greater than that of controls in 0.6, 2.5, and 10 mg/m3 males at 13 weeks (42 to 736 mg nickel/g lung).
Two-year study in rats
Survival, body weights, clinical findings, and hematology
Groups of 65 male and 65 female F344/N rats were exposed to 0, 0.62, 1.25, or 2.5 mg nickel oxide/m3 (equivalent to 0, 0.5, 1.0, or 2.0 mg nickel/m3) by inhalation for 6 hours per day, 5 days per week for 104 weeks. Survival of exposed male and female rats was similar to that of the controls. Mean body weights of 1.25 mg/m3 females and 2.5 mg/m3 males and females were slightly lower than those of the controls during the second year of the study. No chemical-related clinical findings were observed in male or female rats during the 2-year study. No chemical-related differences in hematology parameters were observed in male or female rats at the 15-month interim evaluation.
Pathology findings
Absolute and relative lung weights of 1.25 and 2.5 mg/m3 males and females were significantly greater than those of the controls at 7 and 15 months. At 2 years, there were exposure-related increased incidences of alveolar/bronchiolar adenomas alveolar/bronchiolar adenoma or carcinoma (combined) in males and females. Incidences of atypical alveolar epithelial hyperplasia in the lungs generally increased with increasing exposure concentration in male and female rats. Chronic inflammation of the lung was observed in most exposed rats at 7 and 15 months and at 2 years; the incidences in exposed males and females at 2 years were significantly greater than those in the controls, and the severity of the inflammation increased in exposed groups. The incidences of pigmentation in the alveolus of exposed groups of males and females were significantly greater than those of the controls at 7 and 15 months and at 2 years.
Pigmentation in the bronchial lymph nodes similar to that in the lungs was observed in all exposure groups with the exception of 0.62 mg/m3males and females at 7 months. Lymphoid hyperplasia was observed in the bronchial lymph nodes of 1.25 and 2.5 mg/m3 males and females at 7 and 15 months, and the incidence at 2 years generally increased with exposure concentration.
At 2 years, there was an exposure-related increase in the incidence of benign pheochromocytoma in males and females. The incidences of benign pheochromocytoma and adrenal medulla hyperplasia in 2.5 mg/m3 females and the incidence of benign or malignant pheochromocytoma (combined) in 2.5 mg/m3 males were significantly greater than those in the controls.
Tissue burden analyses
Nickel concentrations in the lungs of exposed rats were greater than those in the controls at 7 and 15 months (7 months, 173 to 713 mg nickel/g lung; 15 months, 262 to 1,116 mg/g lung), and nickel concentrations increased with increasing exposure concentration and with time.
Two-year study in mice
Survival, body weights, clinical findings, and hematology
Groups of 74 to 79 B6C3F1 mice were exposed to 0, 1.25, 2.5, or 5 mg nickel oxide/m3 by inhalation for 6 hours per day, 5 days per week for 104 weeks. Survival of exposed male and female mice was similar to that of the controls. Mean body weights of 5 mg/m3 females were slightly lower than those of the controls during the second year of the study. No chemical-related clinical findings were observed in male or female mice during the 2-year study. No chemical-related differences in hematology parameters were observed in male or female mice at the 15-month interim evaluation.
Pathology findings
At 2 years, the incidence of alveolar/bronchiolar adenoma in 2.5 mg/m3 females was significantly greater than that of the controls, as was the incidence of alveolar/bronchiolar adenoma or carcinoma (combined) in 1.25 mg/m3 females. Generally, incidences of chronic inflammation increased with exposure concentration in males and females at 7 and 15 months. Bronchialization of minimal severity in exposed animals and proteinosis were first observed at 15 months. At 2 years, the incidences of chronic inflammation, alveolar epithelial hyperplasia, and proteinosis in exposed groups of males and females were significantly greater than those of the controls. The severity of chronic inflammation increased with exposure concentration in females, and proteinosis was most severe in 5 mg/m3 males and females. Pigment occurred in the lungs of nearly all exposed mice at 7 and 15 months and at 2 years, and the severity increased with exposure concentration.
Lymphoid hyperplasia occurred in two animals after 7 months; at 15 months, lymphoid hyperplasia occurred in males exposed to 2.5 and 5 mg/m3 and in all exposed groups of females. At 2 years, lymphoid hyperplasia occurred in some control animals, but this lesion was still observed more often in exposed males and females and the incidence increased with exposure concentration. Pigmentation was observed in the bronchial lymph nodes of exposed males and females at 7 and 15 months and in nearly all exposed animals at 2 years.
Tissue burden analyses
Nickel concentrations in the lungs of exposed mice were significantly greater than those in the controls at 7 and 15 months (7 months, 162 to 1,034 mg nickel/g lung; 15 months, 331 to 2,258 mg/g lung), and nickel concentrations increased with increasing exposure concentration and with time.
Genetic toxicology
No increase in the frequency of micronucleated normochromatic erythrocytes was observed in peripheral blood samples from male or female mice exposed to nickel oxide.
Conclusions
Under the conditions of these 2-year inhalation studies, there was some evidence of carcinogenic activity of nickel oxide in male F344/N rats based on increased incidences of alveolar/bronchiolar adenoma or carcinoma (combined) and increased incidences of benign or malignant pheochromocytoma (combined) of the adrenal medulla. There was some evidence of carcinogenic activity of nickel oxide in female F344/N rats based on increased incidences of alveolar/bronchiolar adenoma or carcinoma (combined) and increased incidences of benign pheochromocytoma of the adrenal medulla. There was no evidence of carcinogenic activity of nickel oxide in male B6C3F1 mice exposed to 1.25, 2.5, or 5 mg/m3. There was equivocal evidence of carcinogenic activity of nickel oxide in female B6C3F1 mice based on marginally increased incidences of alveolar/bronchiolar adenoma in 2.5 mg/m3 females and of alveolar/bronchiolar adenoma or carcinoma (combined) in 1.25 mg/m3 females.
Exposure of rats to nickel oxide by inhalation for 2 years resulted in inflammation and pigmentation in the lung, lymphoid hyperplasia and pigmentation in the bronchial lymph nodes, and hyperplasia of the adrenal medulla (females). Exposure of mice to nickel oxide by inhalation for 2 years resulted in bronchialization, proteinosis, inflammation, and pigmentation in the lung and lymphoid hyperplasia and pigmentation in the bronchial lymph nodes.