Long-term inhalation of NiO is damaging to the lungs, causing lesions and in some cases cancer.
The calculated half-life of dissolution of NiO in the blood is more than 90 days. NiO has a long retention half-time in the lungs; after administration to rodents, it persisted in the lungs for more than 3 months. Nickel oxide is classified as a human carcinogen based on increased respiratory cancer risks observed in epidemiological studies of sulfidic ore refinery workers.
In a 2-year National Toxicology Program green NiO inhalation study, some evidence of carcinogenicity in F344/N rats but equivocal evidence in female B6C3F1 mice was observed; there was no evidence of carcinogenicity in male B6C3F1 mice. Chronic inflammation without fibrosis was observed in the 2-year studies.