The Ni2+ ion is a known carcinogen in both humans and mice, possibly by entry into cells via phagocytosis. In the CHO cell line, Ni(OH)2, the LC50 dose has been shown to be 3.6 μg/ml. This high level of toxicity relative to other Ni2+ containing compounds is hypothesized to be due to the insoluble nature of the compound, and concentration in the nucleus. Toxicity and related safety concerns have driven research into increasing the energy density of Ni(OH)2 electrodes, such as the addition of calcium or cobalt hydroxides.