9. Nickel, Ni

Nickel (Ni) is a transition metal whose average concentration in the Earth’s crust is not high. Very high concentrations are thought to be found in the Earth’s inner core and have been discovered in soils based on serpentine rocks. The most common forms of Ni are oxides, and of the five stable isotopes, ⁵⁸Ni is the most abundant. The present occurrence of Ni in the environment is mainly connected with industrial use, especially in metallurgy. Because of its density, Ni is considered a heavy metal. It is an essential element for plants and microorganisms, but its physical role in vertebrates is still not fully understood. On the other hand, its toxic, genotoxic, and carcinogenic properties are known. Nickel concentrations considered normal in birds and mammals fall into a range of 0.05–0.5 for kidneys and 0.04–0.3 mg kg⁻¹ for liver, but other materials, such as feathers and fur, usually accumulate it in higher amounts. Studies on the evaluation of possible biomarkers of Ni exposure have been carried out mainly on humans, but no clear and measurable relationship has been found so far. Some initial findings linked Ni exposure with a decrease in δ-aminolevulinic acid dehydratase activity, but the most useful methods of its detection continue to be based on the measurement of concentrations in select tissues or materials.