"In the US, the minimal risk level of nickel and its compounds is set to 0.2 µg/m3 for inhalation during 15 364 days.[62] Nickel sulfide fume and dust are believed carcinogenic, and various other nickel compounds may be as well.[63][64] Nickel carbonyl, [Ni(CO)4], is an extremely toxic gas. The toxicity of metal carbonyls is a function of both the toxicity of the metal as well as the carbonyl's ability to give off highly toxic carbon monoxide gas, and this one is no exception; nickel carbonyl is also explosive in air. ... Sensitized individuals may show an allergy to nickel affecting their skin, also known as dermatitis."

"DRI Tables"

"Recommended Dietary Allowances (RDA) - The daily dietary intake level of a nutrient considered sufficient by the Food and Nutrition Board to meet the requirements of 97.5% of healthy individuals in each life-stage and sex group. It is calculated based on the EAR and is usually approximately 20% higher than the EAR. ... Tolerable upper intake levels (UL) - To caution against excessive intake of nutrients (like vitamin A) that can be harmful in large amounts. This is the highest level of daily consumption that current data have shown to cause no side effects in humans when used indefinitely without medical supervision."