Nickel [Ni, 28]
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(generally present as an impurity), household cleaning and bleaching agents, and
medical products (joint implants, intrauterine devices, and acupuncture needles).
Nickel exposure can also be from inhalation of cigarette smoke (ATSDR 2005a).
Nickel and its compounds are absorbed from the respiratory tract and, to a lesser
extent, from the gastrointestinal tract and skin. About 20%–35% of the inhaled Ni that
is retained in the lung is absorbed into the blood, whereas only 1%–10% of ingested
Ni is absorbed, depending largely on the composition of the diet. Absorption of Ni
following oral exposure varies between 3% and 40%, being higher from drinking
water. Most of the absorbed Ni is excreted in the urine, regardless of the route of
exposure (Health Canada 2010b).
Oral Ni absorption is greater from more soluble compounds (in % of Ni content):
34, NiNO 3 ; 11, NiSO 4 ; 10, NiCl 2 ; 0.5, Ni 3 S 2 ; 0.09, metallic Ni; 0.04, black Ni oxide;
and 0.01, green Ni oxide. Its absorption was reduced by some ligands, such as cysteine and histidine, and, to a lesser extent, by proteins. Oral absorption of NiSO 4 ,
given in drinking water, was 27%; and it was 0.7%, when the NiSO 4 was given in
food (WHO 2006a).
At current levels of exposure, Ni and its compounds are not of a concern for
human health, according to Health Canada and Environment Canada. However, several Ni compounds are entering the environment in a quantity or concentration and
amounts and forms, which may constitute a danger to human life or health (Health
Canada 2010).
Acute Ni exposure is associated with a variety of clinical symptoms and signs,
which include gastrointestinal disturbances (nausea, vomiting, abdominal discomfort, and diarrhea), visual disturbance (temporary left homonymous hemianopia),
headache, giddiness, wheezing, and cough. The most acutely toxic Ni compound is
Ni carbonyl, Ni(CO) 4 . This colorless, highly volatile liquid is extremely toxic and
induces systemic poisoning, with the lungs and brain being especially susceptible
targets. Chronic inhalation of Ni and its compounds is associated with an increased
risk of lung cancer. Cigarette smoking can cause a daily absorption of Ni of 1  μg
per pack, due to the Ni content of tobacco. Its contents vary from 2.2 to 2.3 μg per
cigarette. About 10%–20% of Ni in cigarettes is released in mainstream smoke, and
most of Ni is in the gaseous phase (IARC 1990).
Inhalation is an important route of exposure to Ni and its salts, in relation to
health risks. The International Agency for Research on Cancer (IARC) concluded
that inhaled Ni compounds are carcinogenic to humans (Group 1) and that metallic
Ni is possibly carcinogenic (Group 2B). However, there is a lack of evidence of a
carcinogenic risk from oral exposure to Ni (IARC 1990, 2012a).
Nickel is one of the most common contact allergens on the skin, not only in industrial settings but also in the general population exposed, via direct contact of the skin
with Ni-containing items (WHO 2006a).
Tolerable daily intake of Ni was estimated to be 12 μg/kg bw (WHO 2011a).
Nickel is present in most food groups (Table 29.3).
Data from studies in the United States give estimates of daily dietary intakes in the
range of 0.101–0.162 mg/day for adults, 136–140 μg/day for males, and 107–109 μg/day
for females. Estimates for pregnant and lactating women are higher with average daily
intakes of 121 and 162 μg/day, respectively (ATSDR 2005a). Daily dietary intakes of