Chromium [Cr, 24]
83
higher amounts of Cr, mean 0.39 mg/kg, than those grown in the Baltic Sea coast,
mean 0.20 mg/kg (Falandysz et al. 2012).
12.6 HUMANS
The total amount of Cr in the human body (70 kg) is estimated at 1.2 mg, but can be
as much as 12 mg, of which in the blood 6–10 μg/L, in the bone 100–300 μg/kg, and
in the tissue 25–800 μg/kg (Emsley 2011). Its concentration is relatively higher in
newborn children than in adults. At old age, Cr amounts in serum decrease by 42%.
Trivalent Cr tends to accumulate in epidermal tissues (hair, etc.), bones, liver, kidney,
spleen, lungs, and in the large intestine. Accumulation in other tissues, especially
muscles, seems to be strictly limited or nonexistent (Pechova and Pavlata 2007).
Humans can be exposed to Cr by breathing air, drinking water, eating food
Cr, or through skin contact with Cr compounds. The most common health problem in workers exposed to Cr involves the respiratory tract. These health effects
include irritation of the lining of the nose, runny nose, and breathing problems
(asthma, cough, shortness of breath, and wheezing). Workers have also developed
allergies to Cr compounds, which can cause breathing difficulties and skin rashes.
Concentrations of Cr in air that can cause these effects may be different for different types of Cr compounds, with effects occurring at much lower concentrations of Cr 6+ compared to Cr 3+ . However, the concentrations causing respiratory
problems in workers are at least 60 times higher than levels normally found in the
environment.
Rates of Cr uptake from the gastrointestinal tract are relatively low and depend
on a number of factors, including valence state (Cr 6+ is more readily absorbed than
Cr 3+ ), the chemical form (organic Cr is more readily absorbed than inorganic Cr), the
water solubility of the compound, and gastrointestinal transit time. Once absorbed
into the bloodstream, Cr 6+ is rapidly taken up by erythrocytes, after absorption and
reduced to Cr 3+ inside the red blood cells. This reduction occurs by the action of glutathione. In contrast, Cr 3+ does not readily cross red blood cell membranes, but binds
directly to transferrin, an Fe-transporting protein in the plasma.
Hexavalent Cr crosses biological membranes easily, reacting with protein components and nucleic acids inside the cell, while being deoxygenated to Cr 3+ . In general,
Cr 6+ is more toxic than Cr 3+ . Trivalent Cr is the most stable oxidation state, in which
Cr is found in living organisms and does not have the capacity to cross cell membranes easily (Mertz 1993). Regardless of the source, Cr 3+ is widely distributed in
the body and accounts for most of the Cr in plasma or tissues. The greatest uptake
of Cr 3+ as a protein complex is by bone marrow, lungs, lymph nodes, spleen, kidney,
and liver. Cr levels in the lungs are consistently higher than in other organs.
Cr 3+ is an essential dietary nutrient, whereas Cr 6+ poses a significant risk of lung
cancer. Cr 6+ compounds can produce effects on skin and mucous membranes. These
include irritation, burns, ulcers, and an allergic type of dermatitis. Reduction of Cr 6+
to Cr 3+ inside of cells may be an important mechanism for the Cr toxicity, whereas
this process, outside of cells, is a major mechanism of protection (ATSDR 2012a).
Chromium is regarded as essential in humans and animals, which takes part in
various metabolic processes. Especially, stable Cr 3+ is known as biological active