Tungsten [W, 74]
343
There is no clear evidence that W may have a biological function in plants.
However, it displays competitive inhibition of Mo in the enzyme nitrate reductase,
reducing the enzyme’s catalytic activity, what resulted in a slower N-fixation by
plants. It is a component of two dehydrogenases (group of oxidoreductases) of some
bacteria (e.g., Eubacterium acidaminophilum). Tungsten also inhibits synthesis of
plant hormones, such as abscisic acid, which resulted in a lower resistance of plants
to various stresses. Under drought condition this effect may be stronger.
The common range of W in terrestrial plants is established at the range of <1–150
μg/kg. However, higher contents, up to 200 μg/kg, are also reported. Vegetables
growing on soil amended with fly ash, contain W within the range from 700 to
3500 μg/kg (Furr et al. vide Kabata-Pendias 2011). Mean contents of W in wheat and
barley grains from Sweden are 6 and 5 μg/kg, respectively (Eriksson 2011a).
52.6 HUMANS
Tungsten content of human body is estimated to be 20  μg, and its concentrations
in blood is 1 μg/L and in bones 0.2 μg/kg (Emsley 2011). Other sources (ATSDR
2005c) with respect to contents for adults are as follows (in μg/kg): <3–5 in kidney,
<3–36 in liver, and <3–11 μg/kg in lung.
Tungsten can enter the blood through ingestion, inhalation, or injection, and is
predominantly and rapidly excreted in urine, with only a very small proportion being
incorporated into the kidneys, liver, spleen, and skeleton. It is deposited in bones, and
may be slowly released to the blood, and eliminated mainly in the urine. Insoluble
forms of orally administered W are chiefly eliminated in the feces. Under conditions
of chronic uptake, it is projected that W levels in soft tissue will take up to 3 years to
reach a plateau, but its level in bones will continue to increase throughout life (Voet
van der et al. 2007).
Tungsten is irritating to the skin and eyes, on contact. Its inhalation will cause
irritation to the lungs and mucus membrane. Irritation to the eyes will cause watering
and redness. Reddening, sealing, and itching are characteristics of skin inflammation. Apparently, there is no chronic effects of exposure to W. Prolonged exposure to
this metal is not known to aggravate medical conditions. All W compounds should
be regarded as highly toxic (Gbaruko and Igwe 2007).
Individuals, who work in manufacturing, fabricating, and reclaiming industries,
especially individuals using hard metal materials, or W carbide machining tools,
may be exposed to higher levels of W and its compounds. Tungsten has been detected
in hair and nail samples of hard metal workers. Pulmonary fibrosis, memory and
sensory deficits, and increased mortality due to lung cancer have been attributed
to occupational exposure to W dusts. However, W and its compounds are not considered very toxic for humans. Nevertheless, it may play a role in cases of pulmonary fibrosis, related to hard metal lung disease. Recently there is emerging evidence
of W  toxic health effects. Biomedical research indicates, at least, potential toxicity of W (Witte et al. 2012).
Recent toxicological interest in W is based on its increased use as a component of armor-piercing munitions and a replacement for Pb in other ammunition.
Embedded W shrapnel represents a unique source of internal exposure to W. Military