Silicon [Si, 14]
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Rainwater contains Si within the range of 0.5–7.5 mg/L, but its average contents
do not exceed 2.6 mg/L (Table 43.1).
The median Si concentration in bottled waters of the EU countries is 6.64 mg/L,
and in tap waters, it is 4.3 mg/L (Birke et al. 2010).
43.4 AIR
Predominated Si compounds in the atmosphere are oxides and silicates, which are
stemming from both natural and industrial sources. Windblown dust from deserts,
especially from the Sahara, may carry huge amounts of Si into the atmosphere. High
concentrations of Si in moss, up to 470 mg/kg, collected in Sweden in 1975 illustrate
its increased concentrations in air (Rühling and Tyler 2004).
Elevated Si contents in air is of a health concern; thus, estimated probable effect
level (PEL) value for Si is 10 mg/m 3 (Table 43.1).
43.5 PLANTS
Silicon is a common mineral constituent of plants, but it is not defined to be essential.
It is readily absorbed from the growth media, mainly in the form of H 4 SiO 4 , from
the soil solution. It is uptaken by both mechanisms, active and passive. Its biological function is slightly known. It is believed, however, that Si is necessary for the
growth and development of plants, as well as for the resistance to toxicities of some
chemicals, as well as to some diseases, especially fungal diseases. The amorphous Si
impregnates the walls of epidermal and vascular tissues. Thus, Si strengthens plant
tissues and reduces water losses.
Silicon contents vary greatly among plant species: the mean amounts in grasses
range between 0.3% and 1.2%, whereas in leguminous, they range between 0.05%
and 0.2%. Some plants may accumulate much higher Si amounts, up to >10%, for
example, sedges, nettles, horsetails, and diatoms. The residues of these plants contribute to the formation of amorphous Si, which partly can be formed as opal.
Rice plants accumulate especially much Si, up to 10% in hulls and up to 4% in
grains. The deficiency of phytoavailable Si in soils of rice plantations has deterioration impact on the crop. The addition of phytoavailable Si in the form of fertilization
improved rice growth and crops.
43.6 HUMANS
The silicon quantity in a man weighing 70 kg is 1 g (Emsley 2011), which is the third
most abundant trace element in the human body after Zn and Fe. Silicon is present
in all body tissues, but its highest concentrations are in bone and other connective tissues, including the skin, hair, arteries, and nails (Jugdaohsingh 2007). In the
last three decades, it has been suggested that Si may be an essential trace element.
Silicon may have a role in a number of areas of human physiology and metabolism,
especially bone and connective tissue formation, but possibly also gene expression
and cardiovascular health (Powell et  al. 2005). Silicon breast implants may cause
autoimmune disorders and even cancer.