Selenium - Se

Atomic number

34

Atomic mass

78.96 g.mol ^-1

Electronegativity according to Pauling

2.4

Density

4.79 g.cm^-3 at 20°C

Melting point

217 °C

Boiling point

688 °C

Vanderwaals radius

0.14 nm

Ionic radius

0.198 nm (-2) ; 0.042 nm (+6)

Isotopes

9

Electronic shell

[ Ar ] 3d¹⁰ 4s² 4p⁴

Energy of first ionisation

940,7 kJ.mol ^-1

Standard potential

- 0.77 V

Discovered by

Jons Berzelius 1817

Humans may be exposed to selenium in several different ways. Selenium exposure takes place either through food or water, or when we come in contact with soil or air that contains high concentrations of selenium. This is not very surprising, because selenium occurs naturally in the environment extensively and it is very widespread.
The exposure to selenium mainly takes place through food, because selenium is naturally present in grains, cereals and meat. Humans need to absorb certain amounts of selenium daily, in order to maintain good health. Food usually contains enough selenium to prevent disease caused by shortages.

Selenium uptake through food may be higher than usual in many cases, because in the past many selenium-rich fertilizers have been applied on farmland.

People that live near hazardous waste-sites will experience a higher exposure through soil and air. Selenium from hazardous waste-sites and from farmland will end up in groundwater or surface water through irrigation. This phenomenon causes selenium to end up in local drinking water, so that exposure to selenium through water will be temporarily increased.

People that work in metal industries, selenium-recovery industries and paint industries also tend to experience a higher selenium exposure, mainly through breathing. Selenium is released to air through coal and oil combustion.
People that eat a lot of grains that grow near industrial sites may experience a higher exposure to selenium through food. Exposure to selenium through drinking water may be increased when selenium from hazardous waste disposals ends up in water wells.

Exposure to selenium through air only comes about in the workplace usually. It can cause dizziness, fatigue and irritations of the mucous membranes. When the exposure is extremely high, collection of fluid in the lungs and bronchitis may occur.

Selenium uptake through food is usually high enough to meet human needs; shortages rarely occur. When shortages occur people may experience heart and muscle problems.

When selenium uptake is too high health effects will be likely to come about. The seriousness of these effects depends upon the concentrations of selenium in the food and how often this food is eaten.
The health effects of various forms of selenium can vary from brittle hair and deformed nails, to rashes, heat, swelling of the skin and severe pains. When selenium ends up in the eyes people experience burning, irritation and tearing.

Selenium poisoning may become so severe in some cases that it can even cause death.

Overexposure of selenium fumes may produce accumulation of fluid in the lungs, garlic breath, bronchitis, pneumonitis, bronchial asthma, nausea, chills, fever, headache, sore throat, shortness of breath, conjunctivitis, vomiting, abdominal pain, diarrhea and enlarged liver. Selenium is an eye and upper respiratory irritant and a sensitizer. Overexposure may result in red staining of the nails, teeth and hair. Selenium dioxide reacts with moisture to form selenious acid, which is corrosive to the skin and eyes. Carcinogenicity- The International Agency for Research on Cancer (IARC) has listed selenium within Group 3 (The agent is not classifiable as to its carcinogenicity to humans.)

Low levels of selenium can end up in soils or water through weathering of rocks. It will than be taken up by plants or end up in air when it is adsorbed on fine dust particles. Selenium is most likely to enter the air through coal and oil combustion, as selenium dioxide. This substance will be converted into selenium acid in water or sweat.
Selenium substances in air are usually broken down to selenium and water fairly quickly, so that they are not dangerous to the health of organisms.

The behaviour of selenium in the environment strongly depends upon its interactions with other compounds and the environmental conditions at a certain location at a certain time.

There is evidence selenium can accumulate in the body tissues of organisms and can than be passed up through the food chain. Usually this bio magnification of selenium starts when animals eat a lot of plants that have been absorbing large amounts of selenium, prior to digestion. Due to irrigation run-off concentrations of selenium tend to be very high in aquatic organisms in many areas.

When animals absorb or accumulate extremely high concentrations of selenium it can cause reproductive failure and birth defects.