Occurrence and Uses
Selenium (Se) is found in rocks and soils all over the world. There are no true deposits of selenium anywhere, and it cannot economically be recovered directly. Various estimates for selenium in the Earth’s crust range from 0.03 to 0.8 ppm; the highest concentrations known are in native sulphur from volcanoes, which contains up to 8,350 ppm. Selenium does, however, occur together with tellurium in the sediments and sludges left from electrolytic copper refining. The chief world supplies are from the copper-refining industries of Canada, the United States and Zimbabwe, where the slimes contain up to 15% selenium.
The manufacture of selenium rectifiers, which convert alternating current to direct current, accounts for over half the world’s production of selenium. Selenium is also used for decolourizing green glass and for making ruby glass. It is an additive in the natural and synthetic rubber industries and an insecticide. Selenium is used for alloying with stainless steel and copper.
75Se is used for the radioactive scanning of the pancreas and for photostat and x-ray xerography. Selenium oxide or selenium dioxide (SeO2) is produced by burning selenium in oxygen, and it is the most widely used selenium compound in industry. Selenium oxide is employed in the manufacture of other selenium compounds and as a reagent for alkaloids.
Selenium chloride (Se2Cl2) is a dark brownish-red stable liquid which hydrolyses in moist air to give selenium, selenious acid and hydrochloric acid. Selenium hexafluoride (SeF6) is used as a gaseous electric insulator.
The elemental forms of selenium are probably completely harmless to humans; its compounds, however, are dangerous and their action resembles that of sulphur compounds. Selenium compounds may be absorbed in toxic quantities through the lungs, intestinal tract or damaged skin. Many selenium compounds will cause intense burns of skin and mucous membranes, and chronic skin exposure to light concentrations of dust from certain compounds may produce dermatitis and paronychia.
The sudden inhalation of large quantities of selenium fumes, selenium oxide or hydrogen selenide may produce pulmonary oedema due to local irritant effects on the alveoli; this oedema may not set in for 1 to 4 hours after exposure. Exposure to atmospheric hydrogen selenide concentrations of 5 mg/m3 is intolerable. However, this substance occurs in only small amounts in industry (for example, due to bacterial contamination of selenium-contaminated gloves), although there have been reports of exposure to high concentrations following laboratory accidents.
Skin contact with selenium oxide or selenium oxychloride may cause burns or sensitization to selenium and its compounds, especially selenium oxide. Selenium oxychloride readily destroys skin on contact, causing third-degree burns unless immediately removed with water. However, selenium oxide burns are rarely severe and, if properly treated, heal without a scar.
Dermatitis due to exposure to airborne selenium oxide dust usually starts at the points of contact of the dust with the wrist or neck and may extend to contiguous areas of the arms, face and upper portions of the trunk. It usually consists of discrete, red, itchy papules which may become confluent on the wrist, where selenium dioxide is liable to penetrate between the glove and sleeve of the overall. Painful paronychia may also be produced. However, one more frequently sees cases of excruciatingly painful throbbing nail beds, due to the selenium dioxide penetrating under the free edge of the nails, in workers handling selenium dioxide powder or waste red selenium fume powder without wearing impermeable gloves.
Splashes of selenium oxide entering the eye may cause conjunctivitis if not treated immediately. Persons who work in atmospheres containing selenium dioxide dust may develop a condition known among the workers as “rose eye”, a pink allergy of the eyelids, which often become puffy. There is usually also a conjunctivitis of the palpebral conjunctiva but rarely of the bulbar conjunctiva.
The first and most characteristic sign of selenium absorption is a garlic odour of the breath. The odour is probably caused by dimethyl selenium, almost certainly produced in the liver by the detoxication of selenium by methylation. This odour will clear quickly if the worker is removed from exposure, but there is no known treatment for it. A more subtle and earlier indication than the garlic odour is a metallic taste in the mouth. It is less dramatic and is often overlooked by the workers. The other systemic effects are impossible to evaluate accurately and are not specific to selenium. They include pallor, lassitude, irritability, vague gastrointestinal symptoms and giddiness.
The possibility of liver and spleen damage in people exposed to high levels of selenium compounds deserves further attention. In addition, more studies of workers are needed to examine the possible protective effects of selenium against lung cancer.
Safety and Health Measures
Selenium oxide is the main selenium problem in industry since it is formed whenever selenium is boiled in the presence of air. All sources of selenium oxide or fumes should be fitted with exhaust ventilation systems with an air speed of at least 30 m/min. Workers should be provided with hand protection, overalls, eye and face protection, and gauze masks. Supplied-air respiratory protective equipment is necessary in cases where good extraction is not possible, such as in the cleaning of ventilation ducts. Smoking, eating and drinking at the workplace should be prohibited, and dining and sanitary facilities, including showers and locker rooms, should be provided at a point distant from exposure areas. Wherever possible, operations should be mechanized, automated or provided with remote control.