Selenium and Occupational or Industrial Health Concerns

Selenium: Occupational/Industrial Health Concerns

Overview of Industrial Hygiene Concerns and Recommendations for Reduction of the Risks Associated with Selenium in the Workplace a. Statement of the Problem. Selenium is a chemical element in the oxygen family (Group VIa) of the periodic table; the element is closely allied in chemical and physical properties with the elements sulfur and tellurium (Selenium, 2004). Selenium was first recognized as an element in 1818 by Jns Jacob Berzelius, a Swedish chemist. The element is a metalloid (this is an element that exhibits properties that are intermediate between the metals and the nonmetals); it is widely distributed throughout the world; however, such distribution only occurs in small quantities (Selenium, 2004). Prolonged exposure to certain metals such as cadmium, copper, lead, nickel, selenium compounds and zinc can cause deleterious health effects in humans (Reilly 1991). At low levels, selenium is considered a nutrient; however, it is being increasingly shown that at higher levels, the element and many of its compounds are toxic in humans (Dunning, 1993). Furthermore, Harris (1991) believes that numerous undiagnosed situations exist where people are suffering from selenium poisoning and physicians simply do not recognize it.

Selenium can be found in hard grains and garlic, in meats (such as kidney and liver), in seafood (particularly swordfish and tuna) as well as dairy products; the majority of people receive some selenium in their diet; however, some get too much which can result in selenium poisoning, or selenosis (Harvitz, 1997). This condition produces flu-like symptoms, as well as "garlicky breath, loss of hair and fingernails and neurological problems" (Harvitz, 2003, p. 44). In more severe cases, selenosis can result in liver damage, respiratory failure and death. "At one point during Marco Polo's legendary trip to China, for example, the explorer's horses became so ill their hooves fell off. Their affliction was dubbed the 'blind staggers' -- the equestrian form of selenosis" (Harvitz, 2003, p. 45). By sharp contrast, selenium deficiencies in other parts of China have been shown to result in high incidences of severely deforming arthritis and a fatal heart-failure syndrome; in addition, researchers have determined that the cancer death rate in high-selenium regions is about one-third that of the low-selenium regions.

According to Grabowski (1992), many metals, even those with pronounced toxicity at high levels, are essential nutrients. "Selenium is a cogent example. Some parts of China, because of soil conditions, suffer a high prevalence of selenium toxicity (selenosis). Other areas of China, because of inadequate soil concentrations, display a high prevalence of selenium deficiency (Keshan disease)" (Grabowski, 1992, p. 176). How much selenium is essential and how much is likely to be toxic remains unclear, though, and regulators must seek to find a delicate risk-benefit balance. Nevertheless, in many industrial settings, such as XYZ Steel's, the fumes and dust of trace elements such as selenium and its compounds are very toxic and have been related to an increase in the incidence of diseases (Horng & Lin, 1999).

b. Summary of Toxic Properties of Selenium. Selenium exists in several different forms, the three most important being 1) amorphous (noncrystalline) (this form is red when in powder form and black when in vitreous (glassy) form; 2) the red crystalline; and 3) the gray metallic (which is also crystalline) (Selenium, 2004). Of these three forms, the metallic form exhibits the most stable properties under ordinary workplace conditions; the other forms very slowly convert to the metallic form at room temperature. In its various compounds, selenium exists in the oxidation states of -2, +4, and +6 and demonstrates a distinct tendency to form acids in the higher valences. Selenium itself is not poisonous to humans; in fact, there is increasing evidence supports the claim that selenium itself is an essential dietary micronutrient for humans (Pilecki & Zachara, 2000). Many of the compounds selenium forms, however, are extremely toxic (Selenium, 2004).

For example, selenium can combine directly with hydrogen, a process that results in hydrogen selenide, H2Se; this compound is a colorless and odoriferous gas that is a cumulative poison. "It also forms selenides with most metals (e.g., aluminum selenide, cadmium selenide, and sodium selenide)" (Selenium, 2004, p. 3). Selenide ions are being increasingly being used in photoelectrochemical cell applications (Lewis, 1995). Selenium compounds are also found in internal combustion exhaust, and have been positively associated with increased mortality rates in five of six American cities surveyed by (Topeka, Kansas was the exception; the other cities surveyed were Boston, St. Louis, Knoxville, Madison, and Steubenville) (Dockery, Laden and Schwartz, 2000). The combustion of coal combustion is responsible for between 62% (Medical and Biological Effects of Environmental Pollutants- Selenium, 1976) and 85% (Eimutis, Quill & Rinaldi, 1978) of harmful selenium emissions in the United States.

Furthermore, in combination with oxygen, selenium forms selenium dioxide, SeO2; the reaction of this oxide with water results in selenious acid, H2SeO3. Selenium also forms a variety of compounds in which the selenium atom bonds to both an oxygen and a halogen atom resulting in compounds such as selenium (VI) oxychloride, SeO2Cl2, which is an exceedingly potent solvent. Finally, the most important commercial compound formed from selenium is selenic acid, H2SeO4; this acid is as strong as sulfuric acid and more easily reduced (Selenium, 2004).