Selenium health effects

Table 3-8. On-going Studies on Selenium Health Effects...

On contact with moist mucous membrane surfaces, hydrogen selenide is probably oxidized to elemental selenium. Thus, in considering health effects, the possible chronic effects of absorbed selenium should be considered in addition to the acute effects of hydrogen selenide itself (see separate monograph on selenium). 

D. Hausknecht and R. Ziskind, Health Effects of Selenium, NTIS PB-250568, Electric Power Research Institute, EPRI EA-571-V1, Palo Alto, Calif., Jan. 1976. 

Selenium is an essential element for humans and is a constituent of selenoamino acids contained in selenoenzymes, such as glutathione peroxidases. This group of enzymes catalyzes the reduction of peroxides and thereby takes part in the anti-oxidative defense of the body. A severely low Se intake may cause heart disease (Keshan s disease). Therefore, possible adverse health effects are associated with a low intake of this element and justify the focus on the low percentile Se intake in the population. The food groups that contribute the most to the Se intake are meat (including offal) followed by bread and cereals, and fish, as shown in Figure 9.8. 

Selenium is mostly found in arid environments under irrigated agriculture. Selenium has toxic effects at high dose levels, but at low dose levels is an essential nutrient. It is classified in the EPA s Group D (not classifiable), and because of its potential chronic health effects, it is regulated by the U.S. government. 

Fluoride, arsenic and (to a lesser extent) selenium are naturally occurring chemicals that have been responsible for severe health effects due to exposure through drinking-water in many countries. Their distribution in groundwater is widespread and their possible presence in surface water should not be ruled out, because groundwater discharge is frequently a major contributor to surface water bodies. 

Fluoride, arsenic, selenium and, in certain circumstances, nitrate should be given high priority. As noted in Chapter 2, the presence of these chemicals in drinking-water has been shown to cause health effects. The natural occurrence of these chemicals is relatively common in water supplies around the world in both developing and developed countries therefore, they should be assumed to be potentially present, and consideration should be given as to whether they are actually present in concentrations of concern. 

Selenium is one of the few substances that have been shown to cause adverse human health effects as a consequence of exposure through drinking-water, although it is an essential element and in many parts of the world there is a deficiency. It is, therefore, important to consider selenium in developing new sources in areas where selenium is suspected. Where selenium is present, monitoring at the treatment works would be appropriate. 

Use and exposure Selenium is an essential trace element for human health. However, acute and prolonged periods of exposure to high concentrations of selenium compounds cause adverse health effects in humans. The symptoms include garlicky breath irritation of eyes, skin, nose, and throat visual disturbances headache nausea vomiting chills fever weakness violent cough bitter metallic taste in the mouth nose bleeds dyspnea bronchial spasms bronchitis pulmonary edema gastrointestinal tract disturbance ... 

Toxicity and health effects Selenium causes hair and nail loss, discoloration and decay of the teeth, and CNS disturbances, including pain and anesthesia of the extremities. Inhalation of hydrogen selenide causes pulmonary edema. The dusts of selenium produce respiratory tract irritation, while the fumes of selenium dioxide produce metal fume fever. Dermal exposure and ingestion of selenium oxychloride cause skin burns, corrosive injury to the gastrointestinal tract, stupor, respiratory depression, and refractory hypotension. Ingestion of selenious acid causes corrosive injury to the gastrointestinal tract, stupor, respiratory depression, and refractory hypotension... 

The growing interest in selenium speciation is due to its toxic and/or essential properties that depend on the actual physicochemical form of the element. Strong evidence exists on the adverse health effects related to selenium deficiency. It was also demonstrated that supplementation with different selenium forms might help to remove the symptoms of deficit. Selenium has been found in the active site of different antioxidant enzymes, protecting cells against the action of free radicals. The species-dependent cancer-preventive properties are well documented, yet the specific biological functions have not been ultimately elucidated. The list of small selenium compounds includes inorganic forms (two stable valence states), volatile alkylated species, and... 

Environmental and health effects of radionuclides are summarized by Siegel and Bryan, (see Chapter 9.06). Potential environmental effects of hydrocarbons and organic chemicals are addressed in other chapters, and will not be discussed here. Environmental and health effects of arsenic, selenium, and mercury are addressed in greater detail elsewhere in this volume, including Chapters 9.02 and 9.04. Further information on the environmental geochemistry of metals is presented by Callender (see Chapter 9.03). 

Ongoing investigations into selenium metabolism include state-of-the-art methods such as HPLC/ICPMS in combination with MS/MS. Data on the profile of selenium metabolites will elucidate the element s essential and toxic roles and relate individual Se species with observed health effects. 

Proposed Pathway for Formation of Dimethyl Selenide from Selenite in Animals 3-6. Activation and Reduction of Selenate to Selenite in Yeast Saccharomyces cerevisiae 3-7. Conceptual Representation of a Physiologically Based Pharmacokinetic (PBPK) Model for a Hypothetical Chemical Substance 3-8. Selenite Model, a Kinetic Model for Selenite Metabolism 3-9. Selenomethionine Model, a Kinetic Model for Selenomethionine Metabolism 3-10.Existing Information on Health Effects of Selenium 6-1. Frequency of NPL Sites with Selenium Contamination... 

States with high soil selenium levels show none of the negative health effects associated with selenium that were seen in Chinese villagers who had eaten much larger amounts of selenium for longer periods. 

Low levels of selenium can also be found in drinking water. Most of the water sources tested in the United States have very low levels of selenium compared with the levels found in food. Selenium levels were less than 10 ppb (10 parts of selenium in a billion parts of water) in 99.5% of drinking water sources tested. The 10-ppb concentration is lower than the 50-ppb Maximum Contaminant Level (MCL), which the EPA believes will protect against adverse health effects. Less than 1% of the daily intake of selenium is estimated to come from drinking water. 

Chapter 3 contains more information on the health effects of selenium and selenium compounds in humans and animals. 

We do not know if exposure to selenium will result in birth defects in people. Selenium compounds have not been shown to cause birth defects in humans or in other mammals. We have no information to suggest that there are any differences between children and adults in where selenium is found in the body or in how fast it enters or leaves the body. Studies in laboratory animals have shown that selenium crosses the placenta and enters the fetus. Studies in humans show that infants are supplied with selenium through breast milk, and therefore, women who were exposed to selenium by living near a waste site might transfer selenium to their babies. However, babies in areas of China with high selenium in the soil did not show any signs of health effects due to selenium, even though some of their parents did. 

Many methods are available to measure selenium levels in human tissue and the environment. However, none of the methods that are routinely available can measure or detect each selenium compound in one test, and better tests that measure lower levels of different selenium compounds are needed. Also, these tests cannot determine the exact levels of selenium you may have been exposed to or predict whether health effects will occur. 

Selenium is known to be an essential micronutrient for humans and other animals both inadequate and excessive selenium intake can cause adverse health effects. However, most people in the United States are unlikely to suffer from selenium deficiency. The current recommended dietary allowance (RDA) for selenium established by the Food and Nutrition Board of the National Research Council is 55 pg/day for adults. Adverse health effects due to selenium are generally observed at doses at least 5 times greater than the RDA. 

No MRLs were derived for inhalation exposure to selenium because of insufficient quantitative data concerning both human and animal exposures. Data on the health effects of inhaled selenium in humans are derived from studies of occupationally exposed workers. These studies suggest that the respiratory system is the most sensitive end point for inhaled selenium dust, but they do not provide quantitative measurements of exposure and are frequently confounded by concurrent exposure to other chemicals. Animal studies support the respiratory system as the target of selenium toxicity, but these are acute studies of exposure to high concentrations of selenium that also produced serious health effects and death. 

Estimates of exposure levels posing minimal risk to humans (Minimal Risk Levels or MRLs) have been made for selenium. An MRL is defined as an estimate of daily human exposure to a substance that is likely to be without an appreciable risk of adverse effects (noncarcinogenic) over a specified duration of exposure. MRLs are derived when reliable and sufficient data exist to identify the target organ(s) of effect or the most sensitive health effect(s) for a specific duration within a given route of exposure. 

Table 3-1 and Figure 3-1 describe the health effects observed in experimental animals that inhaled elemental selenium dust or hydrogen selenide. Studies using other forms of selenium were not used in the LSE tables and figures (Table 3-1 and Figure 3-1) because either the reporting of the studies was incomplete or no studies on other forms were located. All doses are expressed in terms of total selenium. 

In studies of human occupational exposures, it appears that the respiratory tract is the primary site of injury after inhalation of selenium dust or selenium compounds, but gastrointestinal (possibly due to swallowed selenium) and cardiovascular effects, as well as irritation of the skin and eyes, also occur. Little of the available information for humans, however, relates health effects exclusively to measured concentrations of the selenium dust or compounds because of the possibility of concurrent exposures to multiple substances in the workplace. In animals, the respiratory tract is also the primary site of injury following inhalation exposure to selenium dust and hydrogen selenide. Hematological and hepatic effects have also been noted in animals. Inhalation data from laboratory animal studies are available only for acute exposures. 

No studies were located regarding the following health effects in humans or animals after inhalation exposure to selenium or selenium compounds ... 

Table 3-2 and Figure 3-2 describe the health effects observed in humans and experimental animals associated with dose and duration of oral exposure to selenium and selenium compounds (i.e., elemental selenium dust, selenium dioxide dissolved in water [selenious acid], sodium selenate, sodium selenite, potassium selenate, and dietary selenium compounds, which include selenoamino acids). All doses for these compounds are expressed in terms of total selenium. Table 3-3 and Figure 3-3 describe health effects observed in laboratory animals following oral exposure to selenium sulfides (SeS2 and SeS) at varying doses and exposure durations. All doses for selenium sulfide compounds are expressed in terms of the compound, because selenium sulfide preparations often exist as a variable mixture of the mono-and disulfide forms, precluding accurate expression of the dose in terms of total selenium. 

Some epidemiological studies report data from populations exposed to selenium in the food chain in areas with high selenium levels in soil. It is likely that selenite, selenate, and the selenium found in food and in dietary supplements comprise the majority of selenium compounds to which oral, off-site selenium exposures will occur at or near hazardous waste sites. Aside from the variation in effective dose, the health effects from exposure to selenate, selenite, and dietary selenium are not expected to differ greatly. However, oral exposures to many other compounds of selenium could occur (primarily through soil or edible plant ingestion) if those compounds were deposited at the site, or if local environmental conditions greatly favor transformation to those forms. Heavy metal selenides, aluminum selenide, tungsten diselenides, and cadmium selenide are used in industry and may end up in waste sites. 

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