Soil Selenium Levels

Keshan Disease. Conclusive evidence for a role for selenium in human nutrition came with publication of the results of large-scale trials in China that show the protective effect of selenium supplementation on children and young adults suffering from an endemic cardiomyopathy. This was observed in areas of the country (Keshan region) with low soil selenium levels. ... 

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. 

The realization that selenium (Se) may be an essential micronutrient for human diets has arisen only recently, in the second half of the twentieth century. Selenium deficiency, attributable to low soil selenium levels in farm animals, especially sheep that are afflicted by selenium-responsive white muscle disease, has been recognized for at least half a century. However, the more recent identification of Keshan and Kashin-Beck diseases as endemic selenium-responsive conditions, occurring in a central 4000-1— km-wide belt of central China and in areas of Russia, demonstrated conclusively that not only is selenium an essential element for man but also deficiencies occur naturally and require public health measures to alleviate them. Selenium incorporation into plants is affected by the acidity of the soil and by the concentrations of iron and aluminum present so that selenium content of human diets is modulated by these components of the environment. The very recent discovery that these diseases probably arise through the interaction of selenium deficiency with enhanced viral virulence has added a further layer of complexity, but it does not alter the fact that selenium is an essential dietary component that cannot be substituted by any other element. Another complicating factor is that moderately increased soil selenium concentrations result in the opposite condition of seleno-sis, or selenium overload, with equally debilitating consequences. Of all elements, selenium has a very narrow safe intake range, and unlike some other potentially toxic elements, it is absorbed efficiently by the intestine over a wide range of concentrations and across a variety of different molecular forms. 

Several supplementation and epidemiological case-control studies have suggested a possible link between increased selenium intakes or status and protection against certain cancers. First, in intercountry comparisons and studies comparing different regions of the United States having different soil selenium levels, there was a consistent correlation between lower selenium levels and higher risk of cancer. A study of 34,000 male health professionals in the eastern United States... 

There are areas (22) where selenium levels in the soil are very low these include regions of volcanic activity like that adjacent to the Cascade mountains in the Pacific Northwest states of the United States and the central north island of New Zealand. There, because the heat of emption volatilized the selenium, the residual soil parent material is virtually devoid of selenium. Other areas of low soil-selenium reflect leaching of selenium out of the top soil, as in the Canterbury plain on New Zealand s south island. Areas of selenium deficiency have negative implications for animal and human health. 

There is considerable evidence to suggest that selenium compounds are anticarcinogens. For one thing, tests with laboratory animals show that the incidence and size of malignant tumors are reduced when a solution containing Na2Se03 is injected at the part per million level. Beyond that evidence, statistical studies show an inverse correlation between selenium levels in the soil and the incidence of certain types of cancer. 

High concentrations of selenium may occur in groundwater in semiarid or arid areas, near known mineral deposits containing sulfide minerals of uranium and vanadium. Irrigated agriculture may substantially increase concentrations in groundwater in areas with high selenium levels in soil. 

Selenium enters the food chain mainly as selenomethionine from plants that take the element up from the soil but do not appear to use it. The soil content of selenium is highly variable and can be low in volcanic soils when soluble salts are leached out by ground water. Soils in parts of China and New Zealand are particularly low in selenium. Acid soils, where insoluble selenium complexes can be formed with iron and aluminum, occur in some parts of Europe, resulting in low available soil selenium. The geographical source of plant and animal foodstuffs determines the level of dietary intake. In the United States and Canada, wheat and other cereal products are a good source of selenium average intakes in North America range from 80 to 220 fig Se per day, whereas in the UK dietary intake is about 30 to 60 Llg/day. Intakes in China are as low as 11 lg/day and in New Zealand 28 fig/day. ... 

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. 

An examination of thyroid hormone levels in lactating women residing in areas of Venezuela with high levels of selenium in the soil (selenium intake ranged from 250 to 980 pg per day as estimated from selenium content of breast milk) revealed a significant decrease in serum T3 levels, as compared with... 

The disposal of selenium contaminated waste water has resulted in elevated selenium levels in sediments of Lake Belews, North Carolina. The concentration of selenium in sediments ranged from 4 to 12 pg/g (pre-1986), but has dropped to 1 1 pg/g (1996) due to the discontinued release of selenium laden waste water from a local coal fired power plant (Lemly 1997). Selenium was measured in 445 surface soil samples from Florida with a concentration range of 0.01 1.62 pg/g and an arithmetic mean of 0.25 pg/g (Chen et al. 1999). Selenium was detected in soils and bed sediment from the South Platter River Basin at concentrations of 0.30-3.80 pg/g (Heiny and Tate 1997). The highest levels were observed in areas consisting of a high degree of Precambrian rock formation. 

Many of the basic analytical methods used for determining selenium in biological media are also used for determining selenium levels in soil, water, and air. Precautions in the collection and storage of environmental samples, however, are necessary to prevent loss of the volatile selenium compounds to the air. The destruction of organic matter before selenium measurement is also often necessary. 

NAA has been used to determine selenium levels in environmental samples. Dams et al. (1970) reported a detection limit of lxlO 10 g/m3 selenium using nondestructive NAA for determining selenium in air particulate matter. For determining selenium levels in soil, radiochemical variants of NAA have been commonly employed (Bern 1981). Instrumental neutron activation analysis (INAA) is frequently used to determine selenium concentrations in water and can also be used to distinguish between selenium(IV) and selenium(VI) oxidation states (Bern 1981). INAA is also used to determine selenium concentrations in air (Bern 1981). 

It is possible to detect selenium levels as low as 1 ng per cubic meter of air using neutron activation analysis. Standardized methods for selenium determination in different environmental samples such as water, soil, sludge, and industrial waste are available in the above-mentioned literature. 

Extensive literature exists on the inverse associations between soil selenium concentrations or environmental levels of selenium and the occurrence of major forms of cancer. Shamberger and Frost (1969) and Sham-... 

Selenium presence in the human food chain is mainly affected by its levels in agricultural soils. Hence, its daily uptake varies depending heavily on the geographical area. 50-200p.g/day were proposed as normal uptake selenium levels, with the exception of infants and children under 6 years of age where it is suggested as 10-40 g/day and 20-120 J,g/day, respectively. 

In areas of acid or neutral soils, the amount of biologically available selenium should steadily decline. The decline may be accelerated by active agricultural or industrial practices. In dry areas, with alkaline soils and oxidizing conditions, elemental selenium and selenides in rocks and volcanic soils may oxidize sufficiently to maintain the availability of biologically active selenium. Concentrations of selenium in water are a function of selenium levels in the drainage system and of water pH. In Colorado, for example, streams with pH 6.1-6.9 usually contain <1.0p,g Se/L,... 

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