Page 3: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Selenium
Part I. Overview and Application (continued)
Selenium is a naturally occurring element which is ubiquitous in the environment. It is generally present in elemental form or in the form of selenide (Se2-), selenate (SeO42-), or selenite (SeO32-). It is widely distributed in the Earth's crust and is found in trace quantities in most plant and animal tissues. Selenium is not directly mined, but rather is a by-product of the production of other metals. Selenium is used in the manufacture of organic chemicals, reducing agents, glass, paint, ceramic, electronic components, gun bluing agents, nutritional supplements, fertilizers, metallurgical applications and plumbing (as a replacement for lead).
This guideline technical document reviews and assesses all identified health risks associated with selenium in drinking water. It assesses new studies and approaches and takes into consideration the availability of appropriate treatment technology. Based on this review, the guideline for selenium in drinking water is a maximum acceptable concentration of 0.05 mg/L.
Selenium is an essential trace element in the human diet. It is a component of several proteins and enzymes in the body that are known to play important roles, including regulation of thyroid hormones and antioxidant defences. A deficiency in selenium may lead to chronic diseases such as Keshan disease (characterized by cardiomyopathy) and Kashin-Beck disease (characterized by rheumatism) and may also be associated with a form of cretinism related to hypothyroidism. Selenium has minimum daily dose requirements set by international organizations. Health Canada adopted the recommended daily intake for selenium established by the Institute of Medicine (2000) which varies between 15 and 55 µg per day as a minimum selenium intake, depending on the age group. Selenium deficiency is not expected to be a concern in Canada.
Selenium has been classified by the International Agency for Research on Cancer in Group 3: not classifiable as to its carcinogenicity to humans. The vast majority of the literature does not demonstrate an increase in cancer incidence following selenium exposure; a protective effect has even been suggested. A non-cancer approach was used in this assessment, and the MAC for selenium in drinking water is based on chronic selenosis symptoms in humans. Selenosis symptoms resulting from chronic exposure to high levels of selenium are characterized by hair loss, nail anomalies or loss, skin anomalies, garlic odour of the breath, tooth decay and, more severely, disturbances of the nervous system. Links have also been found between selenium exposure and other diseases such as diabetes and glaucoma, but results need to be confirmed before conclusions can be drawn.
Canadians can be exposed to selenium through its presence in food, air, soil, drinking water, as well as through the use of specific consumer products or in occupational settings, with food being the main source of exposure. Selenium levels are generally low in Canadian drinking water supplies. Inorganic forms of selenium that are normally found in drinking water are not volatile and very little quantitative information is available on the absorption of selenium compounds through the lungs or skin. Selenium deficiency is not likely to be a concern in Canada.
There are several analytical methods available for the analysis of total selenium in drinking water at levels well below the MAC. The speciation of selenium in the raw water plays a critical role in the effectiveness of treatment methods used for the removal of selenium. The removal of excess selenium from drinking water has not been studied on a full-scale treatment plant basis, and limited data exist on laboratory and pilot plant tests. Nevertheless, there are several technologies that can remove selenium from drinking water. There are drinking water treatment devices certified for the removal of selenium. The treatment processes that are capable and able to be certified for selenium removal at the residential scale include adsorption, reverse osmosis and distillation.
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