Selenium: A global contaminant of significant concern to environment and human health

Selenium (Se) is one of the most interesting and controversial elements found in the environment. It has long attracted interest due to its strong toxicity and potential carcinogenicity, while more recently it garnered interest as an essential nutrient, as a cofactor of selenoproteins, and due to the claim of a beneficial effect against cancer and other chronic diseases, which was recently found to be unwarranted. Selenium is ubiquitous in the environment, being present particularly in rocks, soils, combustion sources, such as smoking and industrial emissions, and water bodies, and therefore living organisms can be overly exposed to it. Selenium is found in several valence states and organic and inorganic compounds, and exposure is generally done by determining overall Se, its species and the selenoproteins in environmental and biological samples. Examples are serum and plasma, urine, hair and nails, and cerebrospinal fluid Se and Se species concentrations. The environmental effects and health outcomes attributable to both Se deficiency and excess, in animals and humans, have been the subject of a large number of studies, and attracted strong interest in environmental sciences and environmental health, also fueling considerable debate. While little evidence of adverse effects of Se deficiency has been detected so far, except for a cardiomyopathy described in some parts of China, a number of serious effects have been reported or suggested by human and animal studies, particularly those carried out in seleniferous areas and by those with experimental design, such as randomized controlled trials. Legislation and recommendations to control Se exposure and avoid the risk of overexposure widely differ throughout the world, and most of them need to be updated taking into account the most recent epidemiological and toxicological evidence. This is particularly due to the toxic effects on human health of even unexpectedly small doses of Se and some selenoproteins, such as type 2 diabetes, dermatological alterations, liver toxicity, endocrine alterations, and neurological disease. Due to the potential contamination of the environment, particularly soil, food, and water with excessive amounts of Se, effective remediation approaches and technologies are being devised and need to be implemented.