It is clear that the human body does accumulate or concentrate selenium with respect to the environmental levels of selenium. Selenium has been found in marine fish meal at levels of about 2 ppm. This amount is around 50,000 times greater than the selenium found in seawater. Selenium dioxide is the primary source of problems from industrial exposures since the dioxide forms selenious acid with water or sweat, and the acid is an irritant. Selenium compounds released during coal or petroleum combustion may be a significant source of exposure.
Selenium is a metal which exists in nature only in the combined form.
Usually found in the sulfide ores of the heavy metals, such as pyrite, clausthalite, naumannite, tiemannite. Also found in coal.