For instance, Se, which otherwise is an electrical insulator, becomes an electrical conductor when exposed to visible light. Selenium is a necessary micronutrient. We need 50 to 200 micrograms per day of Se, an amount found in a serving of fresh vegetables. In these quantities Se may have a role in preventing heart disease and cancer. Selenium is in most soils.
Selenium occurs naturally as a trace element in most soils, rocks, and waters.
A few places in the western U.S., Australia, and China don't have enough natural Se. Keshan Disease, an Se deficiency that affects the heart, is named for the province in China where it was first documented. Only rarely, however, do people need to artificially add Se to their diets. Too much Se is toxic. The most common cause of Se poisoning in humans is overdosing on vitamins.