Short answers, pulled from the story.
Selenium was discovered in 1817 by Jons Jacob Berzelius and Johan Gottlieb Gahn. Berzelius initially thought the substance found in pyrite residue from the Falun Mine was tellurium, but in 1818 he confirmed it was a new element and named it after the Moon (Greek: selene) because of its similarity to tellurium, which had been named for the Earth.
The largest commercial use of selenium is glassmaking, accounting for roughly half of all consumption, where selenium compounds neutralize green or yellow tints from iron impurities. Other major uses include pigments, photoconductors in flat-panel X-ray detectors, DC power surge protection, and anti-dandruff shampoos such as Selsun.
Selenium is an essential trace element for humans, with a US Recommended Dietary Allowance of 55 micrograms per day for teenagers and adults. It acts as a cofactor for antioxidant enzymes including glutathione peroxidases and is required by the thyroid gland for hormone regulation. The total selenium content in the human body is estimated at 13-20 milligrams.
Selenosis causes diarrhea, fatigue, hair loss, joint pain, nail brittleness or discoloration, nausea, headache, tingling, vomiting, and fever. Both elemental selenium and especially selenium salts are toxic in small doses.
William Grylls Adams and his student Richard Evans Day demonstrated the first solid-state solar cell using selenium as the photoabsorbing layer in 1876. Charles Fritts also fabricated the first thin-film solar cell using selenium shortly after. Research on selenium solar cells declined with the rise of silicon in the 1950s, but a new record efficiency of 6.5% was achieved by IBM researchers in 2017, and in 2024 the first selenium-based tandem solar cell was demonstrated.
Selenium bioaccumulates in aquatic food chains; organoselenium compounds can be concentrated more than 200,000 times by zooplankton. A water concentration of 2 micrograms per liter is considered highly hazardous to sensitive fish and aquatic birds. At Belews Lake in North Carolina, selenium wastewater discharged from 1974 to 1986 eliminated 19 fish species from the lake.