Selenium: The Essential Trace Element from Scalp Treatments to Solar Cells

What is selenium and why does it matter?

In this Chemistry World episode, Bernie Balkan explains that selenium sits just beneath sulfur in the periodic table and is essential for life in small amounts but highly toxic in larger quantities. The discussion covers safe intake levels, dietary sources, and the implication of selenium in enzymes such as glutathione peroxidase and thyroid function. A memorable cautionary tale involves polo horses, where a selenium overdose led to fatalities through a dosing error, underscoring the need for careful management of micronutrient supplements.

• Essential in small amounts, toxic in large amounts with a safe upper intake around 400 micrograms per day.
• Dietary sources include nuts, tuna, and lobster; human deficiency is rare but some animal diets can lead to deficiency.
• Selenium has a broad set of uses beyond biology, from shampoo ingredients to brass alloys and insecticides.
• Environmental and health links include selenosis from selenium-rich coal and potential roles in cancer prevention and immune function.

The episode also covers the history and chemistry of selenium, including the discovery by Berzelius and the moon-inspired name selene, and ends with a memorable note on the skunk odor compound butyl selenomercaptan.

Introduction and elemental context

The podcast opens by situating selenium as an element adjacent to sulfur in the periodic table, but one that often receives less attention in inorganic chemistry textbooks. The host, Chris Smith, narrates how selenium behaves in a way that is reminiscent of sulfur yet also distinct, with multiple oxidation states and allotropic forms that influence its biological and material roles.

Selenium in biology and nutrition

Selenium is an essential trace element needed for several enzymes, notably glutathione peroxidase, and plays a key role in thyroid health. The discussion highlights that humans require selenium in small amounts, with a commonly cited safe upper intake level around 400 micrograms per day. The narrative emphasizes the balance between sufficiency and toxicity, and notes the variability in selenium content across foods such as nuts, tuna, and lobster. It also mentions that while selenium deficiency is rare in humans with varied diets, certain animals with restricted diets, like horses, are more prone to deficiency and may require supplementation under careful supervision.

Case study: selenium toxicity in horses

A dramatic recent example is the reported death of 21 polo horses in Florida due to selenium overdose, traced to a veterinary pharmacist who may have confused micrograms with milligrams. This incident underscores the importance of precise dosing in veterinary contexts and the potential consequences of miscalculation when managing trace elements.

Discovery, naming, and chemical forms

Berzelius is credited with discovering selenium in 1817 as an impurity in sulfuric acid. The element was named after the Greek moon goddess selene, contrasting with tellurium whose name derives from earth. Selenium exists in several minerals alongside sulfur, and its atmospheric release is partly linked to coal combustion. The discussion covers different allotropes, including gray selenium, which forms extended helical chains and is a photoconductor used in early photocells, and red selenium that is employed in glassmaking to introduce or remove color depending on the context.

Environmental and health connections

The podcast notes that selenium toxicity in humans was established through studies of Chinese victims who grew selenium-rich crops on contaminated coal rocks (selenosis). It also discusses how selenium compounds, notably selenates and selenites, can be highly toxic, whereas elemental selenium is less readily incorporated into biological processes. The narrative touches on research into cancer prevention and the interplay between selenium and antioxidant vitamin E, suggesting possible combined effects in health contexts.

Selenium in technology and industry

Beyond biology, selenium has a long industrial history. Gray selenium served as a photoconductive material in early devices, and selenium-based compounds have found applications in photoconductors and photovoltaics. The episode highlights the rise of copper indium gallium selenide (SIGs) solar cells as a promising mass-produced technology. Selenium is also used to improve abrasion resistance in synthetic rubber, and it has historical relevance in brass as a substitute for lead to address toxicity concerns in pipes. In agriculture, sodium selenate is used as an insecticide to protect flowering plants such as chrysanthemums and carnations.

Health research, cancer, and immune function

The host notes ongoing, though not conclusive, studies on the possible role of selenium in cancer prevention and in boosting the efficacy of chemotherapy, with indications that selenium may act in concert with vitamin E as antioxidants. A provocative idea discussed is that selenium deficient soils may influence susceptibility to infections such as HIV/AIDS, reflecting broader links between micronutrient status and immune competence. The episode emphasizes that these areas require careful, nuanced interpretation rather than definitive conclusions.

Skunk odor and the elemental edge

Concluding the exploration, the podcast introduces butyl selenomercaptan as a key ingredient for skunk odor, a compound whose very pungency illustrates selenium chemistry in the realm of everyday sensory experiences. The host hints at the richness of the element's chemistry and its surprising reach across disciplines.

Takeaway

selenium is a versatile and essential micronutrient whose roles span biology, medicine, and materials science, underscoring the importance of dose control, environmental awareness, and interdisciplinary research in fully appreciating this elemental player.