Arsenic, element 33, has been an assassin’s tool, a cosmetic, and a medicine, sometimes in the same era. The explanation starts on the periodic table, where it sits directly below phosphorus. The two elements bond in nearly identical ways, so cells that depend on phosphorus for DNA and energy let arsenic in by mistake. Once inside it cannot do the job, energy molecules fall apart, and the cell dies. The element is odd in other ways too: heated past 600 degrees Celsius it never melts but turns straight into toxic gas, and it can give off a strong smell of garlic.

This episode traces what people did with that chemistry. For centuries arsenic poisoning passed for cholera or dysentery and earned the name inheritance powder, until chemist James Marsh devised a test in the 1830s that made it traceable. Manufacturers then sold it as brilliant green dye for gowns, toys, sweets, and wallpaper. Modern medicine learned to aim the poison, from an early syphilis drug to a leukemia therapy, while millions today drink it unknowingly in groundwater.

• The Marsh test converts arsenic in a tissue sample into arsine gas, which leaves a silvery black mirror on heated glass.
• Napoleon, who died on St. Helena in 1821, likely lived among arsenic green wallpaper that fungi in the damp air converted into toxic gas.
• Hair analysis decades later showed that the Australian racehorse Phar Lap, who died in 1932, was killed by a massive arsenic overdose.
• In 2000 the FDA approved arsenic trioxide for a resistant form of acute promyelocytic leukemia, using it to trigger programmed cell death in cancer cells.
• People native to Chile’s Atacama Desert carry a variant of the AS3MT gene that clears arsenic faster. A 2010 claim that a Mono Lake bacterium built its DNA with arsenic was refuted by 2012 and retracted in 2025.