Forensic Toxicology

Modern forensic chemistry can detect poisons at parts per trillion — like finding one specific grain of sand on an entire beach. The Marsh test for arsenic, developed by James Marsh in 1836, was the first reliable chemical test for a poison. Before it, arsenic was nearly undetectable and was nicknamed "inheritance powder."

Hair tells the rest of the story. It grows about 1 cm a month, and toxins in the blood get locked into each new strand. A 12 cm sample can reveal a month-by-month timeline of what entered someone's body.