Arsenic

It was the presence of higher than expected levels of arsenic in the remains of Ötzi, the 5000 year-old iceman found in the Italian Alps, which suggests he was a coppersmith because the smelting of this metal was often from ores rich in arsenic. We all contain some of this element, and the average person has around 7 mg of arsenic in their body and this varies according to the foods we eat. A dish of rice and prawns may provide more than 20 mg but the arsenic is present as arsenobetaine, (CH₃)₃ As⁺ CH₂ CO^2-, which poses no threat to health. 

John Emsley expores arsenic: its uses in medicine and the speculation that Napoleon was taken down by the gas in his wallpaper.