Lead's a useful product that poisons its users. The Romans used it extensively in water pipes, and to take the acidic edge off low-quality wine. Later generations also used it for water pipes - some of them still in use - and in paint. Most damaging of all, though, was its use, from the 1930s onwards in petrol, as tetraethyl lead (increasingly powerful engines couldn't run smoothly without it). This made city streets, polluted by exhaust fumes, dangerously poisonous.
The trouble with lead is that the body welcomes it. Lead binds to red blood cells which take it to every organ, where it proceeds to do damage. The main effect is on the brain, where it causes behavioural and developmental problems and may be linked to schizophrenia.
All this has been known from the start. The Romans knew about lead poisoning, and the details of its metabolic effects was being studied in the early 20th-century. Lead petrol, though, wasn't fully banned in the UK until 1999, and it is still in use in some countries.