Many buildings built before 1950 still have paint that is lead based and have plumbing with lead pipes. These paints were phased out but it is very expensive to replace and it is better in the long run just to build new housing to replace the paint. The paint can flake off and when ingested by children can cause intellectual or developmental disabilities (the now politically correct term for mental retardation) and or learning disabilities.
In the lab (on animal studies) and in correlational studies no dosage level of lead has been found that is completely benign on the brain. It has a negative effect on the blood brain barrier which protects it from other toxins and has many other negative effects on intelligence test scores and brain function.
The negative effects of lead exposure were recognized by Greek physicians as early as the 2nd century BC. There is a theory that it contributed to the decline of the Roman Empire though that is controversial. It was used heavily in the industrial revolution and in gasoline in the 1920s. A way to test the theory would be to measure residual lead levels in the remains (teeth, bones, and if lucky hair) of the Roman citizens relative to those in the surrounding Barbarian tribes which sacked the empire from AD 410-476. If the levels were higher in the Roman citizens it would support the theory.
It was phased out starting in 1978 in the US but the effects are still felt with leaded gasoline getting into the atmosphere through auto emissions, then into rain water, and finally the soil where it can remain for many years. Other countries do not have as strict regulation on lead as the US does now.
Episode 7 of Cosmos is devoted to lead poisoning and it's history with scientist Clair Patterson being voiced by Richard Gere. I can only embed the whole episode here while Hulu makes it available.