Same as tobacco, pesticides, AGW and so on, and so on, and so on.

Lead pipes were used extensively by the Romans but also known to be a potential source of soluble lead. For this reason, pipes made of clay were preferred—as Vitruvius, who wrote during the time of Augustus, explains.

“Water conducted through earthen pipes is more wholesome than that through lead; indeed that conveyed in lead must be injurious, because from it white lead [ceruse or lead carbonate, PbCO3] is obtained, and this is said to be injurious to the human system. Hence, if what is generated from it is pernicious, there can be no doubt that itself cannot be a wholesome body. This may be verified by observing the workers in lead, who are of a pallid colour; for in casting lead, the fumes from it fixing on the different members, and daily burning them, destroy the vigour of the blood; water should therefore on no account be conducted in leaden pipes if we are desirous that it should be wholesome. That the flavour of that conveyed in earthen pipes is better, is shewn at our daily meals, for all those whose tables are furnished with silver vessels, nevertheless use those made of earth, from the purity of the flavour being preserved in them”

http://penelope.uchicago.edu/~grout/encyclopaedia_romana/wine/leadpoisoning.html

In spite of this knowledge, the Romans continued the extensive use of lead in industry, irrigation and public works and viniculture for the same reasons use of known chemical toxins continues today, It is cheap, it is easy to use, and those most likely to be negatively affected by it, particularly those who extract it and work with it, are considered expendable. Not much has changed in 2000 years.

Everybody is on board with the dangers of lead these days. There are no cycles of rise and fall of environmental lead content over the millennia.