The big question you're missing there is who is doing it, why they may be doing that, and if they ever considered the possibility of secondary and tertiary effects. A man who discovers this dense, heavy metal that can easily be formed into nice sturdy pipes that can thus enable a vast plumbing system to be constructed to easily deliver water to all at trivial expense can't exactly be blamed for the side effects of said pipes becoming a widely adopted staple of his civilization.
If he knew what "Lead" was and did to the human body/mind upon ingestion, yet pushed forth the lead plumbing system plan anyway well, that SOB gets a special place in hell. If he was prudent and tried it in a small test case and saw that oh yes, bad things are happening to these people very coincidentally after I built them that plumbing system and chose to just not inform society at large about his discovery... He gets a pass. I mean, mass indoor plumbing and easy access to water for all is a great thing. Yet, as with all things, costs vs. benefits. He was kinda a huge, huge dick in playing antiquity's version of Mengele but by the standards of those days the man was downright fucking Christian!