Mining and smelting minerals like iron represented technological highs at the Roman Empire’s peak. But those activities also produced enough lead pollution to impair its citizens’ IQs, according to a ...

For most of the Roman Republic and Empire's history ... lasting between 250 BCE and 450 CE (with its peak at 150 CE). During this time, temperatures were warmer, with more precipitation.

“Our data suggests that lead pollution during the 180 years of the peak of the Roman Empire had about one-third as much of an impact on cognitive decline as during the height of 20th century ...

Why can we say that 146 BCE was crucial in the formation of the Roman Empire, perhaps more than any other single year?