[1/2] https://www.theguardian.com/science/2025/jan/06/roman-empires-use-of-lead-lowered-iq-levels-across-europe-study-finds

Roman mining activities put a substantial amount of lead into the air, which shows up in Arctic ice cores. This could have reduced the average IQ by up to 3 points. That decrease could have had an effect on society overall. If this atmospheric lead arrived in the Arctic, it may have arrived in Asia and North America too. So the effect could have reached there too.