[coffeecat]

I've got two things to say about that:

1) Multiple researchers have obtained U-shaped lead-IQ curves which regress toward the mean at extreme values. This, in my opinion, strongly implies that the lead-IQ correlation only occurs in the "ordinary exposure through dirt and dust" regime.

2) The effect sizes reported back in the 70s, when lead levels were an order of magnitude or two higher than they are now, were significantly lower than the effect sizes reported in contemporary research. In both cases, however, they can be broadly summarized as "3-5 IQ points per standard deviation of exposure".