[coffeecat]

Not nearly as hilarious as the idea that lead is more harmful at levels <5 mcg/dL than it is in the 10-20 mcg/dL range. But that's the only way to explain the available data while attributing the observed correlation to Pb neurotoxicity. And indeed, it's the mainstream view among lead-IQ researchers.

Back when lead levels were mostly in the 10-20 mcg/dL range, the observed correlations with IQ were typically around 3-5 IQ points per standard deviation of lead exposure. Now that lead levels are an order of magnitude or two lower, the observed correlation is still... 3-5 IQ points per standard deviation of exposure. Blood levels of other environmental toxins have also been correlated with IQ. These studies typically come in around... wait for it... 3-5 IQ points per standard deviation of exposure.

The common-sense explanation, in my opinion, is that developmentally delayed children eat more dust and dirt.