[coffeecat]

There's been an astonishing amount of research done on the lead-IQ relationship over 50+ years. It's no exaggeration to say that this is one of the most thoroughly investigated public health topics. And one of the most interesting.

My belief is that lead is a noisy metric for dirt/dust ingestion, which correlates with IQ for a variety of reasons. Homes of poor families tend to be dirtier, and wealthier families tend to live in newer suburban housing farther from curbs for example, but that sort of thing is pretty easy to control for. The big thing that can't be controlled in a straightforward way, is developmentally delayed children eating more dust/dirt. To a much lesser extent, this also correlates negatively with IQ in adults due to pica, disregard for cleanliness, etc correlating negatively with IQ. But the lead-IQ correlation is strongest at age 2, and that's no coincidence.

Observable symptoms of lead poisoning occur at blood levels above ~60 mcg/dL, which is 3-4 times higher than the levels studied in 1970s-era lead IQ research, and about 2 orders of magnitude higher than the levels studied in contemporary lead-IQ research. The latter body of research reports significantly larger effect sizes than the former.