[coffeecat]

Please do share if you're aware of any sources associating population-wide lead level decreases to population-wide IQ increases. While we can't be certain about blood lead levels before the mid-20th century, my impression is that the golden age of the Flynn effect (mid-20th century through the mid-70s) was concomitant with explosive growth in the use of leaded gasoline, which should be far and away the primary driver of elevated lead levels, and that cognitive scores have been relatively flat from the mid-70s onward as lead levels have decreased precipitously.

Edit: It's worth noting that the study described by the OP does not demonstrate an effect on IQ. They estimate population-wide lead exposure. The lost IQ point calculation is based on correlation coefficients obtained through observational studies.