Lead free => 5 point IQ increase

From The possible societal impact of the decrease in U.S. blood lead levels on adult IQ. by Kaufman AS, et al.

It is a longstanding belief that lead in the environment causes a degradation of mental ability and many of our environmental policies from the 1970s onward have had lead reduction among their goals. But has it made a difference in population IQ? Kaufman's research suggests that the policies have reduced exposure to lead and that reduced exposure has resulted in an average 4-5 point increase in population average IQ (confounded perhaps by the Flynn Effect, medical advances, possibly some aspects of nutrition, etc.) But five points is a pretty material increase.

From the abstract:

BACKGROUND:
The dramatic decrease in U.S. blood lead levels (BLLs) since the 1970s has been documented--however, the anticipated societal impact on intelligence quotient (IQ) has not. The objectives of this study were to determine whether mean IQs of American adults, adjusted for demographics, have increased in concert with society's decreasing BLL.
METHODS:
Mean IQs of eight normal adult cohorts (N=800), tested individually in 2007 by trained clinicians, were compared using ANCOVA and correlation analysis. Cohorts ranged in mean societal BLLs from 4 1/2 μg/dL (born 1985-1987) to 19 μg/dL (born 1963-1972). IQs were adjusted for confounders-education, gender, ethnicity, region, urban status. To control for age, we analyzed IQ data for a second adult sample (N=800), tested in 1995-all born when BLLs were high (1951-1975, BLL ≥ 15 μg/dL).
RESULTS:
When controlling for education, gender, ethnicity, and region, the regression of IQs on BLLs was significant (r=-0.84, p<0.01); the modeled change in BLLs from 20 to 4 μg/dL suggests predicted increases of 3.8 IQ points (95% CI, 1.4-6.2). Also controlling for urban status produced significance (r=-0.88, p<0.01) with predicted increases of 5.2 points (95% CI, 2.4-8.0). Control analyses ruled out aging as a confounder. CONCLUSIONS:
The dramatic societal decreases in BLLs in the U.S. since the 1970s were associated with a 4-5-point increase in the mean IQs of Americans. This effect is consistent with researchers' predictions; however, other variables (e.g., medical advances) may have contributed to the IQ gains.