Lead was a widely distributed developmental neurotoxicant with unknown but likely substantial consequences for population-level IQ.

Evidence

Half of US population exposed to adverse lead levels in early childhood (PNAS)

corroborated: 2 independent passages support this claim
XREF: connects to: Intelligence is malleable and shaped by environment, not fixed by heredity — Harriet A. Washington, A Terrible Thing to Waste, 54; Changes in childhood lead exposure are responsible for a 56% drop in violent crime in the 1990s, via effects on impulsivity, aggression, and IQ. — NBER Working Paper Series (Reyes, lead exposure and crime); Lead poisoning costs the US billions of dollars and millions of IQ points yearly — Harriet A. Washington, A Terrible Thing to Waste, 37 (+4 more)

Changes in childhood lead exposure are responsible for a 56% drop in violent crime in the 1990s, via effects on impulsivity, aggression, and IQ. (similarity 0.89)
Previous studies reported economic losses of $165-319 billion in lost wages from lead exposure (similarity 0.86)
Childhood lead exposure impairs cognition and raises aggression and dropout (similarity 0.84)
Leaded gasoline exposure cost Americans born before 1996 an estimated 824 million IQ points, averaging 2.6 points per person, much smaller than iodization’s claimed 15-point gap-closing effect. (similarity 0.83)
Lead poisoning costs the US billions of dollars and millions of IQ points yearly (similarity 0.83)
National infectious disease burden strongly predicts average national IQ (similarity 0.82)
A small average IQ drop hugely swells rates of disability and shrinks giftedness (similarity 0.79)
Children absorb lead more readily and face higher exposure risk than adults (similarity 0.78)

National infectious disease burden strongly predicts average national IQ #qualifies (0.70) — Harriet A. Washington, A Terrible Thing to Waste, 937