Proving any connection between the wide range of potential pollutants and a wide range of educational difficulties is a nightmare for scientists, educators, and parents alike.
Lead poisoning provides perhaps the clearest evidence of individual harm. No reputable physician can dispute that even small amounts of lead can reduce intellectual functioning and diminish the capacity of a child to learn. Much of this damage is permanent. And while lead poisoning is in decline, the Centers for Disease Control estimates that some 300,000 children in the United States are poisoned by lead every year, mostly poor children of color.
Lead has long been known as a hazard. Between 1909 and 1934, 11 European nations and Cuba recognized lead's danger and banned or restricted its use in interior paint. But in the United States, paint companies fought regulation, even touting the value of lead in advertising targeted at children.
Lead use became more widespread with its addition to gasoline. In 1925, even after public health doctors sounded the alarm and the surgeon general convened a commission to examine the public-health implications of leaded gasoline, the commission ruled in favor of industry on the dubious strength of a few poorly done studies showing no ill-effects. (The commission also recommended that Congress fund further study, but that never happened.) The case laid the basis for modern environmental policy, in which the burden of proof rests most heavily on the polluted, not the polluter. It took another five decades to amass the data necessary to change the status quo. Lead in gasoline was not phased out until the 1970s and 1980s; it remained in paint until 1978.
Lead was not a trivial additive to paint. Until the 1950s, as much as half, by weight, of a can of paint was lead. Coat by coat, room by room, house by house, America's housing stock became increasingly toxic. As the housing stock has aged, outright decay and renovation keep the lead in circulation, through paint chips and lead-laden dust. It takes only a grain or two of lead a day to poison a child. Those aged six months to five years are most at risk, either by directly ingesting peeling paint, or through putting hands tainted with lead dust in their mouths.