Leaded gasoline creates a natural experiment in long–term study.
A long–term study of more than 500 children who grew up in the era of leaded gasoline has shown that their exposure to the powerful neurotoxin may have led to a loss of intelligence and occupational standing by the time they reached age 38.

The effects are slight, but significant, showing that the higher the blood lead level in childhood, the greater the loss of IQ points and occupational status in adulthood.

The study appeared in the Journal of the American Medical Association.

Study participants are part of a life–long examination of more than 1,000 people born in Dunedin, New Zealand in 1972 and 1973. During their childhood, New Zealand had some of the highest gasoline lead levels in the world.

From birth to adulthood, these people have regularly been assessed for cognitive skills such as perceptual reasoning and working memory. At age 11, blood samples were collected from 565 of them which were then tested for lead.

Participants who were found to carry more than 10 micrograms of lead per deciliter of blood at age 11 had IQs at age 38 that were, on average, 4.25 points lower than their less lead–exposed peers. They were also found to have lost IQ points relative to their own childhood scores.

The study found that for each 5–microgram increase in blood lead, a person lost about 1.5 IQ points.

The mean blood lead level of the children at age 11 was 10.99 micrograms per deciliter of blood, slightly higher than the historical “level of concern” for lead exposure. Today’s reference value at which the CDC recommends public health intervention is half that, 5 micrograms per deciliter, a level which 94 percent of children in the study exceeded. No safe blood lead level in children has been identified.

“This case is different from the one in Flint, Michigan and other cities where lead in the drinking water has led public health officials to begin special interventions for those children,” Moffitt said. Flint’s children are receiving regular blood monitoring and expanded early childhood education, behavioral health services and special nutrition with the federal government’s support. “Interventions of this sort are intended to forestall the sorts of effects we’ve measured in this study,” she said.

What makes the New Zealand case an important natural experiment is that automobile traffic goes through all neighborhoods. Unlike exposures to leaded paint or lead pipes in older structures, which pose more of a threat to poorer families, the exposure to leaded gasoline fumes was distributed relatively evenly across all social strata.

Leaded gasoline was phased out in the U.S. and New Zealand between the mid–1970s and the mid–1990s, but is still used in some Asian and middle eastern countries.

The study also compared changes in social standing using a measure from the New Zealand government that plots families on a 6–point scale. The childhood social status of each child’s family was compared to their adult standing at age 38. Children who were over 10 micrograms of lead attained occupations with socioeconomic status levels four–tenths lower than their less–exposed peers.