What le(a)d to their loss of intelligence?
The effects may be slight but they are significant. A long-term study of over 500 children who grew up in the era of lead-ed gasoline has shown that their exposure to powerful neurotoxin may have led to a loss of intelligence and occupational standing by the time they touched the age of 38.
The effects indicated the higher the blood lead level in childhood, the greater the loss of IQ points and occupational status in adulthood. 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 their 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 per decilitre of blood at age 11 had IQs at 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 decilitre 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 decilitre, a level which 94 per cent of children in the study exceeded. No safe blood lead level in children has been identified.
What makes the New Zealand case an important natural experiment is that automobile traffic goes through all neighbourhoods. 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.