Extremely high levels of lead in Kiwi cities in the 1970s and 1980s lowered the IQ and life prospects of today's adults, research has found.
Research from the University of Otago's internationally recognised Dunedin Study released today showed that among more than 500 Dunedin children who grew up in the era of leaded petrol, those exposed to lead had a lower IQ and social standing by the age of 38, relative to peers who had less exposure.
It showed the effects were "slight but significant", also revealing the higher the blood-lead level was in childhood, the greater the loss of IQ points and job prospects as adults.
New Zealand lead levels were consistently higher than international standards during the 1970s and 1980s, largely due to vehicle exhaust fumes.
Leaded petrol was finally phased out completely in 1996.
An Otago University researcher not involved in the study blamed oil companies and complicit politicians for lead not being banned earlier and said the research had important lessons for today's politicians.
Of the just over 1000 Dunedin Study participants, who were all born in 1972 and 1973, 565 had blood-lead tests at age 11. About half of those tested had more than 10 micrograms of lead per decilitre of blood.
That half had IQ scores on average 4.25 points lower than their less lead-exposed peers when tested again 27 years later at age 38.
They also attained occupations with a lower socio-economic status than those of their parents.
"The results suggest that cognitive impairment associated with childhood lead exposure can persist and may worsen ... across decades."
The paper's senior author, Dunedin Study associate director Professor Terrie Moffitt, of Duke University, North Carolina, said the finding was significant but 4.25 IQ points was not worth very much. "This is not like the kind of thing that would keep people from completing their education or from getting a job."
But it was a discernible difference, and the fact the findings would be spread across the whole population made them more significant.
It differed from other studies because participants exposed to lead were from across the socio-economic spectrum and because it showed the effect of lead persisted for decades.
"As of the start of this year, the long-term adult cognitive outcomes of children exposed to lead were unknown, due to a lack of research.
"Our new paper fills that gap."
If everyone takes a hit from environmental pollutants, society as a whole suffers.
Co-author Professor Avshalom Caspi, of Duke University, said wherever you started in life "lead is going to exert a downward pull".
"If everyone takes a hit from environmental pollutants, society as a whole suffers."
Main author Aaron Reuben, a Duke PhD candidate, said the lower job status is "partially but significantly explained by the loss of IQ".
Moffitt said other studies had linked lead with poor and aggressive behaviour, which could also partly explain why those exposed to higher amounts had poorer job prospects.
She said the data was from an era when such high lead levels were viewed as normal, not dangerous.
The participants - likely comparable to all kids in NZ towns before lead was removed from petrol - had an average blood level of 10.99 mcg of lead per decilitre of blood at age 11, slightly higher than the 1990s "level of concern" for lead exposure.
This research was published today in the Journal of the American Medical Association.