Childhood exposure to high levels of lead petrol in New Zealand cities in the 1970s and 1980s has been linked to lower IQ and socio-economic status in adulthood.

The effects are "slight but significant", says a paper published in the Journal of the American Medical Association.

Researchers used data from Otago University's Dunedin Multidisciplinary Health and Development Study, which has been tracking 1000 people born in 1972 and 1973.

About half of the participants were lead-tested when they were 11.

Those with more than 10 micrograms of lead per decilitre of blood at that age 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 mean blood-lead level of the children at age 11 was 10.99mcg per decilitre.

Ninety-four per cent had levels greater than 5mcg per decilitre, at which the US Centre for Disease Control and Prevention now recommends a public-health intervention.

The paper's senior author, Dunedin Study associate director Professor Terrie Moffitt, of Duke University in North Carolina, says the data came from an era when such high lead levels were viewed as normal.

When the Dunedin children were tested, only a level in excess of 35mcg per decilitre signalled a need for medical investigation.

"This research shows how far-sighted New Zealand was when the country banned leaded petrol in 1996," Prof Moffit said.

"Lead exposure is very rare in Kiwi children today. But the findings suggest the importance of keeping up our vigilance against other environmental pollutants."

The researchers also compared changes in social standing using the New Zealand Socio-economic Index.

Children whose lead levels were over 10mcg per decilitre attained occupations with socio-economic status levels lower than those of their parents.