Exhaust fumes exposing high levels of lead could be to blame for thousands of Kiwi adults battling mental health problems such as antisocial behaviour and hyperactivity, a study says.

Lead was removed from petrol in 1996 in New Zealand after a Dunedin study revealed the chemical's harmful effects on children.

Previous studies have identified a link between lead and intelligence, but now researchers say changes in personality in adulthood could be a result of exposure to the heavy metal.

The Dunedin Multidisciplinary Health and Development longitudinal study tracked 579 New Zealanders born in 1972 and 1973.

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Today, blood lead levels above 5 ug/dL will trigger additional clinical follow-up of a child but when the study was done 94 percent of participants in the study had blood lead levels above this cut-off at age 11.

Researchers found that the people in the study with greater lead exposure were rated as more neurotic, less agreeable, and less conscientious than their less-exposed peers.

These personality characteristics were linked to a number of problems, including worse mental and physical health, reduced job satisfaction, and troubled interpersonal relationships.

Researchers stressed there was not enough evidence to say lead exposure was a direct cause to these mental disorders but there was a link.

Co-author Aaron Reuben, a graduate student in clinical psychology at Duke University, said the connection was "modest" but important to identify.

"At one point in time everyone was exposed to lead, and now, certain people in certain cities and countries are still exposed it," Reuben said.

Physical and mental health evaluations of people involved in the study were conducted periodically until they hit the age of 38 years.

After adjusting for other possible variables, the study found that for every 5 ug/dL above the 11ug/dL average, the likelihood of developing a mental disorder was increased by 1.34 times.

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Reuben said those findings suggested that the health impacts of lead "really can last a long time and this case nearly four decades".

"Lead exposure from decades ago may be harming the mental health of people today who are in their 40s and 50s," he said.

Today, high lead exposures are rarer, and most often found in children who live in older buildings with lead plumbing and paint.

Researchers said although the risks to mental health in adulthood may be very small further research was needed to get more "concrete conclusions."

In the future, the Dunedin Study team is interested in whether lead exposure might be linked to the development of later-life diseases such as dementia or cardiovascular disease.

Director of the University of Otago's Dunedin study, Professor Richie Poulton, said independent replication was required before more concrete conclusions could be drawn.

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