Children in New Zealand who were exposed to high levels of lead have grown up to be less intelligent adults than their counterparts who were not as affected, researchers said Tuesday.
The decades-long study published in the Journal of the American Medical Association (JAMA) shows that the effects of lead, a metal and potent neurotoxin, can be long-lasting.
The research involved more than 500 people living in the southeastern city of Dunedin in the 1970s and 1980s, an era when leaded gasoline (petrol) was common and the exhaust from vehicles meant most people were exposed to high levels of lead.
New Zealand's lead levels were consistently higher than international standards during this time.
A full 94 percent of children in the area had blood-lead levels that were higher than five micrograms per deciliter, the level at which the US Centers for Disease Control and Prevention now recommends a public-health intervention.
On average, kids' blood level was 11 micrograms of lead per deciliter -- twice today's warning level.
Children with this high amount of lead in their blood were given IQ tests as 38-year-old adults, and their IQ was on average 4.25 points lower than peers who had been exposed to less lead.
This is a "slight but significant" difference" that also affected their ability to gain well-paying jobs, said the study.
"The cognitive deficits associated with lead persisted for decades, and showed in the kinds of occupations people got," said lead author Aaron Reuben, a researcher at Duke University in North Carolina.
New Zealand eventually banned leaded gasoline in 1996, and it has also been phased out elsewhere in the world.
However, experts say there is no safe level of lead exposure, and the recent water crisis in Flint, Michigan -- where lead in pipes leeched into drinking water -- shows that risks remain.
Researchers say the Dunedin study is particularly useful because it tracked people for four decades.
Also, it showed "high blood lead levels were observed among children from all socioeconomic status levels," allowing a clearer look at the long-term effects of lead exposure.
Often, exposure to peeling paint in older buildings -- which also tend to be inhabited by poorer people -- is considered a key risk for lead exposure.
"It is routine to treat socioeconomic status as a potential confounder, yet this study suggests that lower socioeconomic status in adulthood is an outcome of early-life exposure, not merely a confounder," said an accompanying editorial by David Bellinger, a researcher at Boston Children's Hospital.
"The study results also provide support for the estimate that interventions since the 1970s to reduce the lead exposure of the US population have raised the mean IQ in adults by as much as 4.5 points."