People exposed to even just small amounts of lead earlier in life can suffer cognitive effects into adulthood without realising it, a long-running New Zealand study shows.
Researchers in the US looked at data collected as part of the ongoing Dunedin Study, which has been tracking the lives and wellbeing of about 1000 Kiwis since the early 1970s.
Of those, 564 were tested for levels of lead when they were 11 - which would have been in the mid-1980s. Almost all - 94 percent - had levels above five micrograms per decilitre thanks largely to leaded petrol, which wasn't banned in New Zealand until the mid-1990s.
Participants in the study had their brains scanned using MRI when they were 45. The researchers found for each five micrograms of lead per decilitre of blood, the total area of their cerebral cortex - the outer layer of the main part of the brain - was just over a square centimetre smaller.
A greater surface area in the cerebrum is vital for improved brain function, scientists say.
The hippocampus was also smaller, the more lead in the blood a person had - potentially affecting memory and learning - and a BrainAGE index score about nine months older than their biological age, meaning their brain appeared to be older than it should.
Each five micrograms of lead per decilitre of blood correlated to a two-point IQ drop - with no safe level of exposure. When the measurements were taken in the 1980s, lead levels below 35 micrograms per decilitre weren't even deemed worth investigating.
"We find that there are deficits and differences in the overall structure of the brain that are apparent decades after the exposure," study co-author Aaron Reuben of Duke University told medical news site MEDizzy.
"That's important because it helps us understand that people don't seem to recover fully from childhood lead exposure and may, in fact, experience greater problems over time.
While the participants affected by lead exposure didn't report any loss of cognitive abilities, their friends and family did, the researchers said.
The Dunedin Study has had far more success in retaining the cooperation of its participants than other long-running studies into human health, with 96 percent of those still alive continuing to take part.
Previous analysis By Duke University scientists when the participants were 38 found they lost 1.5 IQ points for each five micrograms of lead per decilitre. The new data - showing this rising to two points by age 45 - suggests the early exposure to lead continues to have negative effects throughout life. Whether this results in increased rates of conditions such as Alzheimer's is suspected, but yet to be proven.
People with exposures over 10 micrograms per decilitre were also previously found to achieve occupations "with socioeconomic status levels lower than those of their parents", which was partially blamed on their lower IQ scores.
Other research using Dunedin Study data has found correlations between a person's BrainAGE and their facial appearance and busted claimed links between cannabis use and violence.
The latest research was published recently in the Journal of the American Medical Association.