Removing lead from petrol may have sparked a decline in dementia cases, according to new research.
A promising downward trend in rates of the devastating brain disease has been reported across North America and Europe.
This has coincided with an increase in risk factors such as middle age spread and diabetes.
Unleaded petrol has been shown to boost the mental faculties of children and it could have done the same for their parents and grandparents.
Lead author Professor Esme Fuller-Thomson believes the phenomenon is down to “generational differences in lifetime exposure.”
Leaded petrol was finally banned under EU law in 2000 – four years after the US did the same.
Prof Fuller-Thomson, director of the Institute of Life Course and Ageing at the University of Toronto, said: “While the negative impact of lead exposure on the IQ of children is well known, less attention has been paid to the cumulative effects of a lifetime of exposure on older adults’ cognition and dementia.
“Given previous levels of lead exposure, we believe further exploration of this hypothesis is warranted.”
If her team is right it offers hope of the devastating condition becoming less of a problem in the coming decades than experts currently fear.
A growing body of evidence has linked changes to behaviour, including criminal offending, to tetraethyl lead.
The compound was added to petrol almost a century ago to make car engines more efficient.
As it was phased out, beginning in 1973, lead levels in peoples’ blood plummeted.
Research has found Americans born before 1925 had about twice the lifetime exposure as those born between 1936 and 1945.
Prof Fuller-Thomson said: “The levels of lead exposure when I was a child in 1976 were 15 times what they are today.
“Back then, 88 per cent of us had blood lead levels above 10 micrograms per deciliter.
“To put this numbers in perspective, during the Flint water crisis of 2014, one per cent of the children had blood lead levels above 10 micrograms per deciliter.”
Nearly 100,000 residents of the Michigan town were left without safe tap water after lead began leaching into the supply.
Lead is a neurotoxin that crosses the blood-brain barrier. Studies on animals and humans occupationally exposed suggest a connection to dementia.
A higher incidence of the illness has also been reported among older adults living closer to major roads or with a greater exposure to traffic related pollution.
Prof Fuller-Thomson and her pharmacy student Judy Deng are particularly interested in a potential link between lifetime lead exposure and a newly identified type of dementia.
Known as LATE (Limbic-predominant Age-related TDP-43 Encephalopathy), it is distinct from Alzheimer’s disease and mainly effects the over 80s. It has been identified in 20 per cent of this group of dementia patients.
Other plausible explanations for the improving trends in incidence include better education, less smoking and healthier blood pressure levels compared to previous generations.
But, even when these factors are taken into account, many studies still find falling rates of cases.
The next steps could include comparing 1990s assessment of blood lead levels to current health insurance records.
Lead can also be absorbed into teeth and shinbones. These would also serve as proxies for life-time exposure when conducting post-mortems on dementia patients.
The association between particular gene variants associated with higher lead uptake and dementia incidence should also be investigated, said the researchers.
Added Ms Deng: “If lifetime lead exposure is found to be a major contributor to dementia, we can expect continued improvements in the incidence of dementia for many more decades as each succeeding generation had fewer years of exposure to the neurotoxin.”
At the moment there are about 850,000 people in the UK living with dementia, a figure expected to rise to two million by 2050 because of the ageing population. Currently, drugs can only treat the symptoms, and not the cause.
The study published in the Journal of Alzheimer’s Disease follows research linking lead to aggressive or dysfunctional behaviour.
Many Western nations have experienced significant declines in crime in recent decades. It has been suggested unleaded petrol may be the reason.
Exposure to lead during pregnancy can reduce the head circumference of infants, say scientists. In children and adults it causes headaches and inhibits IQ.
It has a range of effects on the brain that have been demonstrated through hundreds of different biological studies.
It reduces the grey matter in areas responsible for things such as impulse control and executive functioning – meaning thinking and planning.
In 2016 a study suggested the proportion of elderly people developing dementia is falling in the US – backing up similar findings in the UK and Europe.
Data from 21,057 people over the age of 65 in the US showed the proportion with dementia fell from 11.6% in 2000 to 8.8% in 2012.