As a neurotoxin, lead is extremely harmful to the brain. It can cross the blood-brain barrier and damage or kill brain cells. Past studies involving people exposed to lead at work have linked lead exposure to dementia.
In their study, U of T researchers hypothesized that lead contributes to the development of LATE, or Limbic-predominant Age-related TDP-43 Encephalopathy.
LATE is a recently identified type of dementia and affects around 20 percent of dementia patients over the age of 80. Much remains unknown about LATE, but people with this disease exhibit symptoms similar to Alzheimer's, including memory loss.
The researchers pointed to fewer dementia cases to support their hypothesis. Many studies have shown that the number of dementia patients has decreased over the past decades. This could be attributed to the lower prevalence of smoking, higher educational attainment and better management of hypertension among today's older adults compared to previous generations.
However, these factors were not sufficient to explain the downward trend in dementia cases. As such, the researchers suggested that generational differences in lifetime lead exposure could have contributed to the decline.
The researchers explained that leaded gasoline was a ubiquitous source of air pollution from the 1920s to the 1970s. At the same time, dementia was more common among older adults who live close to major roads and among those with greater exposure to traffic-related pollution, as past studies showed.
However, when leaded gasoline was phased out starting in 1973, the levels of lead in people's blood gradually plummeted. Indeed, studies from the 1990s showed that Americans born between 1936 and 1945 were exposed to half the amount of lead over their lifetime as those born before 1925.
"The levels of lead exposure when I was a child in 1976 were 15 times what they are today," said Esme Fuller-Thompson, a professor of medicine at the U of T and one of the study's researchers. (Related: 94% of Americans who grew up during the era of leaded gasoline found to be lead poisoned and brain damaged.)
She noted that around 88 percent of the people back then had blood lead levels above 10 micrograms per deciliter (mcg/dL). Blood lead levels of five mcg/dL and above are considered elevated in children while values of 10 mcg/dL and above are considered elevated in adults.
"To put this number in perspective, during the Flint Michigan water crisis of 2014, one percent of the children had blood lead levels above 10 micrograms per deciliter," she said, referring to the public health crisis that plagued the city of Flint. Tens of thousands of residents were exposed to dangerous levels of lead after drinking contaminated municipal water.
Judy Deng, Fuller-Thompson's co-author who was a U of T pharmacy student at the time of the study, remarked that dementia cases should continue to decline if lifetime lead exposure indeed contributes to dementia. She explained that succeeding generations had been exposed to smaller amounts of lead over their lifetime.
Deng and Fuller-Thompson recommended several tests to assess whether their hypothesis is valid. These included comparing past and present records of blood lead levels and assessing the amount of lead in the teeth and tibia bones when examining a deceased person's brain for dementia.
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