Striking new research published in the journal Proceedings of the National Academy of Sciences estimates lead exposure, primarily from car exhaust, has negatively affected the IQ of about half the population of the United States. The study calculated childhood exposure to exhaust from leaded gasoline, particularly in the 1960s and 1970s, resulted in the loss of up to six IQ points in some people.
Over the last 50 years scientists have found exposure to lead can significantly disrupt healthy childhood development. Our current threshold for unsafe blood lead levels (five micrograms of lead per deciliter of blood) is much lower than previous thresholds, and some researchers have recently gone as far as suggesting anything higher than one microgram per deciliter is unsafe.
Major improvements in public health measures have dramatically reduced childhood blood lead levels in the United States. In 2015 it was estimated only three percent of children had blood lead levels above five micrograms per deciliter.
In contrast, over 90 percent of people born between 1951 and 1980 are estimated to have had childhood blood lead levels in excess of that threshold. That equates to over 170 million people currently alive in the US. Even more stark is the estimates claiming more than 54 million people in the country experienced childhood lead levels over 15 micrograms per deciliter.
Historic lead exposures came from a number of sources, including paints and water pipes, but one of the most pervasive sources was exhaust from cars using leaded gasoline. Aaron Ruben, an author on the new study, said automotive exhaust is one of the most direct ways for lead to get into the brain and exert neurotoxic effects.
“Lead is able to reach the bloodstream once it’s inhaled as dust, or ingested, or consumed in water,” said Reuben. “In the bloodstream, it's able to pass into the brain through the blood-brain barrier, which is quite good at keeping a lot of toxicants and pathogens out of the brain, but not all of them.”
The new research set out to quantify the effect of lead exposure on IQ levels from a total population perspective. Using a number of US demographic datasets including leaded gasoline use and historic childhood blood lead levels, the study calculated lead exposure in the 20th century ultimately reduced the country’s cumulative IQ score by 824 million points.
On average, the researchers estimate this means lead exposure has reduced every adult citizen’s IQ by about three points. However, homing in on certain periods in history and places with the highest levels of leaded gasoline use (meaning higher levels of leaded automotive exhaust and high childhood blood lead levels) the study estimates lead exposure resulted in a drop of up to six IQ points for some people.
“I frankly was shocked,” said study co-author Michael McFarland when he first saw the results. “And when I look at the numbers, I'm still shocked even though I'm prepared for it."
The researchers point out that a few IQ points may initially seem insignificant but studies have consistently found correlations between IQ and everything from general health to economic success. Plus, while these calculations estimated overall population exposures to lead, the researchers point out lead exposure is not evenly spread among all members of a community. There are disproportionately higher childhood blood levels seen in African American communities, for example.
“On the individual level, even relatively small deficits in achieved IQ can have a meaningful impact on people’s lives, as cognitive ability, described by IQ, meaningfully predicts a person’s educational and occupational attainment, health, wealth, and happiness,” the researchers wrote in the study. “Notably, in the multidecade Dunedin Study, Reuben et al. reported a dose-response relationship between childhood lead exposure and mild downward social mobility by midlife. They estimated that 40 percent of the association was explained by IQ loss.”
Although improvements in public health standards, such as banning leaded gasoline, have significantly reduced childhood blood levels, Reuben is concerned about the lack of research into the long-term impacts of childhood lead exposure. One of Reuben’s prior investigations found a correlation between childhood blood lead levels and lower structural brain integrity at the age of 45.
So the next question Reuben is keen to investigate is whether high childhood blood lead levels influence the pace of age-related cognitive decline. Are increasing rates of dementia and Alzheimer’s disease influenced by 20th century exposures to lead?
“Millions of us are walking around with a history of lead exposure,” Reuben said. “It's not like you got into a car accident and had a rotator cuff tear that heals and then you’re fine. It appears to be an insult carried in the body in different ways that we're still trying to understand but that can have implications for life.”
The new study was published in the journal PNAS.
Source: Duke University
