Lead Exposure Risk in US Children Far From Gone

Damian McNamara

January 20, 2020

Despite long-standing efforts to ban lead-based paint, some children in the United States remain at risk for the neurotoxic effects of lead exposure, particularly those living in low-income areas, new research shows.

In a large cross-sectional study, investigators found children living in areas with the highest risk of lead exposure scored significantly lower on cognitive tests and had lower brain volume than their counterparts who lived in areas with the lowest risk of lead exposure.

"What is clear is that the risk of lead exposure is a nationwide problem that may affect lower income children more than their higher income peers living in the same neighborhoods," senior author Elizabeth Sowell, PhD, an investigator at the Saban Research Institute at Children's Hospital Los Angeles, California, told Medscape Medical News.

The study was published online January 13 in Nature Medicine.

Clear and Present Danger

For those who believe lead exposure is a thing of the past, Sowell noted that although lead-based paint and lead in gasoline were banned decades ago, poorer children are still more likely to live in older homes that are not as well maintained, and therefore "continue to be at risk of environmental lead exposure from old lead-based paint, lead plumbing, as well as lead contamination from soil and other sources."

Previous research has linked family income level with gray matter volume in children as well as found an association between income levels and neurocognitive ability. However, these studies did not consider lead exposure.

To address this gap in the evidence, Sowell, lead author Andrew Marshall, PhD, and colleagues studied data from 9712 children participating in the ongoing Adolescent Brain Cognitive Development (ABCD) study. Children were 9 and 10 years old at enrollment in this longitudinal, multicenter study designed to follow participants over 10 years, which was sponsored by the National Institute of Mental Health.

The ABCD study does not include serum lead measures, so investigators used geocoded lead-risk scores based on each participant's census tract location. They calculated risk based on poverty levels and the age of homes.

Almost 41% of the children in the study were classified as low risk for lead exposure, 32% were at intermediate risk, and the remaining 27% were high risk.

A Heavy Legacy

The results showed that higher income was associated with significantly higher scores on cognitive tests (F (2, 9699) = 49.62; P < .001).

At the same time, cognitive performance scores were significantly lower in children living in areas with a low- vs high-risk of environmental lead exposure (F (1, 9699) = 4.70; P = .030).

Researchers also found a signification interaction between family income and lead risk (F (2, 9699) = 7.34; P = .001).

"Specifically, the negative association between lead risk and cognitive test scores was significant in the low-income group (P < .001), but not in the mid- or high-income groups (P ≥ .127)," the researchers note.

In addition, mean cognitive test scores in the low-income group were 9% lower than those in the high-income group. For the low-income group living in areas with the highest lead-risk scores, there was an additional 3.1% reduction in test performance.

Lower income was also associated with lower cortical thickness, surface area, and volume, although the researchers found no significant effect of lead risk levels on these structures.

For example, overall mean cortical surface area was 4.5% lower in the low- versus high-income group, with an additional 2% reduction for the low-income group living in high lead-risk areas.

Researchers also went beyond income and looked for similar associations using the Area Deprivation Index (ADI), which includes 17 variables associated with socioeconomic status. 

However, although the ADI confirmed some overall trends, the measure did not fit the model of cognitive function or family income.

Financial Disadvantage

It's important to note that because of the study's cross-sectional and observational design, the findings do not prove causality. An additional limitation of the ABCD study data is its inability to rule out other potential confounders.

"Based on the data we currently have for the 11,000+ individuals in the ABCD cohort, we cannot definitively say whether our results come from higher bodily lead levels in poorer children or whether other factors associated with higher family income protect children in some way from the same level of exposure," Marshall, who is also affiliated with the Saban Research Institute, told Medscape Medical News.

Lead-based paint in older housing units may continue to pollute the home environment, and wealthier families may have greater financial resources to maintain and/or remediate their homes, the researchers note.

"We shouldn't be surprised that we found alterations in brain volume with lead exposure because of the extensive research linking low-level lead exposure with IQ deficits and attention deficit hyperactivity disorder in children," study coauthor Bruce Lanphear, MD, MPH, professor of health sciences at Simon Fraser University in Burnaby, British Columbia, told Medscape Medical News. "Brain imaging is simply a new technology to measure the impact of lead [and other insults] on the brain."

On a positive note, lead exposure remains a modifiable risk factor.

"Children do not have to live with lead in their environments," Sowell added. "Lead can be remediated, and may improve cognitive and brain outcomes for those at highest risk."

Going forward, the investigators are seeking funding to test lead blood levels in ABCD study participants.

"We have collected naturally shed baby teeth from approximately 4000 participants in the ABCD study and are archiving them until funding can be secured for analysis," Sowell added.

New Questions

Commenting on the study for Medscape Medical News, Madhav P. Bhatta, PhD, MPH, said that although the findings confirm the negative effects on brain development and cognitive function in children, they also raise new questions.

These include whether there are potential protective factors in children from high-income families or other elements that exacerbate the effect of lead exposure in children from low-income households.

Further studies to identify such modifiers and to measure serum levels of lead are warranted, added Bhatta, professor of epidemiology and global health, College of Public Health at Kent State University in Ohio.

"This study with a large sample and assessing multiple childhood brain outcomes clearly provides further evidence of the detrimental effects of lead exposure," added Bhatta, who was senior author of a study examining elevated lead blood levels among immigrant children new to the United States between 2009 and 2016 in Ohio.

"Unfortunately, millions of low-income, minority, immigrant, and resettled refugee children in the United States continue to face the adverse consequences of lead exposure because of the poor housing conditions and the inability of their families to afford preventive measures," he added.  

Ongoing Global Hazard

Also commenting on the study, Aaron Reuben, a PhD student in the department of psychology and neuroscience, Duke University, Durham, North Carolina, said legacy lead in the environment remains an ongoing hazard for children in the United States and around the world.

Despite decades of research, not enough is known about the structural changes in the brain that underlie these disruptions or to what extent they are lasting, modifiable, or likely to worsen over time, Reuben told Medscape Medical News.

Until future research confirms the findings among children with serum lead level measures, the findings "must be interpreted with caution," added Reuben, who was lead author of a previous study that examined potential links between childhood lead levels and cognitive function and socioeconomic status in adulthood.

The current study, he added, "is an important reminder about the potential immediate and lasting vulnerability of the brain to childhood lead exposure."

Sowell, Marshall, Bhatta, and Reuben have reported no relevant financial relationships.

Nature Med. 2020;26:91-97. Full text

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