Manganese is an essential mineral, but too much of it may have neurotoxic effects.
The new study - which was led by Dr. Erin Haynes, an associate professor in the Department of Environmental Health at the University of Cincinnati College of Medicine in Ohio - was carried out among the residents of East Liverpool, OH.
East Liverpool is a city whose levels of air manganese have been exceeding those recommended by the United States Environmental Protection Agency (EPA) for "over a decade." In fact, a 2010 report from the EPA confirmed that "airborne manganese concentrations consistently exceeded" their guideline values "at all monitoring locations."
Manganese is an essential mineral, key to brain development and growth. But the EPA warn that it can be toxic "at excessive exposure levels."
Manganese is often used to make steel and other alloys, batteries, fertilizers, and ceramics, among other things.
The EPA refer to studies that have found that chronic exposure to air manganese correlated with Parkinson's-like neurological changes, and living in areas with excessive levels of the metal has been linked to neurological deficits - especially in men over 50 years old.
Children and young adults are also at risk, the EPA caution, as excessive manganese may impact learning and behavior.
The residents of East Liverpool may be particularly vulnerable to the risks posed by manganese, given that the city is home to a waste incinerator and a manganese processor. For this reason, research into the potential effects of manganese exposure on the population was long overdue.
Excessive manganese linked to low IQ
Dr. Haynes and team analyzed blood and hair samples from 106 children aged between 7 and 9, all of whom were enrolled in the Communities Actively Researching Exposure Study (CARES) between March 2013 and June 2014.
CARES is a larger research project that started in 2008. A previous CARES study overseen by Dr. Haynes suggested that both excessive and insufficient levels of manganese may inhibit neurodevelopment.
For this study, the scientists tested the blood and hair for manganese and lead. They also analyzed serum for cotinine. The study participants received questionnaires that the researchers used to perform cognitive assessments.
To calculate potential associations between the biological measures of manganese levels and scores on IQ tests, Dr. Haynes and colleagues used linear regression models.
After they had adjusted for potential confounders, the researchers found that hair manganese levels correlated negatively with full-scale IQ scores, as well as with processing speed and working memory.
Weighing in on the significance of the findings, Dr. Haynes says, "There are socioeconomic issues at play. [But] they are also compounded by potentially significant environmental exposures."
"Children may be particularly susceptible to the neurotoxic effects of ambient [manganese] exposure, as their brains are undergoing a dynamic process of growth and development."
Dr. Erin Haynes
However, the authors also note some limitations to their research. Firstly, the study examined a small number of children, and secondly, the association found may be due to historic exposure to manganese rather than exposure to chronic levels of the metal.
"[Manganese] exposure in this community should be further evaluated in comparison to other pediatric [manganese] cohorts," the authors say.
"Environmental justice issues such as psychosocial stressors should be included in these analyses as they may play a role in neurotoxicity," they add.
Dr. Haynes says that in future, she plans to include neuroimaging data in her and her team's studies, "as [they] continue to advance [their] understanding of the impact of manganese on neurodevelopment [and] define the lines between essential benefit and toxicological harm."