A new study finds a strong link between high levels of lead in topsoil and cognitive problems in 5-year-old boys. No such link was found in girls, however.
The findings, published in the journal Economics and Human Biology, show that these harmful effects in boys were found even in counties where the government considers the levels of lead to be low.
“These findings strengthen our understanding of the adverse effects of lead exposure on children’s cognitive development,” said Dr. Edson Severnini, assistant professor of economics and public policy at Carnegie Mellon University’s Heinz College. “They are concerning because they suggest that lead may continue to impair cognition.”
The researchers looked at data on preschoolers from the 2000 Census, which had recorded whether individuals ages 5 and older had experienced any cognitive difficulties lasting at least six months.
The researchers focused on 5-year-olds because they wanted to examine the effects of lead before the start of formal schooling, and because age 5 is within the range described by the U.S. National Toxicology Program as having the highest rate of ingestion of lead from soil due to hand-to-mouth behavior.
The team also examined data from the U.S. Geological Survey on topsoil lead in the 252 largest counties in the United States, counties with 100,000 people or more and that represent 45 percent of the U.S. population. Researchers also considered several other factors, including issues associated with climate, the economy, demographics, and housing, as well as other topics related to counties and children’s attributes.
Because lead emissions by vehicles were an important source of topsoil contamination in the United States before lead additives were removed from gas in 1996, the study used the 1944 Interstate Highway System Plan as an instrument.
The plan predicted the placement of interstate highways, and therefore predicted where lead was accumulated in soil over several decades. As a result, children growing up in counties near those new highways had more exposure to lead in the soil in the 2000s than children in counties that did not have any part of the highway system.
Higher lead in topsoil considerably increased the likelihood that 5-year-old boys would have cognitive problems such as learning, remembering, concentrating, or making decisions. Living in counties with a concentration of lead in topsoil that was above the national median roughly doubled the probability of these boys having cognitive difficulties.
But 5-year-old girls were not affected, perhaps because they are naturally protected by estrogen, the authors suggest.
The researchers found negative effects on the boys even in counties with levels of lead concentration in topsoil that are considered low according to guidelines from the U.S. Environmental Protection Agency (EPA) and state agencies that address these issues.
“Our study provides new evidence of the damaging effect of lead on cognitive development, even in areas with low lead concentration,” said Dr. Karen Clay, professor of economics and public policy at Carnegie Mellon University’s Heinz College, who coauthored the study. “This indicates the need for further monitoring of soil in urban areas and suggests that the EPA should revise its standards for acceptable levels of lead in soil.”
The researchers warn that the study did not observe children’s blood lead levels to estimate a direct relationship between blood lead levels and cognition. Instead, they relied on an indirect measure that has been shown to have a clear relationship with blood lead levels, namely the concentration of lead in topsoil.
Source: Carnegie Mellon University