Exposure to lead, whether it's from paint or household and municipal water pipes, is a big health risk. But lead contamination in topsoil is also a major source of lead exposure, and a concerning new study found that when boys are exposed to lead through the soil at age five, cognitive problems often follow.

The researchers used data from the U.S. Census in 2002, focusing on a question about whether a child age five and older in the household had experienced any memory, attention or other cognitive difficulties that lasted six months or more. Children under the age of five are the most likely to ingest lead from dirt via hand-to-mouth action, and age five is also when most kids start school in this country. These two reasons led the team to target this age group.

New evidence of the damaging effect of lead on cognitive development, even in areas with low lead concentration, suggests the EPA may need to revise its standards for acceptable levels of lead in soil.

They correlated the cognitive data with lead levels in different geographic locations, measured by a couple of different methods. One used calculations from the U.S. Geological Survey of topsoil lead in the 252 largest counties in the U.S. Each county included at least 100,000 people, and together the counties represented 45 percent of the U.S. population. Since a lot of lead contamination in the soil comes from cars, the researchers relied on the 1944 interstate highway plan, which predicted that areas nearer to the highways would have higher lead levels,

It turned out that higher levels of topsoil lead significantly raised the risk that 5-year-old boys would have problems with concentration, learning, memory and decision-making. But the same connection wasn’t seen in girls. The researchers think this may be because girls’ brains are “protected” by estrogen. Previous research has also suggested a gender difference in the effects of lead.

“These findings strengthen our understanding of the adverse effects of lead exposure on children's cognitive development,” said study author, Edson Severnini, in a statement. “They are concerning because they suggest that lead may continue to impair cognition.”

Equally alarming was the fact that the team also found some risk of cognitive problems in boys living in counties where the lead levels are considered low by Environmental Protection Agency guidelines.

“Our study provides new evidence of the damaging effect of lead on cognitive development, even in areas with low lead concentration,” said study author Karen Clay. “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.”

One limitation of the study is that the researchers only used an indirect method of looking at the connection — they didn’t measure the kids’ blood lead levels — so theoretically there could be other variables at play. But since many other studies have found connections between environmental lead, blood levels and cognitive problems, it seems likely that the link in the current study is legitimate. Hopefully, this study and others will prod the EPA and other recommending agencies to rethink how lead levels are conceptualized and monitored.

The study is published in the journal Economics and Human Biology.