Lead in children's' blood may be linked to increased emotional and behavioral problems, according to a recent study.

Researchers found that blood lead concentrations were associated with increased risk of behavioral and emotional problems, such as being anxious, depressed, or aggressive.

Lead is a naturally occurring toxic metal, but sources of lead exposure are often due to human activities, including burning fossil fuels, mining, and manufacturing. In the United States, lead exposure usually comes from lead-containing products, such as paint, caulking, and pipe solder, in older homes. In China, lead exposure is more often related to air pollution.

"This research focused on lower blood lead levels than most other studies and adds more evidence that there is no safe lead level," said Kimberly Gray, health scientist at the National Institute of Environmental Health Sciences. "It is important to continue to study lead exposure in children around the world, and to fully understand short-term and long-term behavioral changes across developmental milestones. It is well-documented that lead exposure lowers the IQ of children."

For the study, researchers measured blood lead levels in more than 1,300 preschool children in China between the ages of 3 and 5. Behavioral problems were assessed at age 6 using standardized questionnaires. The questionnaires were filled out by the children's teachers and parents, which the authors noted is both a strength and limitation.

"The study used scores from two sources, but the ratings do not provide a clinical diagnostic measure of behavioral problems," Jianghong Liu, senior author of the study, said in a statement.

The average blood lead level in the children was 6.4 micrograms per deciliter.

The Centers For Disease Control and Prevention now uses a reference level of 5 micrograms per deciliter, to identify children with blood lead levels that are much higher than normal, and recommends educating parents on reducing sources of lead in their environment and continued monitoring of blood lead levels.

Previous studies have reported that lead exposure causes what psychologists call externalizing behavior problems, such as aggressiveness and bullying, which may lead to truancy and even jail time as children get older.

"Young children are particularly vulnerable to the toxic effects of lead, because lead can affect children's developing nerves and brains," Liu said.

The findings were recently published online June 30 in the journal JAMA Pediatrics.