Even low exposure to lead correlates with emotional and behavioral problems among young children, a new National Institutes of Health study suggests.

Research conducted among preschool children in China reveals that even small amounts of lead are associated with increased risk of problems such as anxiety, depression and aggression.

The study, whose results were published today in the journal JAMA Pediatrics, builds on previous research showing a correlation between lead exposure and lower IQ levels. However, earlier studies have largely focused on blood lead levels at or above 10 micrograms per deciliter.

Pollution from an industrial plant in China.

Gustavo Madico/Flickr

The latest NIH study looked at more than 1,300 children ages 3 through 5 in China with an average blood lead level of 6.4 micrograms per deciliter. The Centers for Disease Control and Prevention now uses a blood lead level of 5 micrograms per deciliter to identify children with excessive lead exposure.

“This research focused on lower blood lead levels than most other studies and adds more evidence that there is no safe lead level,” said NIH health science administrator Kimberly Gray in a statement.

The new research also sheds further light on discrepancies between the causes and repercussions of lead exposure in children in China and the U.S. In China, blood lead concentrations appear to increase with age in preschool children, according to researcher Jianghong Liu of the University of Pennsylvania’s School of Nursing. Meanwhile, in the U.S., concentrations tend to increase with age in children up to 2 or 3 years old and then decline.

While lead is a naturally occurring toxic metal, sources of its exposure often trace back to human activities such as mining and manufacturing. In the U.S., lead exposure typically comes from lead-containing products in older homes such as paint, caulk and pipe solder. In China, the source is often air pollution.

While studies in the U.S. have shown links between lead exposure and “externalizing behavior problems” — aggressiveness and bullying, for example — this study primarily suggests correlations with internalizing problems such as anxiety.

In the recent NIH study, researchers analyzed one blood sample per child. Behavioral problems were assessed when the child turned 6 years old using standardized questionnaires.