CHICAGO – About one-third of attention deficit cases among U.S. children may be linked with tobacco smoke before birth or to lead exposure afterward, according to provocative new research.

Even levels of lead the government considers acceptable appeared to increase a child’s risk of having attention deficit hyperactivity disorder, the study found. It builds on previous research linking attention problems, including ADHD, with childhood lead exposure and smoking during pregnancy, and offers one of the first estimates for how much those environmental factors might contribute.

“It’s a landmark paper that quantifies the number of cases of ADHD that can be attributed to very important environmental exposures,” said Dr. Leo Trasande, assistant director of the Center for Children’s Health and the Environment at Mount Sinai School of Medicine in New York.

More importantly, the study bolsters suspicions that low-level lead exposure previously linked to behavior problems “is in fact associated with ADHD,” said Trasande, who was not involved in the research.

The study’s estimate is in line with a National Academy of Sciences report in 2000 that said about 3 percent of all developmental and neurological disorders in U.S. children are caused by toxic chemicals and other environmental factors and 25 percent are due to a combination of environmental factors and genetics.