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.

The study bolsters suspicions that low-level lead exposure previously linked to behavior problems “is in fact associated with ADHD,” …

Based on study estimates, more than 5 million 4-to-15-year-olds nationwide have levels higher than 2 micrograms per deciliter, Lanphear said.

Children with blood lead levels of more than 2 micrograms per deciliter were four times more likely to have ADHD than children with levels below 0.8 microgram per deciliter. The government’s “acceptable” blood lead level is 10 micrograms per deciliter, and an estimated 310,000 U.S. children ages 1 to 5 have levels exceeding that.

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