Children who breathe in secondhand smoke at home are more likely to develop attention-deficit hyperactivity disorder (ADHD), learning disorders and other behavior problems, compared with children who grow up in smoke-free homes, a new study suggests.

The study does not prove a cause-and-effect relationship between secondhand smoke and these disorders, and does not define how much smoke a child must be exposed to in order to be at increased risk, HealthDay reports.

The researchers, from the Harvard School of Public Health in Boston, investigated the results of a telephone survey of families that included 55,358 children under age 12. Six percent of the children were exposed to secondhand smoke in the home. The researchers found that 8.2 percent of those children had learning disabilities, 5.9 percent had ADHD, and 3.6 percent had behavioral and conduct disorders. Children exposed to secondhand smoke at home had a 50 percent increased risk of having two or more of these disorders compared with children who were not exposed to secondhand smoke.

Boys had a significantly higher risk, the researchers reported in the journal Pediatrics.  Older children, especially those aged 9 to 11 years, and those living in households with the highest poverty levels, were at greater risk. The researchers estimate that secondhand smoke may account for 274,100 excess cases of these disorders in the United States.