LOS ANGELES - Children and teens exposed to secondhand smoke are more likely to develop symptoms of a variety of mental-health problems, including major depressive disorder and attention-deficit/hyperactivity disorder, according to a study published yesterday in the journal Archives of Pediatrics and Adolescent Medicine.

At this point, it should come as no surprise to anyone that exposure to tobacco smoke is unhealthy. Plenty of studies have linked secondhand smoke to respiratory problems, asthma, sudden infant death syndrome, middle-ear infections and other physical health problems.

However, the link between secondhand smoke and mental health has not been examined as closely.

The new study is thought to be the first that looks at how secondhand-smoke exposure - as measured by the presence of a nicotine metabolite in the blood - is associated with mental health in a nationally representative sample of American kids and teens.

Researchers from the National Institutes of Health, the University of Miami and Legacy, a nonprofit organization that fights tobacco use, used data on 2,901 youths between the ages of 8 and 15 when they were part of the National Health and Nutrition Examination Survey from 2001 to 2004.

As part of the study, the kids were asked to provide blood samples; those who were exposed to secondhand smoke had higher levels of cotinine, which is produced as the body metabolizes nicotine. The kids also were assessed for a variety of mental-health disorders as defined by the National Institute of Mental Health's Diagnostic Interview Schedule for Children Version IV.

Here's what the researchers found: On average, the kids had almost five symptoms of major depressive disorder, almost four symptoms of ADHD, almost three symptoms of generalized anxiety disorder, and more than one symptom of conduct disorder.

After taking into consideration the kids' health history and other factors, the researchers determined that levels of cotinine in the blood were strongly correlated with ADHD symptoms and weakly linked with symptoms

of major depressive disorder, conduct disorder and generalized anxiety disorder. Overall, the links between cotinine and psychiatric symptoms were greater for boys than for girls, and for whites than blacks and Mexican Americans.

But none of those symptoms added up to a single diagnosis of a mental-health disorder that could be linked to exposure to secondhand smoke in the children and teens in the study.

At first, it appeared that higher cotinine levels might be associated with a higher risk of ADHD. But it turned out that the increased ADHD risk was actually because of smoking by mothers during pregnancy