Despite years of graphic anti-smoking campaigns and public smoking bans, people continue to light up — especially at home, where more than half of American children and teenagers are exposed to secondhand smoke. Scientists believe that passive smoke either causes or exacerbates lung cancer, asthma, learning disabilities, heart disease and other conditions and now a new study now finds that teenagers exposed to tobacco smoke may also have significant hearing loss — and not even know it. Here, a brief guide:

How was this study conducted?
Researchers from the New York University School of Medicine tested over 1,500 adolescents aged 12 to 19 for blood levels of cotinine, a byproduct of nicotine that forms when people are exposed to tobacco smoke. They also tested the teens' ability to hear high-, medium- and low-frequency sounds.

What did the researchers find?
Teenagers who've been exposed to secondhand smoke have roughly double the risk of hearing loss compared to those with little or no exposure. And the more these teens have been exposed to tobacco smoke, as measured by their blood levels of cotinine, the greater their risk of significant hearing loss. Teenagers with the highest levels of exposure had nearly triple the risk of hearing loss. 

How bad is their hearing?
They weren't deaf, but their hearing was measurably impaired. And a startling number of these teens — over 80 percent — had no idea that they had significant hearing loss. "It's kind of surprising," says Dr. Joseph DiFranza, as quoted by Reuters Health. "We already knew that passive smoking is bad for children," but "this just piles on another reason" to keep tobacco smoke away from kids.

How does hearing loss affect teens?
Hearing impairment was especially noticeable in the mid- to high-frequency range, which means the adolescents might have difficulty understanding human speech. That could result in problems at school, where they may be singled out as troublemakers, fail at social interactions, or be misdiagnosed with ADHD or other behavioral problems.

What can parents and teachers do?
The obvious first step is to minimize children's and teen's exposure to tobacco smoke. Testing is another important step: Infants are regularly screened for hearing loss, but there are no guidelines for hearing tests past the early school years. One researcher noted that all children and teenagers exposed to secondhand smoke should be regularly screened for hearing loss, in case hearing aids are warranted.

Sources: EurekAlert, MedPage Today, Reuters, Time, WebMD