THE CANADIAN PRESS -- TORONTO - Second-hand smoke exposure has been linked to a host of health-related issues, but new research is pointing to another potential side-effect: hearing loss.
"We know that second-hand smoke is associated with ear infections and that's been well-established for a while. And there's also a lot of effect of second-hand smoke on other systems," said study co-author Dr. Anil Lalwani, a professor of otolaryngology and pediatrics at New York University Langone Medical Center.
"One of the factors associated with early hearing loss in adults is exposure to noise as well as smoking," he added. "So we wanted to see if there was detrimental consequences to second-hand smoke exposure to kids who do not actually smoke but are exposed to smoke."
Lalwani and fellow researchers from the centre looked at the risk factors of sensorineural hearing loss among U.S. teens aged 12 to 19 for the study, published in the July issue of Archives of Otolaryngology -- Head and Neck Surgery.
Sensorineural hearing loss is irreversible and either stays the same or progresses over time, said Lalwani.
The sample included 1,533 youngsters who took part in the National Health and Nutrition Examination Survey from 2005 to 2006.
Participants were interviewed about a number of things related to their health status and family medical history, exposure to second-hand smoke and their own recognition of hearing impairment.
Youngsters also underwent a physical exam which included testing for cotinine -- a by-product of nicotine exposure -- and hearing tests.
Both low and high frequency hearing were tested. Low frequency hearing gives a sense of how loud things sound to individuals and is where vowels are contained, said Lalwani. High frequency hearing is where consonants are contained, allowing individuals to distinguish between words that sound similar, and it provides clarity to hearing.
"What we found was in both the low and high frequency averages, the hearing was worse in individuals exposed to smoking, statistically so for low frequency," said Lalwani. "What this, again, speaks to is individuals exposed to second-hand smoke are at risk for hearing loss."
"If you now extend to the prior finding in adults that adults who smoke have greater or accelerated hearing loss, what this says is the injury starts to occur very early on, even when they're not directly smoking, but are simply exposed to second-hand."
The study found 82 per cent of the kids who had hearing loss didn't complain of any hearing difficulty and therefore were unaware they had a hearing loss -- placing them potentially at risk for progressive hearing loss if their exposure is continuous, said Lalwani.
Researchers outlined a number of limitations to the study's data and analysis. The history of prenatal exposure to second-hand smoke was available for some and not all participants. The duration of second-hand smoke exposure and the noise exposure history of the teens was unavailable.
Lalwani said the work sets the stage for future studies. Researchers have submitted proposals to the national funding agency and they are looking at other databases for other years, he said.
Robert Harrison, a senior scientist with Toronto's Hospital for Sick Children, said he was surprised by the findings because he can't see how second-hand smoke could affect the audiology of the inner ear to the point of producing hearing loss.
While he is a little skeptical about the direct effect of second-hand smoke, Harrison said the survey data stands and is "statistically significant."
"There is some relationship, but I suspect it's a non-causal relationship because of this recreational noise exposure possibility.
Or, it is something to do with very early parental exposure during pregnancy or those early years in relation to otitis media (middle ear infection)," said Harrison, who sits on the board of directors with the Hearing Foundation of Canada.
Roberta Ferrence, a senior scientist with the Centre for Addiction and Mental Health in Toronto, said generally with health problems associated with second-hand smoke, duration of exposure seems to be the biggest factor.
"People always think active smoking is far more serious -- and generally it is. But it depends on what age you're exposed and how long," said Ferrence, who is also deputy director of the Ontario Tobacco Research Unit at the University of Toronto.
"Most kids don't start smoking until they're at least 12 or 14 or 16 whereas they may be exposed from in utero, so they may start off with an extra 12, 14 years of exposure to second-hand smoke. And particularly at times when their body is developing, an infant is certainly much more vulnerable than a 14-year-old."
Little kids also breathe more quickly, so there may be internal exposure as a result of the smoke, she added.
In the U.S., Lalwani said hearing screening is done at birth and then again when children enter elementary school, but there isn't any mandated screening of adolescents.
In Canada, there is very little testing of hearing in children other than early hearing tests in high-risk instances, said Harrison.
"It used to be that the schools did an assessment on occasion.... but there are very few opportunities when a child will actually get a hearing test unless there is a problem with the hearing."
Harrison said he would like to see hearing tests being done when children first enter school and later when entering high school.
"I think it would be useful for a routine hearing test at those stages because a mild sensorineural hearing loss can interfere with the child's educational achievement," he said, adding that those students may be missing things if they sit in the back of a crowded, noisy classroom.
Lalwani said what's clear over the last few decades is the realization of the "incredible negative consequences" to bystanders of second-hand smoke, whether it's to their own cognitive development, sensory deprivation, or cardiac or respiratory disease.
"This has huge implications for public health."