The Daily Mail reports today that research has found that “teenagers exposed to second-hand smoke are twice as likely to suffer hearing loss”. It said that scientists think that passive smoking affects the blood supply to an area of the inner ear called the cochlea, causing ‘sensorineural’ hearing loss.
This cross-sectional study assessed 1,500 US teenagers and found that a higher proportion of those exposed to second-hand tobacco smoke had hearing loss for low-frequency sounds.
This study had several limitations, including the fact that it only measured tobacco exposure at one point in time. Therefore it is not possible to say whether the hearing loss came before or after the smoke exposure. Exposure to second-hand smoke may also vary considerably on a daily basis. It also relied on the adolescents saying whether or not they smoked, something that many might not want to admit. It also did not assess the potential confounder of exposure to loud noise, a major risk factor for hearing loss, and one which teenagers might be exposed to if they regularly attended clubs or listened to loud music.
As it stands, this study does not show that smoking causes hearing loss. Further studies would be needed to confirm this association and whether this is a causal effect or not.
Where did the story come from?
The study was carried out by researchers from New York University. Funding was provided by the Zausmer Foundation and the National Institutes of Health/ National Center of Minority Health and Health disparities. The study was published in the peer-reviewed medical journal Archives of Otolaryngology Head and Neck Surgery .
Some of the newspapers implied that this study found that smoke exposure causes hearing loss in teenagers. However, this cannot be determined from this cross-sectional study, which only showed an association between the two. The newspapers also mentioned the potential behavioural or learning problems that could arise from impaired hearing. This study did not directly assess what the functional consequences of the hearing loss experienced by the adolescents in this study were. The researchers in their discussion raised this issue but said that the effects of mild hearing loss in adolescence remained to be elucidated.
What kind of research was this?
This study investigated whether there is an association between second-hand smoke and “sensorineural hearing loss” in adolescents aged between 12 and 19 years. Sensorineural hearing loss is mostly caused by damage to the delicate hair cells in the ear, which convert sound waves into brain cell signals, but can also be caused by damage to the sound-processing centres in the brain.
The researchers say that second-hand smoke has been linked to middle ear infections in children. They also speculate that second-hand smoke during pregnancy could be linked to sensorineural hearing loss as it could affect the development of the foetus; or that smoke exposure causes low birthweight, which may affect the child’s subsequent development. Furthermore, they say that exposure in childhood or adolescence may damage the nerve cells in the cochlea or the nervous pathways in the brain that are needed for hearing.
This was a cross-sectional study, which assessed the relationship between different factors, at one point in time. Therefore it cannot determine whether smoking causes the effects seen. To do this, the researchers would need to do a prospective study where they followed the individuals from a time before hearing loss had occurred.
What did the research involve?
The researchers collected data from 2,288 adolescents aged between 12 and 19 years who had completed the National Health and Nutritional Examination Survey between 2005 and 2006. The survey had been sent to a nationally representative sample of the non-institutionalised civilian population in the US.
Participants were interviewed to determine their family medical history, current medical conditions, medication use, self-report of the presence of smokers in the household, and socioeconomic and demographic information. The participants were also given physical tests, and they gave blood and urine samples.
The participants also underwent a series of hearing tests and were asked whether they thought they had a hearing impairment.
There are three main types of hearing loss:
- sensorineural hearing loss involving structures of the inner ear (i.e. the cochlea) or nervous pathways transmitting sound to the brain
- conductive hearing loss, which may be caused by problems with the outer ear, the eardrum or the bones in the middle ear that transfer sound waves
- or a mixture of conductive or sensorineural hearing loss
Based on the hearing tests, the researchers excluded 32 participants that had mixed or conductive hearing loss.
Alongside the participants’ self-reports of smoking exposure, the researchers used blood tests to determine the amount of nicotine by-products (cotinine) in the blood. The smoking categories were:
- Active smokers: cotinine levels of 15.0 µg/L or higher, or those who reported smoking in the past five days.
- Exposed: cotinine levels were detectable but less than 15.0 µg/L, and those who did not report smoking in the past five days.
- Unexposed: undetectable cotinine levels, and without self-defined smoking.
To study the effect of second-hand smoke only, the 229 active smokers were excluded from the study. In total, this left the researchers with data from 1,533 adolescents.
The researchers used a technique called logistic regression to model the association between hearing loss and second-hand smoke. The model included the influence of gender, age, race/ethnicity and socioeconomic characteristics.
What were the basic results?
The researchers found that second-hand smoke exposure was associated with an increased rate of sensorineural hearing loss (SNHL) in one ear. They found that 7.5% of adolescents who were not exposed to smoke had SNHL, whereas 11.8% of adolescents exposed to second-hand smoke had SNHL for low-frequency sounds in one of their ears (p<0.04).
Adolescents exposed to second-hand smoke had an 83% increased risk of SNHL for low-frequency sounds compared to adolescents who were not exposed (95% confidence interval 1.08 to 3.41). There was no difference in the proportion of exposed or non-exposed adolescents who had SNHL for high-frequency sounds. Higher levels of cotinine in the blood among adolescents exposed to second-hand smoke were associated with the prevalence of low-frequency SNHL.
The researchers found that 82% of adolescents with SNHL did not recognise that they had hearing difficulties.
How did the researchers interpret the results?
The researchers said, “second-hand smoke is associated with hearing loss in US adolescents. Furthermore, this risk to auditory function is directly related to serum (blood) cotinine level, a biomarker for tobacco exposure”.
This cross-sectional study showed an association between second-hand smoke exposure and hearing loss for low-frequency sounds in adolescents. However, there are considerations that must be made when interpreting these results.
- As the survey was cross-sectional, it cannot say whether exposure to second-hand smoke directly causes hearing loss or whether it is just associated with it. Determining whether smoke caused hearing loss in teenagers would require long-term follow-up of adolescents from before the time of hearing loss to see when and how often they were exposed to smoke. It is not possible to say whether these effects were associated with impaired development or damage to the hearing system.
- The study only included adolescents who were exposed to second-hand smoke, rather than adolescents who actively smoked. The study determined exposure by measuring blood levels of a marker of nicotine at one point in time, and it determined whether the adolescent was a smoker by asking them. It is likely that some adolescents may not admit to smoking. Also, they may sporadically smoke, meaning that although they may not have smoked in the five days preceding the test, they may have smoked at other times.
- The researchers highlighted that their survey did not ask about exposure to excessive noise, a known risk factor for hearing loss, and that adolescents may be exposed to recreational noise. In particular, exposure to loud noise could have been an important confounder for this study. For instance, if the teenager regularly attended clubs, pubs, etc. this is likely to have been associated with exposure to both loud music and second-hand smoke. Therefore it could be noise that is causing the observed hearing loss, rather than smoke.
The newspapers do make the point in their reports that some teenagers may have hearing loss without being directly aware of it, but that this may affect their behaviour or ability in the classroom. However, though this study found that a high proportion of adolescents with hearing loss were unaware they had it, and discussed potential implications of hearing loss on behaviour, it did not directly assess whether their hearing loss affected their functioning or behaviour.
In conclusion, though this study suggests an association between exposure to second-hand smoke and the risk of losing hearing of low-frequency sounds, further studies would be needed to confirm this finding and whether this is a causal effect or not. Smoking and second-hand smoke are associated with numerous health risks, and avoiding exposure to smoke as much as possible is advised.