Smoking damages blood flow to the ear
Smoking and obesity could both cause permanent hearing damage, say scientists.
Either could threaten blood flow to the ear, they say, with damage levels clearly linked to the level of obesity or the length of a smoking habit.
However, the Antwerp University-led study found that high levels of work noise remained the biggest risk.
In a separate study, smoking in middle age was linked to worse memory, which could hasten the arrival of dementia.
A link between smoking and hearing problems has been suggested by others, but the conclusions of the latest research, involving more than 4,000 men and women aged between 53 and 67, offer the most convincing evidence to date.
All the study participants were given a hearing test, then asked about their lifestyle and where they worked.
Dr Erik Fransen, of the University of Antwerp in Belgium, one of the lead researchers, said that the ability to pick out high frequency sounds was damaged in smokers and the obese, although to not as great an extent as those exposed to very loud noise in the workplace.
He said: "The hearing loss is proportional to how much you smoke and your body mass index (BMI).
"It starts getting worse once you have smoked regularly for more than one year."
He said that, unlike some parts of the body, once damage had occurred, there was no prospect of recovery.
"Once the damage is done, it's done. It does not repair."
The theory behind the hearing damage is similar to the reason smoking and obesity can harm other organs.
Both can disrupt the flow of blood around the body, and Dr Fransen suggested that the resulting lack of oxygen, coupled with the failure to remove toxic waste from the ear, can be damaging.
Amanda Sandford, from the pressure group Action on Smoking and Health (ASH), said that the results, published in the Journal of the Association for Research into Otolaryngology, should serve as a warning particularly to younger smokers.
She said: "There are so many young people who think that they can give up in middle age and escape some of the other diseases associated with smoking.
"In this case, some of the damage may already have been done."
The study was part-funded by the Royal National Institute for the Deaf (RNID), and Dr Mark Downs, from the charity, said that, with an ageing population, age-related hearing loss could be a major problem.
"Losing your hearing in later life can make it harder to maintain contact with friends and families and lead to isolation and depression, so making small concessions now could have an enormous effect in the long term."
A separate research project involved more than 5,000 civil servants, who completed memory and reasoning tests and then repeated them five years later.
It found that smoking in middle age was linked to a decline in memory and verbal reasoning ability.
Memory problems at this age have been linked to a swifter onset of the symptoms of dementia.
This study was conducted jointly between the University of Paris and University College London.