You can add an increased risk of hearing loss to the long list of reasons why smoking is bad for you, according to a new study that tracked the health of 50,195 people in Japan for a period of up to 8 years.
After allowing for other health conditions, smokers were found to be up to 70 percent more likely to develop hearing loss compared with those who had never smoked.
Given the large sample size and the length that the study covers, as well as the included adjustments for separate issues like high BMI and diabetes, it's the most comprehensive link yet between smoking and hearing loss, say the researchers from Japan.
"The more one smokes, the higher the risk of hearing loss," one of the researchers, Huanhuan Hu from the National Centre for Global Health and Medicine in Japan, told Lisa Rapaport at Reuters.
"Quitting smoking virtually eliminates the excess risk of hearing loss, even among quitters with short duration of cessation. Because the risk of hearing loss increases with the number of cigarettes smoked per day, if quitting is impossible people should still smoke as little as possible."
Data for the study was pulled from the annual health checks that regularly take place in Japan, and looked at both high-frequency hearing (someone talking to you) and low-frequency hearing (the rumble of a car).
Those who smoked up to 10 cigarettes a day were found to be 40 percent more likely to develop high-frequency hearing loss and 10 percent more likely to develop low-frequency hearing loss, the figures showed.
For 11-20 cigarettes a day, those numbers went up to 60 percent and 20 percent respectively. For more than 20 cigarettes a day, the risks went up even further, to 70 percent for high-frequency hearing and 40 percent for low-frequency hearing.
According to the researchers, those statistics "suggest that smoking may be a causal factor for hearing loss" without definitively proving the link – further studies are needed for that.
Such an association has been proposed before, but the overall evidence has been inconclusive. Earlier studies have used smaller sample sizes or relied on self-reporting to determine hearing loss, which isn't as reliable as a clinical examination.
That said, this study has limitations of its own – in this case the subjects were self-reporting their smoking habits, rather than having them independently monitored, and data for noise exposure at work wasn't available for all the participants.
But even after taking those caveats into account, the connection turned up by the results is still a strong one, especially as hearing loss risk increased in tandem with the number of cigarettes smoked each day.
What we still don't know for sure is why this might be happening. One possible reason is that toxins in cigarette smoke could damage the inner ear; another idea is that the way nicotine and carbon dioxide reduce oxygen in the blood impairs the function of our ears.
Whatever the biological processes going on, data from more than 50,000 people is a compelling case study to add to the evidence that the more you smoke, the greater your risk of hearing loss.
And if you need another little push to give up the cigarettes, the research also found that the risk of hearing loss associated with smoking started decreasing again within five years of quitting.
"With a large sample size, long follow-up period, and objective assessment of hearing loss, our study provides strong evidence that smoking is an independent risk factor of hearing loss," says Hu.
The research has been published in Nicotine & Tobacco Research.