Passive smoking nearly doubles a teenager's risk of hearing loss, research reveals.
Investigators say the findings, from a study of over 1,500 US teens aged 12 to 19, suggest that secondhand tobacco smoke directly damages young ears.
And the greater the exposure the greater the damage.
Often it was enough to impair a teen's ability to understand speech, Archives of Otolaryngology - Head & Neck Surgery reports.
It is still unclear how much exposure could be harmful and when the damage might occur.
Experts already know that smoke increases the risk of middle ear infections.
And they believe it may also harm the delicate blood supply to the ear causing "subtle yet serious" changes.
For these reasons, as well as other smoke-related health risks, they say the best advice is to avoid any exposure to tobacco smoke as far as is feasible.
Dr Ralph Holme of the charity Action on Hearing Loss, formerly RNID, said: "We already knew from our own research that regular active smoking is a significant risk factor leading to hearing loss.
"This research strongly suggests that children exposed to tobacco smoke are at increased risk of hearing loss.
"Further research is needed to demonstrate a causal link, but in the meantime to protect your child's hearing, and health, it would be advisable to avoid smoking around them."
Lead researcher Professor Anil Lalwani, from the New York University School of Medicine, said: "We need to evaluate how we deal with smoking in public places and at home, as well as how often and when we screen children for hearing loss."
In the study, around 40% of the 800 teens who had been exposed to secondhand smoke had detectable hearing problems, compared to about 25% of the 750 teens who had not had this exposure.
Yet very few - less than a fifth - of the affected teenagers were aware that they had a problem with their hearing. This is because mild hearing loss is not necessarily noticeable to the individual.
But hearing tests revealed that they struggled with high and low frequency sounds.
Co-researcher Dr Michael Weitzman said: "It's the type of hearing loss that usually tends to occur as one gets older, or among children born with congenital deafness."
He said this could make it difficult for children at school because they might find it hard to follow lessons and be wrongly labelled as "troublemakers".