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People who are exposed to the second-hand smoke from others' cigarettes are at increased risk of hearing loss, experts believe.

Doctors already know that people who smoke can damage their hearing.

The latest study in the journal Tobacco Control, involving more than 3,000 US adults, suggests the same is true of passive smoking.

Experts believe tobacco smoke may disrupt blood flow in the small vessels of the ear.

This could starve the organ of oxygen and lead to a build up of toxic waste, causing damage.

The harm is different to that caused by noise exposure or simple ageing.

In the study, the researchers from the University of Miami and Florida International University looked at the hearing test results of 3,307 non-smoking volunteers - some who were ex-smokers and some who had never smoked in their lifetime.

Listening problems

The tests measured range of hearing over low, mid and high noise frequencies.

To assess passive smoke exposure, the volunteers had their blood checked for a byproduct of nicotine, called cotinine, which is made when the body comes into contact with tobacco smoke.

This revealed that people exposed to second-hand smoke were far more likely to have poorer hearing than others, and to a degree where they might struggle to follow a conversation in the presence of background noise.

Passive smoking increased their risk of hearing loss across all sound frequencies by about a third.

Dr David Fabry, who led the research, said: "We really do not know exactly how much smoke you need to be exposed to in order to be at increased risk. But we do know that the threshold for damage is very low.

"Really, the safe level of exposure is no exposure."

Dr Ralph Holme, head of biomedical research at the RNID (Royal National Institute for Deaf People), said: "We already knew from our own research that regular active smoking is a significant risk factor leading to hearing loss and this new study is important as it highlights the increased risks posed by passive smoking too.

"Hearing loss can often be very frustrating and lead to social isolation, if not quickly addressed.

"Before you next light up a cigarette, consider how it could impact not only on your own long-term hearing but your friends' and relatives' too."