The number of teenagers with permanently damaged hearing as a result of listening to loud music is reaching "alarming" levels, warns new research.
Detailed hearing tests on 170 students between the ages of 11 and 17 showed that almost all of them engage in "risky listening habits" - at parties, clubs and on personal listening devices.
And more than a quarter of them were already experiencing chronic, persistent tinnitus - a ringing or buzzing in the ears that more typically affects people over 50.
Study co-author Doctor Larry Roberts, of McMaster University in Canada, said: "It's a growing problem and I think it's going to get worse.
"My personal view is that there is a major public health challenge coming down the road in terms of difficulties with hearing."
Further testing of the same subjects - all students at the same school in São Paulo, Brazil - showed that even though they could still hear as well as their peers, those experiencing tinnitus were more likely to have a significantly reduced tolerance for loud noise.
Lack of tolerance to loud noise is considered a sign of hidden permanent damage to the nerves that are used in processing sound, damage that can foretell serious hearing impairment later in life.
Dr Roberts said that when the auditory nerves are damaged, brain cells increase their sensitivity to their remaining inputs, which can make ordinary sounds seem louder.
Increased loudness perception is an indication of nerve injury that cannot be detected by the audiogram, the standard clinical test for hearing ability.
Research indicates that such "hidden hearing loss" caused by exposure to loud sounds in early life deepens over the years, worsening a person's hearing ability later in life.
Dr Roberts said: "The levels of sound exposure that are quite commonplace in our environment, particularly among youth, appear to be sufficient to produce hidden cochlear injuries.
"The message is, 'Protect your ears.'"
Dr Roberts and colleagues at the University of São Paulo School of Medicine in Brazil were able to gather detailed figures, which he said created a more complete picture of what's happening to young people who may not be aware that they are hurting themselves when they listen to loud music.
He said it's common after listening to loud music to experience a ringing in the ears for the next day or so. More than half the students in the study said it had happened to them.
But Dr Roberts said brief tinnitus is an early warning sign. Testing showed that 28 per cent of the participants had already developed persistent tinnitus.
Dr Roberts said the 28 per cent with persistent tinnitus also showed heightened sensitivity to loud sounds, indicating that the neurons that transmit sounds to the brain may have been damaged.
But while some other forms of hearing loss can be repaired, such nerve damage cannot be undone.
Dr Roberts, a veteran researcher, said the only solution is prevention.
And he compared the evolving campaign against loud music to the early years of the campaign against smoking, in the sense that many people have no idea that they are hurting themselves.
The findings were published in the journal Scientific Reports.