WE ALL know that too-loud sounds – a jackhammer drilling, an airplane taking off, a rock concert – can make one lose their hearing.

These days, with every other person sporting mobile phone or iPod earphones in their ears, it's a danger that is very real.

But how really does an odourless, colourless, shapeless entity like sound affect the ear?

In the inner ear, there are three layers of microscopic “hearing” cells or outer hair cells. When sound waves hit them, these cells can convert the sound energy into electrical pulses that are transmitted by the nerves to the brain. The brain then interprets the sound.

However, every person is only born with 15,000 of these cells, a finite number, said Singapore General Hospital’s Associate Professor Low Wong-Kein.

The director of the hospital’s Centre for Hearing and Ear Implants said the body cannot re-grow these cells when they are damaged, unlike skin cells, where new skin can grow to replace the sloughed-off old ones, for example.

That means that if one spoils too many of these cells such that they cannot work to transmit the electrical pulses, the amount of hearing that can be perceived by the brain will drop drastically.

Prof Low recently did some experiments in the United States where he exposed guinea pigs to loud sounds above the human acceptable limit. He found that initially healthy cells would become “disorganised”, burst and become scar tissue. And that was after just 10 hours of exposure to noise at 120 decibels – equivalent to front-row seats at a rock concert. 

At 120 decibels, the amount of exposure considered safe by the US National Institute for Occupational Safety and Health is less than 10 seconds. The safe level for workers exposed to loud sounds is usually 85 decibels for eight hours of exposure.

However, many MP3 players today are able to deliver more than 10 hours of playback time. 

While music players sold in Europe are governed by the continent’s 100 decibel limit, gadgets in other countries, including Singapore, may go beyond that.

Mr Tan Boon Hai, a senior audiologist from Changi General Hospital, said it wasn’t the patients that had already come to the clinic to seek help he was worried about.

“It’s the ones who are out there and still exposed that are unaware of the damage to their ears,” he said.

Remember that the next time you reach out for the volume wheel on your iPod!

Read the full story in The Straits Times' today.