Noise exposure is the main cause of preventable hearing loss worldwide. It now accounts for more than a third of all cases of hearing loss in developed countries – and city dwellers are most at risk. A study published recently in The Lancet revealed that living in a noisy city increases your risk of hearing damage by 64%.
Noise is measured in decibels (dB). Zero decibels is almost complete silence. It is the quietest thing someone with healthy hearing can hear above absolute silence (which is -9 dB). A typical conversation is around 60dB, and anything you need to raise your voice to be heard over is probably above 87dB. Prolonged exposure to anything above 85dB, without adequate ear protection, is assumed to be potentially damaging.
People can be exposed to noise at work, such as a construction site (up to 96dB), or socially, such as a music festival or nightclub (up to 110dB). But you might be exposed to loud noises so constantly throughout the day that you don't even realise they are there, perhaps from road works (75–105dB) or a noisy pub (around 90dB).
Many countries have health and safety regulations to prevent people from damaging their hearing at work. For example, in the UK, anyone exposed to noise levels of 85dB or more in the workplace must be given hearing protection and be monitored for damage to their hearing. But mp3 players and mobile phones allow us to exceed that noise level with little more than a disclaimer from the manufacturers.
A typical personal audio device will produce around 100dB, and specialist headphones could take this even higher. If someone were employed to work in a noise level of 100dB, they would not be allowed more than five minutes' exposure per shift. After about 15 minutes, they would risk serious damage to their hearing.
Hidden hearing loss
Until recently, it was believed that noise-induced hearing loss resulted from damage to the sound-sensing cells in the cochlea, the part of the inner ear that converts vibrations into electrical impulses. But animal studies show that even relatively moderate amounts of noise exposure can cause damage to the auditory nerve – the nerve connecting the inner ear to the brain.
People with suspected hearing loss are usually referred to an audiologist. The audiologist measures hearing by finding the quietest sound a person can hear in a quiet environment. In a soundproof room, the patient wears headphones and the audiologist plays beeps at a number of different frequencies to determine the threshold at which the person can hear "in quiet".