Noise-induced hearing loss is permanent. Damage to the hearing system can be caused by continuous exposure to loud sounds over an extended period of time or by a one-time exposure to an intense “impulse” sound, such as an explosion. It is important to understand the noises that can be harmful to you, and what you can do to protect your hearing when you encounter these noises.
Sounds that you hear on a daily basis vary in decibels. When someone whispers you are generally hearing sound that is about 30 decibels. When you are listening to a normal conversation you are generally hearing sound that is approximately 60 decibels. An ambulance siren however, is much louder and is typically 120 decibels. When you hear sound above 85 decibels you should take steps to protect your hearing as this level of noise can cause damage. Prolonged exposure to sound above 85 decibels can lead to noise-induced hearing loss.
(source: Listen Up!)
Noise induced hearing loss is almost always preventable. If you find yourself around loud noise, take action to protect yourself and your hearing.