A sound needs to be over 80 dB before it can potentially cause permanent hearing damage. However, it is not as simple as just the loudness of the sound. The duration of exposure and how the sound enters the ears are equally important.
The louder the sound, the less time it takes before damage can occur. For example, it is safe to listen to sounds under 80dB indefinitely. Sounds at 85dB can start to cause damage after 8 hours of exposure. This time drops by half to 4 hours just by increasing the sound level to 90dB. Increasing the sound level to 95dB reduces this safe listening time to just 2 hours. The average preferred volume of portable media players is about 94dB. Sounds over 100dB may cause hearing damage within 15 minutes of exposure and sounds louder than 130dB damage hearing instantly.
As well as the sound level and the duration of exposure, how the sound enters the ear also plays a part in how likely the sound will cause hearing damage. Supra-aural headphones (the larger headphones that completely cover the entire ear) are safest. Tests show in-ear earphones are more than three times more likely to cause hearing damage than supra-aural earphones.