The effects of loud environmental noise and loud music playback on the ear are similar. According to various studies, permanent hearing loss may occur for prolonged exposures over 85 decibels (85 dB).

Decibel level reference: 

However, it is not only the loudness (measured in dB) that matters, but also the duration – the longer the exposure to the loudness, the greater is the damage:

*Maximum hearing period is only a reference. Seek medical advice if you have any doubt.

A notice ‘Long hour and high volume damage hearing’ will be displayed when the volume of this MP3 player attains 80dB-85dB (you can take this Decibel level as reference of a medium truck passing by). This notice is to comply with a regulation in the European Union for protecting your hearing safety (EN 60950-1). It is not suggested to adjust beyond this volume level because long exposure to this volume level may damage your hearing. If you really need to increase the volume level beyond this level (80-85dB), shorten the hearing period to protect your hearing.

Reference and extended reading:

• US government (Texas) – Noise and Hearing Protection Fact Sheet 
  http://www.tdi.texas.gov/pubs/videoresource/fsnoise.pdf
• EUROPA-DG Health and Consumer Protection
  http://ec.europa.eu/health/opinions/en/hearing-loss-personal-music-player-mp3/l-3/3-hearing-protection-limits.htm
• Canadian Academy of Audiology
  http://www.canadianaudiology.ca/consumer/how-loud-is-too-loud.html