Threshold of Pain   

The level of sound at which the listener begins to feel physical pain. 

This occurs between approximately 115 and 140 dB.

 At these listening levels a significant risk of long-term hearing damage exists and extreme caution should be used such as 

using ear protection devices,

 limiting the time of exposure to such high levels of sound,

 or lowering the sound level. 

Before the threshold of pain is reached the listener usually experiences the threshold of discomfort which should act as an early warning that listening levels are too high. You should not depend on this alone, however, due to the fact that most listeners will become quickly habituated to such high sound levels and the threshold of discomfort will rise to dangerous levels before pain occurs. 
See Also:
monitoring levels.