We have to accept the fact that the level at which noise will start to cause damage in a particular individual is not known since people respond differently to noise. Anything over normal environmental levels could cause a problem.

Authorities seem to agree that, as a good guide, a safe work environment should be a workplace where the noise exposure is less than approx 85 dB(A) averaged over an eight hour period, or a peak noise level of 140 dB(C). Where these values are exceeded, all practicable measures should be taken to reduce the noise level by engineering noise control. Failing this, ways should be explored to reduce the exposure time by half for every 3 dB the level is above the exposure standard.

The above chart shows the recommended maximum exposure times for a given SPL (sound pressure level). Be aware that your hearing protection, be it plugs or muffs, does not reduce by as much as you may think. What sounds like a dramatic reduction to you, may in fact just be a rolling off of the top end and not that much reduction in overall acoustic energy. Check the specs on your ear plugs or hearing muff and read what the sound reduction is reported to be. Be very wary of specs that are based on a single frequency. If no specs are available, assume the worst and seek out better hearing protection.