Sound is measured in units called decibels (dB). While there are laws guiding the amount of noise a person can be exposed to in the workplace, noise is considered too loud if the sound is over 85 dB. To put noise in context the wind in the trees can be 20 dB; a person when talking can be 55 dB; a chainsaw can be measured at 95 dB; and a single handgun blast can be as much as 160 dB.
As a rule of thumb, if you have to raise your voice over the noise, you should be wearing hearing protection. No person should be exposed to sounds over 120 dB for even very short periods of time without hearing protection. Levels over 120 dB can cause permanent damage even after one brief exposure.