At what level does sound become unsafe ?

It is best, where possible, to avoid any unprotected exposure
to sound pressure levels above 100dB(A). Use hearing protection when
exposed to levels above 85dB(A), especially if prolonged exposure is
expected. Damage to hearing from loud noise is cumulative and is
irreversible. Exposure to high noise levels is also one of the main
causes of tinnitus. The safety aspects of ultrasound scans are the
subject of ongoing investigation.

There are other health hazards from extended exposure to vibration. An
example is “white finger”, which is found amongst workers who use hand-
held machinery such as chain saws.