Noise.  Hearing levels in people declines when exposed to certain levels of noise over a period of time.  The extent of hearing decline depends on the level of noise and duration of exposure.  OSHA says 85 decibels (dBA) or higher over an 8-hour work shift is excessive.  If you have a smartphone you can download a hearing meter app and determine, pretty closely, dBA levels.

When addressing a workplace hazard using Hierarchy of controls is required: This means first look to engineer out the hazard, next use administrative means to reduce/eliminate the hazard and finally, use personal protective equipment (PPE).  PPE (hearing protection) is the most common means of controlling noise hazards. Earmuffs or earplugs are the PPE means of controlling hazardous noise levels.

Once hearing is lost there is no means medically to get it back.  Hearing loss cannot be fixed.  So protect employees from excessive noise with the use of hearing protection.  Preserving hearing is simple.