Employees exposed to excessive in-plant industrial noise may be at risk to suffer a variety of physiological and psychological consequences. Other intangible effects have also been hypothesized to be caused by stress associated with noise exposure. Prevention of hearing loss is the focus of hearing conservation programs. It is the only physiological effect that has an undisputed, well documented association to noise exposure in humans. The table below summarizes some of the possible effects that are linked to noise exposure. Total employee exposure includes recreational sources.

Effects Linked To Noise Exposure

Noise Induced Hearing Loss

The basic mechanism of hearing involves converting sound waves hitting the ear drum to structure-borne vibrations transmitting through bones in the middle ear. From there, vibrations are changed into nerve impulses in the cochlea of the inner ear. The fluid filled cochlea contains 40,000 tiny hair cells like the one shown at right (magnified) that initiate the nerve impulse which is transmitted to the brain. With repeated exposure to excessive noise, these hair cells lose some of their resilience and may even break off resulting in sensorineural or noise induced hearing loss. Hearing loss is permanent because once damaged, the hair cells can never be repaired or replaced.

The Federal Noise Standard For Employee Exposure As Developed By OSHA
(Occupational Safety & Health Administration)

Key Points

• 8 hours at 90 dBA equals the permissible exposure level (100% dose)
• 8 hours at 85 dBA equals 50% of the permissible exposure level (PEL)
• The OSHA Standard was formulated to minimize not eliminate the risk of hearing loss
• Dosimeters, not sound level meters, are used to establish employee exposure/ dose
• Feasible engineering controls MUST be implemented when the equivalent dose for 8 hours exceeds 90 dBA or where continuous noise is over 115 dBA

OSHA Derating Instructions For All Hearing Protectors Using NIOSH Method #2

A comprehensive hearing conservation program guideline was added to the original OSHA standard. The hearing conservation amendment outlines requirements for annual audiometric testing, training and documentation/ record keeping.

OSHA Compliance Strategy Summary

Caution!
Hearing Protective Devices' (HPD) Noise Reduction Ratings (NRR) May Over Estimate Performance

A recent study by Dennis A. Giardino and George Durkt, Jr. of the Mine Safety and Health Administration published in the American Industrial Hygiene Association Journal compared field performance to the EPA noise reduction rating (NRR). The results from some 1,265 hearing protective device (HPD) evaluations showed that field performance is significantly less than that specified by the NRR, especially for low frequency noise. Results also showed that the NRR is not a good indicator for comparing relative performance of HPD models.

Conducting An Industrial Noise Control Survey

Listed below are several of many possible questions that need to be researched as part of the noise control survey to determine feasible and practical treatments/solutions. Meeting the acoustical requirements alone is not sufficient to solve the problem in most cases. Identifying all the needs of the noise control treatment can best be accomplished by soliciting input from all disciplines that might be affected. Forming a noise control team with representatives from maintenance, operations, quality control, production, safety, engineering, management, etc. will assure the best possible design to meet the needs of all involved. Preliminary input from the parties involved leads to "ownership" and "buy-in" creating a sense of accomplishment for the team.