Although we often take it for granted, hearing is one of our most important senses. During infancy and childhood, hearing is vital for the normal development of speech and language. Hearing remains equally important throughout life as it enables us to communicate with friends and colleagues, and allows us to stay connected to and be aware of our environment.

The ear is a remarkable structure, capable of detecting sounds with a wide range of frequencies and intensities. Illness, disease and heredity can affect our ability to hear, however, as can aging and exposure to loud noise.

The fact that hearing can be damaged by noise is the reason OSHA requires us to monitor workplace noise levels, establish hearing conservation programs and perform annual hearing testing on our employees. These requirements are detailed in the OSHA noise regulation, 29 CFR 1910.95.

According to the OSHA noise regulation, overall hearing thresholds must be determined for each ear at, or shortly after the time of hire, and annually thereafter. Results of each annual test are compared to the employee’s baseline test to see if a Standard Threshold Shift (STS) has occurred. The regulation defines an STS as “a change in hearing threshold relative to the baseline audiogram of an average of 10 dB or more at 2000, 3000, and 4000 Hz in either ear.”

Chances are that one or two of your employees will show an STS following their annual hearing tests. Since OSHA considers most STS’s to be work-related and recordable on the OSHA 300 Log, unless determined otherwise by an audiologist or a physician, you may, if you wish, simply record these STS’s on the OSHA Log. However, if you wish to minimize your number of OSHA Log entries, a little background regarding the auditory mechanism, the testing process and the purpose of the hearing conservation program may be helpful.

Conductive Vs. Sensorineural Hearing Loss
As a sound wave travels from its source, it enters the listener’s outer ear, passes through the middle ear and then enters the inner ear before traveling along the auditory nerve to the brain. Blockage or damage located anywhere along this auditory pathway can cause some degree of hearing loss. Problems located in the outer or middle ear may result in conductive hearing loss while damage to the inner ear or the auditory nerve results in sensorineural loss.

Conductive hearing loss, caused by problems in the outer ear such as impacted cerumen (ear wax), or in the middle ear, such as fluid accumulation or otosclerosis, can typically be treated successfully either medically or surgically. Sensorineural hearing loss, on the other hand, is usually a result of damage to the tiny sensory hair cells of the inner ear and cannot be treated, although hearing aids may often be of help. Two common causes of sensorineural hearing loss are aging and noise exposure.

A diagnostic evaluation by an audiologist will include pure tone testing by both air conduction and bone conduction. This will determine not only an individual’s overall hearing level, but will also reveal whether the hearing loss is conductive, sensorineural, or mixed (that is having both conductive and sensorineural components). Treatment options are then based upon these findings.

Hearing conservationists, however, are limited by the OSHA noise regulation to pure tone air conduction testing only. This testing assesses the integrity of the entire auditory system, but cannot tell us whether a hearing loss is conductive, sensorineural or mixed. Clearly, OSHA mandated hearing tests are not meant to be diagnostic, but are intended to serve as safety measures, alerting us to the possibility that employee hearing is being affected by exposure to workplace noise.

Determining Recordability and Work Relatedness
An STS, therefore, tells us that an employee’s hearing has worsened compared to baseline test results, but does not tell us why it has worsened. Since OSHA considers most STS’s to be work-related unless determined otherwise by an appropriate professional, you may simply record all the STS’s on the OSHA 300 Log. Did you know, however, that you can have an audiologist or a physician make a determination regarding the work-relatedness of your STS’s? You may, of course, utilize the services of local audiologists or physicians, but an excellent first choice would be the audiologist or medical doctor who provides professional oversight of your hearing conservation program.

OSHA considers most STS’s to be work-related unless determined otherwise by an appropriate professional.

The determination process is straightforward. You will most likely be asked to submit a questionnaire indicating the employee’s 8-hour TWA (time-weighted average) noise exposure and the NRR (noise reduction rating) of his or her hearing protection. Earplug or earmuff NRR’s are determined by the manufacturer and can be found on the product packaging material. Time weighted average noise exposure is best measured by personal dosimetry, although a sound level meter can be used. If the employee works an extended shift, it will be important to provide values normalized to an 8-hour shift as OSHA requires that this information be furnished to the audiogram reviewer. You will also be asked to provide information about non-work noise exposure and any health conditions that may possibly be affecting hearing.

The reviewing professional will check to see if the hearing protection worn by the employee is adequate for the reported noise exposure. The employee’s audiometric history will also be examined in conjunction with information you have provided in the questionnaire.

Certain audiometric configurations may suggest conductive hearing loss and, especially in combination with information suggesting a current cold or a history of outer or middle ear problems, may be determined “not work-related.” Most cases, however, are not so straightforward. More often, it is the additional information provided in the questionnaire regarding health conditions or non-work noise exposure that is critical.

Many audiometric configurations will show high frequency hearing losses, which suggest the sensorineural losses caused by both noise and aging. Of help in distinguishing these two common causes of sensorineural loss is the employee’s pure tone threshold at 8000 Hz. OSHA does not require testing at this frequency, but it is strongly recommended that you include it in your testing program. While both noise and age result in high frequency hearing losses, the pure tone configurations typically differ in that noise induced hearing loss will often show an improvement at 8000 Hz while age-related hearing loss will not.

If the pure tone configuration suggests noise-induced hearing loss, is that loss due to workplace noise or recreational noise, or a combination of both? What about power tools or firearm use? What about family history of hearing loss? Is there a history of diabetes? Without additional information, the reviewer may be unable to make a determination as to whether or not the STS is work-related.

Remember that OSHA considers most STS’s to be work-related. OSHA does give you the option, however, of having an audiologist or a physician review each of your STS’s for work-relatedness. While such review does not guarantee that a given STS will be determined “not work-related,” it will almost certainly reduce the overall number of cases you need to record on the OSHA 300 Log.