Introduction

Hearing is the foundation for communication, which is the essence of humanity. It is how people express themselves, how they can show emotion and how they are able to relate to one another. From a very early age, it is how we learn to communicate, and it is the building blocks of our foundational knowledge and emotional understanding. When this sense is dulled in any way it can decrease both physical and social health; decreased performance in school and reduced quality of life can be observed. Noise is one of the many factors that can result in hearing loss, but other environmental exposures and genetic predispositions in combination with noise may also affect this crucial sensory function. Previous studies have shown that noise exposure, including naturally occurring low levels, could cause permanent damage to the auditory system. The majority of the time, this low-level exposure is overlooked in the world of research. Though the few studies associated with low level noise exposure suggest that it could increase work injury, blood pressure, hypertension, and decreased health in general (Hoffman, 2018; Estill, 2015).

One of the most common causes of disability is hearing loss and the sequalae of this debilitating handicap. It is one of the most common health issues across the world, and the numbers are only predicted to increase in the coming years (The Global Burden of Disease, 2004). A significant portion of the hearing loss is due to occupational auditory stimuli that are harmful to the sensory system (Nelson, et al. 2005). This type of permanent hearing loss is referred to as “noise- induced hearing loss.” Unlike many other severely disabling conditions, such as vision problems or mental health disorders, this specific type of hearing loss is easily treatable and preventable. There are laws that are put into place to prevent this occupationally acquired hearing loss. Although these laws were put into place to protect individuals in the work-force, such laws have hidden risks.

The Occupational Safety and Health Administration (OSHA) has put in standards to ensure the safety and health of all the employees that are working for companies with over 10 staff members. These safety regulations include anything from grain safety in the agriculture sector to hearing protection at construction sites. OSHA has stated that anyone that is exposed to 90 dB TWA (time-weighted average) for 8 hour or more hours, the employer must implement a hearing conservation program. A hearing conservation program can be a simple program that provides hearing protection devices to the employees that are potentially being exposed to these harmful measures. The only regulations that OSHA provides for these hearing protectors are they must be “comfortable to wear and offer sufficient attenuation to prevent hearing loss” (United States Department of Labor). In terms of the policy that is in question, there are several issues with this broad statement that OSHA provides in this prevention policy. Unfortunately, many companies will not provide sufficient protection devices; they can be ill-fitting to the population, and many are not feasible in the work environment. The average noise at a construction site is 102.9 dB, which is well over OSHA standards for noise exposure (Birkner, 2012).  The hearing protection devices that many companies provide will only supply 7 dB of attenuation, making the noise exposure continuously well above the acceptable limit. Another glaring issue with this restriction is attenuation is only feasible if the hearing protection device is properly inserted, fit correctly, and worn at all times during the noise exposure. Studies have shown that although subjects were provided with hearing protection devices, employees in hazardous noise environments only wear the provided prevention 30% of the time (Freuler, 2014). Therefore, OSHA is lacking in staff to implement and carry out the regulations that they have put into place. OSHA needs to carry out these laws and apply them because we know employees will not wear  protection if they are simply provided with no prior education about the noises that they surround themselves with and are potentially harmful. There are obvious flaws in this method for protecting the hard-working men and women of the United States, and that is why the regulations should be tested.

The type of noise-induced hearing loss that many clinicians are presented with are a result of many years of noise exposure, so thought. However, many previous studies have not looked at 24-hour noise exposure, rather 2 hours or 8 hours. Under OSHA restrictions, an 89 dB TWA noise level has no time limit and it is acceptable for employees exposed to this level to not wear any hearing protection devices (United States Department of Labor). This type of long duration noise exposure is quite possibly the case when many employees are working 12, even 48-hour shifts. This leaves the question, can subjects suffer from a permanent hearing loss while still following safety protocol implemented by OSHA?

A large portion of the research that is conducted involving the auditory system uses animal models that can represent the human ear. Numerous animals have varying frequency ranges compared to that of a human, making them less than ideal for an animal model depending on what the researcher is looking for. However, the chinchilla shows a frequency audibility range similar to the range that is observed in humans. There are several other advantages for using chinchillas in auditory research; they can tolerate anesthesia, the cochlea and ear are effortlessly available, electrophysiology measures are attainable and accurate, and this species is less susceptible to ear infections compared to other available animal models (i.e., rat, mouse, guinea pig). All of these factors were taken into consideration when addressing what animal would be best for this research study. Chinchillas were primarily chosen due to the tolerance to anesthesia and the valid electrophysiology measures that can be attained with auditory stimuli.

The electrophysiology measures that are used with animal models are called auditory brainstem responses (ABR). The ABR’s can detect neural transmission in the auditory processing system given a certain acoustic stimulus, indicating that the subject would perceive the stimulus. This measurement can detect reliable auditory thresholds of these animals. These measurements are seen as waves through a software, these waves are symbolic of action potentials. The amplitude of these recorded action potentials indicates the amount of acoustic information it takes for the animal to be aware of it.  With anesthesia and these acoustically evoked potentials the thresholds of the chinchillas can be observed before noise exposure, immediately post noise exposure, and up to four-week post noise exposure to observe permanent threshold shifts and temporary threshold shifts.

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