INTRODUCTION
Background
Nowadays, noise pollution plays a huge negative role in affecting our health generally, and hearing specifically. Our hearing is directly impacted by environmental noises, which can result in temporary or permanent damage. It is well established that loud noises (being 105 decibels (dB) or above) such as a gunshot or airplane takeoff, can result in hearing impairments such as a permanent increase in hearing threshold (1). These permanent threshold shifts (PTS) can be detected through an audiometry (1). The increased hearing threshold is due to the damage of hair cells present in the cochlea. Permanent increases in hearing threshold are also known as “noise-induced hearing loss” (NIHL). Although most NIHL cases are the result of exposure to loud sounds, it is now known that moderate sounds (70-105 dB) and potentially leisurely sounds (<70 dB) can also affect our hearing threshold levels (HTL) by inducing temporary threshold shifts (TTS). Recent physiological studies have shown that sounds at moderate levels, such as blenders, cars or even doorbells can also cause TTS and that the buildup of damage from these can result in NIHL (1,2). Hidden hearing lose (HHL) is considered to be a phenomenon that is resulted of long-term exposure to moderate and leisurely sound levels (3,4,5). Individuals with HHL can still test within the normal range for hearing and claim to experience hearing deficiencies within their life; hence, hearing loss is publicly accepted as a result of aging and due to the damage of the peripheral auditory and outer hair cells overtime. These auditory dysfunctions are normally experienced in public settings with background noise. This is due to the presence of competing noises which can affect the individual’s standard hearing capacity (6). Some researchers have even suggested that HHL may cause a future hearing pandemic which will detrimentally affect our quality-of-life. The current speculated cause is the increased exposure to leisure sounds due to technological advances (7).
2nd Paragraph
– Hidden Hearing loss explained through human based studies
o Discuss lack of outer hair cell damage
Indicates that a central (neuronal signalling) issue is occurring
 Peng et al. 2007 (retrospective study)
• Chinese university students exposed to personal stereo players
o statistically significant difference in pure tone audiometry (3-5 dB)
o How phoneme recognition is decreased
o Small cohort studies on leisure music and effects on human hearing perception
 Tam et al
• Australian young adults exposed to MP3 music, increase in 6 kHz HTL, reduction in otoacoustic emission amplitudes (post-exposure)
Further studies to include in paragraph 2

o Kujala et al. 2004 – Long-term exposure to noise in “clinicallry normal hearing” subjects had a persistent effect on central auditory processing and lead to concurrent beavioral deficits
 showed phoneme discrimination differences in noise exposed individuals (95-100 dB)
 Changes in frequency around 12.5 – 14 kHz
• Ours was more 13 – 15 I believe
 Kumar et al. in 2012 found that noise of 80 dB also resulted in a similar effect
• A follow up by Bharad waj et al. 2015 found that this was not the result of Outer hearing cell loss but rather a change in threshold hearing ability in the brain.
 Stamper and Johnson in 2015 also found that noise between 67 dB and 83 dB can also affect hearing but not through peripheral damage

3rd paragraph (Will most likely be broken into 2 paragraphs)
– Audiometry studies in animals
– Loud noises
o Generally done in short exposure spurts
o Plenty of papers to choose from here, perhaps use 2 primary with a review to show that there’s a lot of data available
o Helps understand the effect of Outer hair cell loss
– Moderate and Leisure noise levels
o Generally done in prolonged exposure situations
o This is what helps us understand what HHL is in humans
o Various papers available in review paper (Eggermont) given by Dr. Yan
o Can also include the effect of exposure (70 dB) for 18 days to infant rats (from 12- 30 days for 12 hours a day)
 Found that mice had an increased hearing threshold in the 4-16 kHz range of around with an increase of about 10 – 15 dB SPL (at 11.3-16 kHz).
 Since ours effect period is shorter (and a lower loudness) we can expect lower but similar results (which is what we saw)
 This paper can be used for our discussion too.
o Helps us understand the retrospective studies but not cohort group studies
o Eggermont’s quote on how we need more research in this field
Definitions of words that should be used based on that meaning:
– Moderate noise – Noise which is (70 dB – 105 dB)
– Loud noise – Noise which is (105 dB or greater)
– Leisure noise – (Noise below 70 dB)
– HHL – Only found in humans, when an audiometry (or any clinical test for hearing loss) reports normal hearing yet the individual still claims to experience hearing issues

References:
1. Eggermont J. Effects of long-term non-traumatic noise exposure on the adult central auditory system. Hearing problems without hearing loss. Hearing Research. 2017;352:12-22.
This article was used as a reference point to gain a basic understanding of the current state of research regarding non-traumatic noise exposure.
2. Gourévitch B, Edeline J, Occelli F, Eggermont J. Is the din really harmless? Long-term effects of non-traumatic noise on the adult auditory system. Nature Reviews Neuroscience. 2014;15(7):483-491.
This article was used to understand what types of noise can induce temporary threshold shifts which can eventually lead to permanent threshold shifts meaning permanent hearing damage.
3. Cheng L, Wang S, Peng K, Liao X. Long-Term Impairment of Sound Processing in the Auditory Midbrain by Daily Short-Term Exposure to Moderate Noise. Neural Plasticity. 2017;2017:1-10.
The article by Cheng and colleagues was used to understand the effects HHL would have on individuals in day-to-day life. It was also used to obtain the clinical definition of what HHL is.
4. Brattico E, Kujala T, Tervaniemi M, Alku P, Ambrosi L, Montillo V. Long-term exposure to occupational noise alters the cortical organization of sound processing. Clin Neurophysiol. 2005;116:190-203.
This study was used to show how there has been a recent growing interest in occupational noise and how it affects the auditory cortex. It was used to specifically understand the issues caused by HHL with one of them being phoneme indiscrimination.
5. Kujala T, Shytrov Y, Winkler I, Saher M, Tervaniemi M, Sallinen M, et al. Long-term exposure to noise impairs cortical sound processing and attention control. Psychophysiol. 2004;41:875-881.
This study was used in a similar manner as the article published by Brattico and his colleagues in where phoneme discrimination was found to be the result of cortical damage.
6. Bharadwaj HM, Masud S, Mehraei G, Verhulst S, Shinn-Cunningham BG. Individual differences reveal correlates of hidden hearing deficits. J Neurosci. 2015;35:2161-2172.
This study was used to explain why individuals with HHL were experiencing issues in daily life and the focused reason was due to environmental noises which would inhibit the attention towards other sounds.
7. Carter L, Williams W, Black D, Bundy A. The Leisure-Noise Dilemma. Ear and Hearing. 2014;35(5):491-505.
The study by Carter and colleagues was used in order to understand the current opinions of individuals regarding the state of technology and the effects it can have on our future. One major concern brought up in this study was that music and sound all together is now easier to find relative to any other point in time which is why some researchers are worried about the future state of hearing around the globe.