الفهرس 
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Abstract
Occupational noise-induced hearing loss (NIHL) is defined as bilateral sensorineural hearing loss that develops slowly over a period of several years as the result of exposure to continuous or intermittent loud noise in the workplace. Tinnitus and NIHL have been commonly reported in personnel who are routinely exposed to occupational noise.
Estimates suggest that large numbers (approximating between 5 and 30 million) Americans are exposed to hazardous noise levels in the workplace. Based on exposure levels, about one out of every four workers will develop permanent hearing loss.
Occupational noise-induced hearing loss can significantly influence worker communication and safety and can have a tremendous impact on the lives of workers.
Occupational NIHL, as opposed to occupational acoustic trauma, is hearing loss that is a function of continuous or intermittent noise exposure and duration, and which usually develops slowly over several years. This is in contrast to occupational acoustic trauma, which is characterized by a sudden change in hearing as a result of a single exposure to a sudden burst of sound, such as an explosive blast.
Noise exposure alone usually does not produce a loss greater than 75 dB in high frequencies and greater than 40 dB in lower frequencies. Nevertheless, individuals with non-NIHL, such as presbycusis, may have hearing threshold levels in excess of these values.
Hearing loss due to continuous or intermittent noise exposure increases most rapidly during the first 10 to 15 years of exposure, and the rate of hearing loss then decelerates as the hearing threshold
Summary
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increases. This is in contrast to age-related loss, which accelerates over time.
Severe noise can cause permanent hearing damage. A recent study now shows that the capacity to recover from noise damage varies with time of day, driven by circadian clock control of a brain-derived neurotrophic factor (BDNF) in the inner ear. The effects of loud noise on our hearing include ‘ringing in the ear’ and temporary deafness, but for severe noise trauma, these effects can be permanent. The cause of these problems resides in the spiral cochlea of the inner ear, specifically involving damage to the delicate hairs of the inner ear, which are tuned to specific frequencies, and also importantly the dendrites of the auditory nerve. The (BDNF) is known to be involved in repair of the auditory nerve following (noise) damage or nerve cell loss. In the day-time, mRNA levels of BDNF rose over 30-fold, but following stimulation at night there was no response. This strongly suggests that BDNF activation is gated by a circadian clock within the cochlea, only allowing activation of this gene at certain specific phases of the circadian cycle. Subsequent studies revealed that the cochlea contains a self-sustained circadian clock.
The aim of this study was to evaluate degree of hearing loss of subjects during work according to time of noise exposure.
This study included 104 subjects divided into two groups. GI included 20 subjects with normal peripheral hearing. GII included 84 subjects with history of noise exposure during work. GII was further subdivided into two subgroups according to time of work; subgroup IIa (Day only group) and subgroup IIb (Day and Night group).
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Both study and control groups were subjected to Full history taking (personal history, history of exposure to noise or ototoxic drug intake and history of diabetes, hypertension or other medical conditions which could affect the hearing. Any patient or control who gives a positive history was excluded from the study), complete ENT examination and pure tone audiometry.
from the results and tables previously mentioned, severity of hearing loss between Day only group and Day and Night group isn’t influenced by some factors like age and noise exposure in years and there was no statistical significance in hearing loss between right and left ears in Day and Night group.
PTA done to the subjects and abnormalities were higher in Day and Night group than Day only group. A significant difference was found as regard severity of hearing loss between both groups. A correlation was found between time of noise exposure and severity of hearing loss.
from this study, we concluded that hearing loss is higher among subjects with occupational noise exposure during day and night time than those exposed during day time only.