Background: Noise as a harmful physical agent for factory workers is one of the most important causes of neurosensory hearing loss. In this study, workers of noise-polluted factories were assessed by audiometric equipment to determine their frequency and intensity of hearing loss (HL). Materials and Methods: In this descriptive study 2004 workers from five factories in Zanjan and suburbs (including Iran-transfo, Pars-switch, Foolad-agfa, Zanjan-choob & Shir-e-pasturizeh) were selected through total sampling and assessed for audiometry. Cases with history of hearing impairment and conductive hearing loss were excluded from the study. Hearing of at least 30 dB in the band of 4KH was considered as the threshold of H.L. The results were classified and presented through descriptive statistics. Results: Out of 2004 cases 1395 (69.6%) were in normal range of hearing (threshold), 1 with previous history of H.L, 58 (2.9%) conductive H.L. and 7 with mixed H.L. 543 (27%) had isolated neurosensory H.L. of whom 311 cases had bilateral involvement and 124 and 108 cases had unilateral involvement of left and right ears, respectively. After excluding the cases with neurosensory H.L. probably related to old age, the final rate of frequency of noised-induced H.L. was obtained as 22.5%. The intensity of H.L. in each one of the ears was as follows: 53.6% mild H.L., 27.3% moderate H.L., 14.9% moderate to severe H.L, 2.9% severe H.L. and 1.3% of cases profound deafness. Conclusion: Similar internal and foreign studies have shown different results which are mainly due to various levels of noise and different cultural conditions of noise control in different factories. The results of this study indicate the necessity of care for utilizing protective instruments against noise in factories. More research concerning the change in temporary or permanent hearing threshold is recommended.

Background and Objective: There are many chemical, physical and biological hazards which are threatening the dentists' health. In this study we assessed the effect of noise on Zanjan dentists hearing loss as one of the dentistry physical hazards. Materials and Methods: This cross sectional study was performed on all zanjan dentists in 2006-7. All dentists were examined by audiometer (CA76 and AC40). The exclusion criteria were hearing accidents and diseases. The data were statistically analyzed by T test ( =0.05). Results: Noise induced hearing loss was observed in most cases. Left ear was more susceptible than right ear. The men had more hearing loss than the women. (P value on Freq 8000 HZ= 0.001). Deutists with seniority of 10 years and over had more hearing loss than who have seniority of less than 10 years (P value on Freq 8000 HZ= 0.004). The dentists in the age of 40 years old and over had more hearing loss than the ages below 40 year (P value on freq 8000 HZ in the left ear = 0.007, in the right ear = 0.019). Left ear had more hearing loss than right ear. Noise induced hearing loss in extra–speech frequencies was more than speech frequencies. The noise frequency of dental equipments was between 70 to 105 db. Conclusion: Noise induced hearing loss which has threatened the dentists' health is an incurable problem that must be prevented. Regular audiometric exam, use of hearing protection devices and also the occupational health regulations must be implemented for dentists in order to reduce the risk of noise induced hearing loss.

Background and Objective: Hyperbilirubinemia is a common neonatal abnormality. Severe hyperbilirubinemia is a risk factor for auditory system injury. Auditory brainstem responses (ABR) are important in early diagnosis of hearing impairments in healthy term infants with elevated bilirubin levels requiring exchange transfusion.
Materials and Methods: During a two- year- period (2007 – 2009), in a prospective descriptive analytical study, in Tehran Milad Hospital, 64 (32 female, 32 male), healthy term (> 37 weeks) infants, who required treatment or were treated with phototherapy or received exchange transfusion for elevated bilirubin levels or jaundice, were studied. After obtaining a written consent from their parents, the infants were tested with auditory brain responses and results were analyzed using SPSS 16 software.
Results: No significant correlation was found between ABR and age, weight, bilirubin level or ABO blood group. Nineteen out of 64 infants received exchange transfusion. Three out of 19 infants (16%) exhibited abnormal ABR and 16 infants (84%) had normal ABR. There was no significant correlation between exchange transfusions and ABR (P>0.05).  
Conclusion: The results pointed out that 14% of the infants with elevated bilirubin who required exchange transfusion had abnormal ABR. This indicates that elevated bilirubin levels even without inducing kernicterus should be considered as risk factors for hearing impairments. Further studies are needed on how long these tests may remain abnormal.

Background and Objective: Mutations in GJB2 gene is the most common cause of autosomal recessive non-syndromic hearing loss in many populations. The aim of this study was to determine the frequency of 35delG, 167delT, M34T, 235delC mutations in West Azarbaijan population.
Materials and Methods: 129 patients from 96 families were studied. Mutations were detected using ASO-PCR and PCR-RFLP methods.
Results: Totally, 65.89% of cases were sporadic and the remaining (34.11%) were familial. Six out of 8 cases with 35delG mutation and one case with 235delC mutation were offspring of consanguineous union. Mutations of 35delG were detected in 8 families. 167delT and M34T mutations were not found but 235delC was detected only in one family. On the other hand, 13 out of 258 chromosomes had 35delG mutations. Five patients were homozygous and 3 were heterozygous for 35delG mutation. It means that, in 5.04% of the patients the major reason for hearing loss was 35delG mutation. One out of 258 (0.39%) chromosomes had heterozygous 235delC mutation.

Conclusion: It can be concluded that the other genes or mutations could result in autosomal recessive non-syndromic hearing loss in West Azerbaijani population.

Background and Objective: Hearing loss as a sensory disorder is among the most common developmental disorders. Based on the universal neonatal screening results, the incidence of hearing deficit is 1–3 per 1000 live births in the well-baby nursery population and 2–4 in 100 newborns in the intensive care community. The aim of this study was to highlight the main risk factors for hearing loss based on the existing evidence.
Materials and Methods: We searched all observational studies related to risk factors of hearing loss from 1990 to 2019 in PubMed, Scopus, Web of Science, Science direct, and google scholar search engines. Quality of the included studies was evaluated by the STROBE checklist. Totally, 17 observational studies met our inclusion criteria.
Results: In most of the reviewed studies, the relations between hearing loss in infants and some maternal and neonatal variables such as ventilatory support, craniofacial anomalies, hyperbilirubinemia, meningitis, Apgar scores, sepsis, asphyxia, stay in intensive care units, respiratory distress syndrome, and pulmonary surfactant were statistically significant.
Conclusion: Our results indicated that ventilator support, craniofacial anomalies, low birth weight, and hyperbilirubinemia were the main statistically significant risk factors for hearing-loss.