According to the Global Burden of Disease report, hearing loss is the fourth leading cause of disability globally . It has been shown that the prevalence of hearing loss doubles with every 10-year increase in age. Approximately half of the persons in their 7th decade (60 to 69 years of age)  and about 80% who are 85 years of age or older  experience hearing loss that is severe enough to influence daily communication. In the most recently published report, WHO estimated that about 466 million people, 6.1% of the world’s population, has been subjected to disabling hearing impairment in 2018 . This report predicted that this estimate would rise to 630 million by 2030 and over 900 million by 2050. This increasing trend has emerged as a serious public health issue that necessitates appropriate, well-designed, and well-coordinated actions at global, regional, and local levels [4–6].
Importantly, the burden of hearing impairments over the life span is significant both individually and socially. It has been demonstrated that hearing loss in adults can adversely influence their health, economic and psychosocial conditions and leads to social isolation and a diminished quality of life [7–10]. As compared with age-matched adults with normal hearing levels, older individuals with hearing loss reveal several negative health outcomes including higher rates of hospitalization , falls and frailty , death  as well as higher rates of depression and dementia . On a societal aspect, hearing-impaired usually indicate lower levels of education compared to those with normal hearing sensitivity. Furthermore, hearing-impaired adults usually show higher levels of underemployment or unemployment and lower income levels compared to their normal-hearing peers [7, 16].
Generally, the hearing acuity reduces with age - physiologically beginning by the third decade of life. The age-relating loss (ARHL) usually shows a symmetrical sensorineural hearing loss (SNHL) in the high frequencies, which influences speech understanding in noisy situations [18, 19]. SNHL, predominately induced by damage to the sensory hair cells in the inner ear, has been reported as the main cause of permanent hearing impairment. In addition to ARHL, the main factors contributing to the rising trend in hearing impairments are the increased rate of non-communicable diseases (NCDs), the use of ototoxic medications, and noise exposure [17, 19]. However, estimations of hearing loss prevalence across various cohort studies differ due to various tonal frequencies used to calculate a pure tone average, hearing threshold cut-offs to define hearing loss and binaural or monoaural definition of hearing loss. Furthermore, differences in cohort characteristics (recruitment of population sample or volunteer cohort) and the age of the participants in the cohort study also constrict comparison across studies [2, 20].
Recent evidence has shown that a significant percentage of hearing loss prevalence in adults could be prevented by taking appropriate measures and interventions such as community-oriented health education [19, 21, 22]. However, many world countries, particularly low, and middle-income countries, lack strategies and programs to reduce exposure to risk factors such as occupational noise exposure, use of ototoxic drugs, and having NCDs [21, 23]. Iran has the second rank of the population in the Middle East with an aging population. There is no published population-based cohort study on ear diseases in southwest Iran and on Arab ethnicity. Moreover, ear disorders' risk factors, etiologies, and development mechanisms vary based on geographical, cultural, and social conditions. The main objective of the HECS are as follow:
To estimate the prevalence and incidence of NCDs to identify the risk and protective factors of the NCDs in the study population
To build a biobank for assessing the possible roles of physiological, genetic, and epigenetic markers in the prognosis and diagnosis of the NCDs
To determine the relationship between the NCDs and hearing impairment