1. Introduction
As major public health issues worldwide, Chronic Respiratory Diseases (CRDs) such as Chronic Obstructive Pulmonary Disease (COPD), pneumoconiosis, asthma, interstitial lung disease, and pulmonary sarcoidosis impose significant socioeconomic burdens on individuals and communities and are severely overlooked compared to other non-communicable diseases, such as cardiovascular disease, cancer, and diabetes [1, 2, 3]. Therefore, respiratory diseases are of particular relevance to public health and are the central focus of the World Health Organization’s (WHO) surveillance and prevention strategies [4, 5].
In the International Statistical Classification of Diseases and Related Health Problems-10th Revision (ICD-10), this group of diseases covers a wide range of acute and chronic respiratory disorders (J00-J99), including acute upper respiratory tract infections, influenza, and pneumonia, other acute lower respiratory infections, other diseases of the upper respiratory tract, chronic lower respiratory diseases, lung diseases due to external agents, other respiratory diseases principally affecting the interstitium, suppurative and necrotic conditions of the lower respiratory tract, other diseases of the pleura, and other diseases of the respiratory system [6].
Acute respiratory infections include a wide range of diseases, such as the common cold, pharyngitis, tonsillitis, influenza, and lower respiratory tract diseases [7, 8]. Clinically, acute lower respiratory infections can be divided into bronchiolitis and pneumonia, which are difficult to differentiate, especially in children under two years of age [9, 10]. The importance of these diseases is reflected in their impact on the healthcare system. 
Seasonal influenza is a major cause of additional outpatient visits each year. Depending on the season, influenza can cause about 30,000 hospital admissions due to primary and secondary infections [11]. Before the outbreak of the COVID-19 pandemic, seasonal influenza killed about half a million people worldwide in 2018 [12].
The morbidity and mortality due to lung diseases are overwhelming. Each year, hundreds of millions of people are affected by chronic respiratory conditions and four million people die prematurely due to these diseases [13]. Respiratory infections are the leading cause of death in developing countries. Although the global burden of disease study in 2020 showed that deaths related to lower respiratory tract infections have decreased compared to 2 decades ago, the mortality rate is still too high [14]. This is especially the case for infants and young children, with nearly three million children, mostly under the age of 5, dying of pneumonia and lower respiratory tract infections each year [14]. Pneumonia kills more children than the human immunodeficiency virus, tuberculosis, and malaria combined [15]. 
Asthma not only kills 180,000 people worldwide each year but also imposes a great socioeconomic burdens due to morbidity and disruption of life [16]. This disease affects 235 million people worldwide and accounts for over 30% of all child hospitalizations in the United States [13] with an increasing incidence in both developed and developing countries. All ages, races, and ethnicities are affected by this disease, but large variances have been observed between countries and within demographic groups. The burden of asthma is higher in urban areas, partly due to environmental exposures and lack of access to effective care and medication, especially in low-income countries [16].
A study in Golestan Province in Iran found that death due to respiratory diseases accounted for 7.58% of all deaths in 2018 [17]. Given that the trend and rate of mortality from respiratory diseases have not been estimated for this province and Iran as a whole, the purpose of this study was to investigate the mortality rate and years of life lost due to respiratory diseases during 2014-2018. Also, according to the mortality rate of respiratory system diseases, studies can be conducted to compare with the mortality rate after the outbreak of COVID-19.
2. Methods
The documentary research method (secondary analysis) was used and data were collected from clinical records. The population included all deaths registered in the health department of Golestan University of Medical Sciences (GOUMS) during 2014-2018. Mortality information was obtained from the statistics and performance analysis unit of GOUMS in the form of an Excel file. In addition to the cause of death, this file also provided information about the demographic characteristics (age, gender) of the deceased. 
Deaths records contain information about the number and cause of death from cities affiliated with GOUMS. These records are regularly compared to the information from the national organization for civil registration, and discrepancies are identified and followed up. In the present research, the International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD-10) was used for disease classification and cause of death coding. Then, to qualitatively examine the death cause, the data were examined and modified in terms of causes of death inconsistent with sex and age, trivial conditions, and codes for ill-defined and unknown causes of death. Global Burden of Disease (GBD) 2010 and 2013 studies were used to modify the codes of inconsistent and trivial causes of death [18, 19, 20]. As for ill-defined and unknown causes of death, it was assumed that each code, at any age and sex, contains the cause of death that follows the distribution of causes of death within that age and sex group [21].
After qualitative correction of the data, the population of the research i.e., the total number of deaths were extracted for the period 2014-2018. The total population of Golestan Province was 1,777,014 and 1,868,819 people in 2011 and 2016, respectively, according to the population and housing census. Population growth rate and population information by sex and age group were used to calculate population estimates for years between censuses. The following equation was used to estimate the population size (Equation 1):

1. P(t+n)=Pt (1+r)n
where is the population size in the 2nd census, is the population size in the 1st census, is the difference between the two censuses, and is the annual population growth rate.
In this study, the methodology used in GBD 2002 was used to maintain comparability [22], and the Standard Expected Years of Life Lost (SEYLL) was adopted to calculate Years of Life Lost (YLL). This measure uses the expectation of life at each age based on an ideal standard derived from a model life table to estimate YLL associated with death [23]. The difference in survival between men and women was determined to be 2.5 years, and the Coale and Demeny regional model life tables (Family Model West, level 26) for men and women were used to determine life expectancy in different age groups. These were calculated with the following formula using age and sex patterns from the original GBD Project in the Excel format as well as the weighting scheme from GBD 2000, which is consistent with the views of Iranian experts (Equation 2) [23, 24]:


where: 
N=Number of deaths for a given age and sex;
L=Standard life expectancy for the same age and sex;
r=Discount rate (GBD standard value is 0.03);
ß=Age-weighting function (GBD standard value is 0.04); 
C=Age-weighting correction constant (GBD standard value is 0.1658).
After modifying and cleaning the data, WinPepi 11.65 was used to calculate the rates, and Microsoft Excel version 2016 was used for frequency distributions and line graphs. 
3. Results
During the period 2014-2018, 2,462 deaths due to respiratory diseases have been recorded in Golestan Province, of which 1,416 people (57.5%) were men and 1,046 people (42.5%) were women. In terms of residence, 1,304 cases (53.5%) were rural and 1,132 cases (46.5%) were in the urban population. The average age of death was 62.28±26.29 years, with an average of 62.56±25.32 years for men and 61.90±27.56 years for women. 
The crude death rate due to respiratory diseases during these 5 years was 26.34 per 100,000 population, with a higher rate in men (30.17 per 100,000 population) than in women (22.49 per 100,000 population). The highest and lowest crude death rates due to respiratory diseases were 33.14 per 100,000 population in 2018 and 20.74 per 100,000 population in 2014 (Table 1). 


The trend of mortality due to respiratory diseases during these 5 years indicates that deaths per 100,000 population have been increasing in both men and women, although it remained constant between 2014 and 2015 (Figure 1). Table 2 presents the frequency and percentage of deaths due to respiratory diseases based on age group and cause of death during 2014-2018.


The highest and lowest deaths due to respiratory diseases were in the age groups 80 years and above (29.7%) and 5-14 years (1.7%), respectively. Most deaths caused by acute upper respiratory tract infections, chronic lower respiratory diseases, and influenza and pneumonia were in the age group of 80 years and older. Also, the highest number of deaths in age groups was due to chronic lower respiratory diseases (918 cases), followed by influenza and pneumonia.
The highest number of deaths due to respiratory diseases in all the studied years was related to men and women aged 80 years and above (28.7% and 31.1%, respectively). Table 3 presents the frequency and percentage of deaths due to respiratory diseases by sex and age group during 2014-2018.


Table 4 presents the YLL due to respiratory diseases in Golestan Province by age group and gender during 2014-2018.


Approximately 6,864 YLL existed due to premature death during this period, with men (3,511 years, 7.2 per 1,000) contributing more than women (3,353 years, 7.2 per 1,000). The highest number of YLL due to respiratory diseases was in the age groups 80 years and above (42.8 per 1,000) and 69-60 years (16.3 per 1,000) in men, and age groups 80 years and above (40.7 per 1,000) and 79-70 years (23.2 per 1,000) in women. 
4. Discussion
Mortality is the most objective measure of health problems, especially in low- and middle-income countries, such as Iran [25]. This study investigated the mortality rate and YLL due to respiratory diseases in northern Iran.
The results indicated that the mortality rate due to respiratory diseases was higher in men than women during the studied period. In a study in Erbil, Iraq, the mortality rate due to respiratory diseases was 42.43 per 100,000 population, which is higher than the present research. Also, consistent with present findings, this study found that the mortality rate was higher in men than in women [26]. 
The results showed that mortality due to respiratory diseases increased in these 5 years. In a study conducted in Khorramabad’s hospital by Mahmoudi et al., respiratory diseases were identified as the Second leading cause of death [27], and in another study, the mortality rate caused by respiratory diseases in the elderly population was 10.3% [28]. A study in India showed an increase in the incidence of chronic obstructive pulmonary disease (COPD) from 1990 to 2016 [29]. Also, a study in Mazandaran Province found that respiratory diseases were the leading cause of death in children under five years of age [30]. The spread of COVID-19 in Iran and the world is a global epidemic that is the cause of severe infectious lung disease. This virus can be transmitted through direct contact with the respiratory droplets of an infected person (cough and sneeze). Its infection can lead to pneumonia, serious respiratory problems, and eventually, lead to the death of infected patients [31]. However, the main complications observed in most of these patients are respiratory, and both parts of the upper and lower respiratory tract can be affected by this virus [32].
According to the results, the highest number of deaths in all age groups was due to chronic lower respiratory diseases, followed by influenza and pneumonia. One study showed that the prevalence of asthma was higher in adults than children and in boys than girls, and after adolescence, the prevalence of this disease was higher in women than men [33]. Influenza is an acute respiratory infectious disease caused by the influenza viruses [34]. Influenza viruses are mainly transmitted from person to person through droplets produced while coughing or sneezing. Influenza infects 5%-15% of the world’s population in annual epidemics, resulting in hundreds of thousands of deaths each year [35]. Some studies have found that the incidence of influenza is higher in women than in men [36, 37]. Pneumonia is an infection of the lung parenchyma that presents not only as a disease but also as a clinical syndrome. It is the sixth leading cause of death in the United States and the most common cause of death due to infection, accounting for more than 50% of outpatient visits [38, 39]. Acute respiratory infections cause 4.5 million deaths among children each year, the vast majority of which occur in developing countries. Pneumonia unassociated with measles causes 70% of these deaths [40]. 
According to the results, the number of years lost due to respiratory diseases increased during the studied period and was higher in men than women, with the highest rate belonging to the elderly. Torkashvand et al. found that YLL due to respiratory diseases among the elderly increased from 2011 to 2017 [41]. Given that the elderly population is projected to increase in Iran as well as worldwide, this reality has to be acknowledged and the Iranian health system should change its policy goals to meet the needs of the elderly population [42].
Due to the increased exposure to the risk factors for respiratory diseases, including smoking, exposure to pollutants, allergens, occupational exposures, unhealthy diet, obesity, and physical inactivity, among others [43], and given the rapid population growth, Chronic Respiratory Diseases (CRDs) are becoming a more significant problem in all regions of the world. Therefore, it seems necessary to conduct more comprehensive research throughout Iran, and an in-depth and expert approach is essential for planning the country’s future health.
Recent data from Germany show that 12.3% of all deaths are caused by respiratory diseases [44] and the direct costs of treating respiratory disorders amount to €14.7 billion [9]. Millions of people die due to lack of access to vaccinations and medications or the inability to access care in the health care system, which can be avoided. Immunization programs can effectively prevent many deadly infectious diseases, including measles, influenza, pertussis, and common bacterial pneumonia. However, many countries do not have certain vaccines in their national immunization programs [45]. 
In addition, this study was designed before the onset of the COVID-19 pandemic, and due to changes that this pandemic has caused in the mortality rate in Golestan province, therefore it is recommended to investigate the trend of mortality by respiratory diseases during this pandemic.
5. Conclusion
As a result, the findings of the study showed that during 2014-2018, 2,462 deaths due to respiratory diseases were recorded in Golestan Province, with a higher prevalence in men than women, which subsequently resulted in a higher number of YLL due to respiratory diseases in men than women. The average age of death was over 60 years in both men and women. The mortality rate due to respiratory diseases was 26.34 per 100,000 population and has increased during these 5 years. 

Ethical Considerations
Compliance with ethical guidelines
This study was approved by  the ethical committee  of Golestan University of Medical Sciences (GOUMS)  (Ethic Code: IR.GOUMS.REC.1398.329). 

Funding
This study was financially supported by Golestan University of Medical Sciences (GOUMS) (Grant No.: 111277).

Authors' contributions
All authors equally contributed to preparing this article.

Conflict of interest
The authors declared no conflict of interest.

Acknowledgments
This research would not have been done without the cooperation of the Health Management and Social Development Research center in Gorgan City, Iran.


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