Background: It is well established that upper and lower airways are often clumped together when diagnosing and treating a disease. This study was designed to determine the prevalence of upper and lower airway diseases and to assess the effect of sociodemographic factors on the prevalence and the comorbidity of these disorders.

Methods: This cross-sectional population-based study included patients with ages ranging between 15 to 65 years, who were referred to allergy outpatient clinics in various provinces of Iran from April to September 2020. A modified global Allergy and Asthma European Network (GA2LEN) screening questionnaire was filled out by local allergists of the 12 selected provinces in Iran. Information about the patients and sociodemographic factors was also recorded. Statistical analysis was done by univariate statistical analyses and multiple logistic regressions in SPSS software Version 26.

Results: Out of 4988 recruited patients, 1078 (21.6%) had the symptoms of allergic rhinitis (AR) and 285 (5.7%) met the criteria of asthma. The prevalence of acute rhinosinusitis (ARS) and chronic rhinosinusitis (CRS) was 21.6% and 22%, respectively. The highest prevalence of AR and ARS was in Tehran with the arateof of 33.9% each. Asthma was more prevalent in Khuzestan (14.2%) and CRS in Baluchestan (57.5%). Our analysis showed that the patients with asthma were most likely to have other allergic diseases as well—CRS (OR= 4.8; 95% CI, 2.02-5.82), AR (OR= 2.5, 95% CI, 2.10-3), ARS (OR= 1.8; 95% CI, 2.10-3), followed by eczema (OR= 1.4; 95% CI, 1.13-1.67). We found that those individuals with CRS were most likely to have painkiller hypersensitivity (OR= 2.1; 95% CI, 1.21-3.83). Furthermore, smoking has been found more than 1.5 folds in patients with ARS. After adjusting variables, there was no correlation between education, occupation, and ethnicity with the studied diseases.

Conclusion: Rhinosinusitis is a common condition among Iranian patients. This study confirmed that inflammation of the upper and lower airways can occur simultaneously. Gender, education, occupation, and ethnicity were found to be irrelevant in the development of either AR, asthma, ARS, or CRS.