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Biological sex differences exist for many airway diseases in which females have either worse or better health outcomes. Inflammatory airway diseases such as cystic fibrosis (CF) and asthma display a clear male advantage in post-puberty while a female benefit is observed in asthma during the pre-puberty years. The influence of menstrual cycle stage and pregnancy on the frequency and severity of pulmonary exacerbations in CF and asthma point to a role for sex steroid hormones, particularly estrogen, in underpinning biological sex differences in these diseases. There are many ways by which estrogen may aggravate asthma and CF involving disturbances in airway surface liquid (ASL) dynamics, inappropriate hyper-immune and allergenic responses, as well as exacerbation of pathogen virulence. The deleterious effect of estrogen on pulmonary function in CF and asthma contrasts with the female advantage observed in airway diseases characterised by pulmonary edema such as pneumonia, acute respiratory distress syndrome (ARDS) and COVID-19. Airway surface liquid hypersecretion and alveolar flooding are hallmarks of ARDS and COVID-19, and contribute to the morbidity and mortality of severe forms of these diseases. ASL dynamics encompasses the intrinsic features of the thin lining of fluid covering the airway epithelium which regulate mucociliary clearance (ciliary beat, ASL height, volume, pH, viscosity, mucins, and channel activating proteases) in addition to innate defence mechanisms (pathogen virulence, cytokines, defensins, specialised pro-resolution lipid mediators, and metabolism). Estrogen regulation of ASL dynamics contributing to biological sex differences in CF, asthma and COVID-19 is a major focus of this review. Plain Language Summary: Sex differences exist in many airway diseases in which females have either worse or better health outcomes. These include cystic fibrosis (CF) and asthma where females post-puberty have more frequent lung exacerbations and higher mortality. Lung infections and difficulty in breathing become worse in post-puberty in females and during the ovulation period of the menstrual cycle, and in pregnancy, indicating a role for sex hormones in underpinning sex differences in CF and asthma. Evidence also exists for sex differences with a female advantage in airway diseases which are characterised by flooding of the airways, as in pneumonia and COVID-19, where females have better lung function and lower risk of death than males. The female sex hormone estrogen plays an important role in determining the role of sex biology in airway disease severity and mortality. Estrogen can control the movement of salt and water in and out of the lung airway tubes and dehydrate the lung surface to make it more sticky with mucus, as observed in CF and asthma, thus worsening the condition. In contrast, estrogen can have beneficial effects in lowering the volume of water in the lungs in COVID-19 thus alleviating flooding of the airways. This review focusses on the biology of sex differences in CF, asthma and COVID-19, and the cellular mechanisms by which estrogen can have either detrimental or beneficial effects in these diseases. Highlights: Airway diseases such as cystic fibrosis (CF), asthma and COVID-19 show marked sex differences in their morbidity and mortality Estrogen and female sex are associated with exacerbated CF and asthma Estrogen aggravates CF and asthma by compromising the optimal airway surface liquid dynamics for effective mucociliary clearance and by exacerbating bacterial virulence Estrogen confers a female advantage in COVID-19 by reducing airway flooding and reinforcing immunity. [ABSTRACT FROM AUTHOR]