HUMAN respiratory diseases such as asthma, allergic rhinitis, and influenza can worsen when individuals are exposed to extreme atmospheric conditions. These conditions include very cold, hot, or dry air, all of which contribute to increased vapour pressure deficit (VPD), a measure of air dryness. The rise in global temperatures over the past century has led to an increase in atmospheric VPD, amplifying its effects on human airways.
VPD influences the evaporation of water from biological surfaces, much like the process of transpiration in plant leaves. In the human respiratory system, upper airway mucosa experiences similar water loss, leading to potential cell compression and inflammation. This effect is particularly pronounced during mouth breathing, a behaviour that has become more common due to factors such as obesity, allergic rhinitis, and ageing. Unlike nasal breathing, which humidifies inhaled air, mouth breathing allows dry air to reach the larynx, exposing airway surfaces to high VPD levels.
This study explored the hypothesis that mucus transpiration contributes to airway inflammation and chronic respiratory disease. Mucus, a hydrogel composed of water, salts, proteins, and mucin macromolecules, exhibits dehydration behaviour similar to plant hydrogels. When exposed to dry air, mucus thins and concentrates, leading to mechanical compression of epithelial cells. This process triggers the release of inflammatory cytokines such as IL-6, TNF-α, and IL-33, which are linked to eosinophilic and non-eosinophilic inflammation in asthma. Additionally, these cytokines interact with sensory neurons, contributing to symptoms such as cough, laryngeal hypersensitivity, and bronchospasm.
Experimental studies with cell cultures and mice demonstrated significant inflammatory responses to dry air exposure. Mice subjected to intermittent dry air inhalation exhibited inflammatory cell infiltration in the lungs, supporting the hypothesis that mucus transpiration plays a role in airway inflammation. These findings align with observations in humans, where exposure to dry air exacerbates respiratory symptoms.
The study highlights the increasing risk of airway inflammation due to climate change, which is raising atmospheric VPD levels. It underscores the need for further research into airway hydration strategies and respiratory disease management in response to global warming. Understanding the long-term effects of chronic exposure to high VPD air could inform healthcare approaches aimed at mitigating climate-related respiratory health risks.
Reference
Edwards DA, Edwards A, Li D, et al. Global warming risks dehydrating and inflaming human airways. Commun Earth Environ. 2025;6:193.
