Document entitled "Global warming increases the risk of dehydration and respiratory inflammation in humans" is a study that examines the impact of dry air caused by increased by water evaporation due to global warming, to respiratory inflammation and potential worsening of lung diseases.

Basic hypothesis The study suggests that evaporation of water from airway mucus (a process called mucus transpiration) in dry environments leads to thinning of the mucus layer and compression of epithelial cells, which in turn triggers inflammation. This hypothesis is based on the assumption that healthy airway mucus behaves as a cross-linked hydrogel that undergoes transpiration similar to plant leaves.

Study methodology combines:

• Continuous mathematical analysis: This analysis predicts that water evaporation thins the mucus layer and compresses epithelial cells during respiration.
• Experiments on human tracheobronchial cells (HTB cells): Cells were exposed to air with different degrees of dryness (95%, 60% and 30% relative humidity at 37 °C). Thinning of the mucus layer and increased secretion of inflammatory cytokines (TNF-α, IL-33 and IL-6) were observed.
• Experiments on a mouse model: Mice with a muco-inflammatory phenotype (Scnn1b-Tg) were exposed to intermittent dry air for 14 days, which resulted in histopathological changes and alteration of inflammatory infiltrates. Wild-type mice (WT) did not show inflammatory changes upon the same exposure.
• Climate model simulations: Based on theoretical and experimental findings, simulations were conducted for the continental United States to examine the future consequences of respiratory inflammation due to global warming. The Community Earth System Model (CESM) version 2.1.3 was used under the RCP 8.5 scenario.

Key findings of the study:

• Mucus transpiration: Upper respiratory tract mucus exhibits transpiration, in which water evaporates from the surface of the mucus. The rate of evaporation is proportional to the water vapor pressure deficit (VPD) in the larynx and the ventilation rate.
• Mucus thinning and cell compression: Dry air causes thinning of the mucus layer and compression of epithelial cells. Experiments on HTB cells have confirmed that with decreasing relative humidity, the mucus layer progressively thins.
• Increased secretion of inflammatory cytokines: Compression of epithelial cells due to dry air leads to increased secretion of pro-inflammatory cytokines TNF-α, IL-33 and IL-6. The highest concentrations of cytokines were observed at the lowest relative humidity (30%).
• Inflammation in a mouse model: Intermittent exposure to dry air induced an inflammatory response in the lungs of mice with dehydrated airways (Scnn1b-Tg), manifested by eosinophil and lymphocyte infiltration and focal parenchymal consolidation.
• Impact of global warming: Climate models predict that global warming will significantly increase the risk of high VPD in the continental United States, particularly during the summer months, in the latter half of this century. This means that most of the U.S. will be at increased risk of respiratory inflammation. The risk of high VPD is generally higher in urban areas, but given the size of the high-risk rural areas, the overall risk will be similar for both urban and rural communities. Winter VPD is low outdoors, but indoor heating in winter increases laryngeal VPD.
• Mouth breathing increases the risk of high laryngeal VPD and inflammatory compression of epithelial cells in the larynx and trachea compared to nasal breathing, which provides a higher relative humidity of the inhaled air. The increased prevalence of chronic mouth breathing contributes to this risk.

Discussion and conclusions:

The study concluded that Mucus transpiration represents a potential chronic source of mechanical compression of airway epithelial cells, which may contribute to the pathogenesis of chronic respiratory diseases.The observed increase in inflammatory cytokines due to exposure to dry air (VPD = 4.4 kPa) is associated with inflammation characteristic of asthma.

The authors emphasize that the risk of respiratory inflammation due to global warming is likely to increaseChronic exposure to air with high VPD is also increasing due to the population moving indoors and the increase in the prevalence of mouth breathing.

The study identifies certain restrictions, such as focusing on time-averaged breathing processes and not addressing the impact of chronic mucus compression in the smaller airways. Also, the risk simulations related to global warming did not take into account extreme weather or the risk of inflammation from indoor air.

Despite these study limitations, highlights the importance of understanding the consequences of breathing air with increased VPD and the need for airway hydration management in the context of climate change. Spring

The study is published in the journal nature

Glossary of Key Terms

• Vapor Pressure Deficit (VPD): Vapor pressure deficit, the difference between the saturated water vapor pressure and the actual water vapor pressure in the air. It characterizes the "dryness of the air" and its tendency to accept additional moisture.
• Mucus transpiration: The process of evaporating water from the mucus layer in the respiratory tract into the inhaled air, similar to the transpiration of water from plant leaves.
• Mucociliary transport: A mechanism for clearing the airways in which the coordinated movement of cilia on epithelial cells propels mucus and trapped particles upward, where they are removed.
• Inflammatory cytokines: Proteins released by immune system cells and epithelial cells in response to injury or infection that trigger and regulate the inflammatory response (e.g., TNF-α, IL-33, IL-6).
• Respiratory epithelial cells: Cells lining the airways that produce mucus and have cilia. They play a key role in airway protection and immune response.
• Muco-inflammatory phenotype: A condition characterized by the presence of excessive mucus and inflammation in the airways, often associated with chronic respiratory diseases.
• Histopathological changes: Microscopic changes in tissues that indicate the presence of a disease or pathological process.
• Bronchoalveolar lavage (BAL): A procedure in which fluid is injected into the lungs and then aspirated to obtain cells and other components from the surface of the lower respiratory tract for diagnostic purposes.
• Representative concentration pathway (RCP 8.5): A scenario for the future development of greenhouse gas emissions, which assumes high emissions and significant warming of the planet by the end of the 21st century.
• Osmotic pressure: The pressure that arises due to the difference in solute concentration between two solutions separated by a semipermeable membrane. In the context of mucus, it affects its hydration and viscosity.