Environmental Changes and Their Effect on the Airway Microbiome: Implications for Airway Diseases

Obstructive airway diseases, including asthma and chronic obstructive pulmonary disease (COPD), represent the two major chronic airway inflammatory diseases. In both conditions, the airways are exposed to a variety of microbial and environmental stimuli. A healthy airway microbiome is characterized not only by microbial diversity but also by the absence of significant quantities of pathogens, and is signified by ecological stability, functional resilience, and balanced interactions with the host immune system. Although microbial diversity is often considered a marker of respiratory health, its association with disease depends on the anatomical site, host factors, and environmental exposures. Thus, microbial diversity alone should not be used as a universal marker of airway health-the occurrence of pathogens and the organ compartment are also factors that are important to consider. Disruption of this microbial homeostasis, known as dysbiosis, may involve reduced diversity of the commensal flora as well as increased quantity of key pathogens, and this condition is believed to stimulate inflammation that is associated with the progression of asthma and COPD, including exacerbations. Environmental changes, including air pollution, climate change, and urbanization, can disrupt microbial balance by reducing diversity and increasing the prevalence of pathogens. Several mechanisms contribute to this interaction, including damage to the airway epithelium, impaired mucociliary clearance, oxidative stress, and alterations in innate and adaptive immune responses. This review examines the interplay between external environmental factors and the airway microbiome, and how they affect airway obstructive diseases, asthma, and COPD to understand their immunopathology, and to improve disease prevention and treatment approaches. This is an area of great interest, and further research is required to elucidate these interactions so we can better help our patients.