Chronic Humidity and Nasal Mucosal Dysfunction in Tropical Climates: Implications for Sinonasal Health in Sub-Saharan...

The nasal mucosa is the primary defense mechanism of the upper respiratory tract. Its functionality relies heavily on the intricate balance of mucociliary clearance (MCC), mucosal hydration, epithelial tight junction integrity, and local immunological responses. While the detrimental physiological effects of low humidity and cold temperatures on nasal function are extensively documented in the literature, the physiological and pathological impacts of chronic exposure to high relative humidity (RH) and high temperatures-the defining characteristics of tropical and equatorial climates-remain significantly underrepresented and poorly synthesised. Given that approximately 40% of the global population lives in these climate zones, understanding these mechanisms is of paramount importance to global health. This evidence gap is particularly critical for sub-Saharan Africa, where sinonasal disorders constitute a significant and underappreciated component of the otolaryngological disease burden, yet region-specific clinical guidelines remain largely absent. Compounding this, rapid urbanisation across African and Asian tropical cities is accelerating the adoption of air-conditioning, creating novel patterns of indoor-outdoor micro-climatic exposure that may be fundamentally altering the epidemiology of chronic rhinitis. This systematic review aims to comprehensively evaluate the correlation between high ambient humidity in tropical climates and nasal mucosal function. The primary endpoints include mucociliary clearance times, ciliary beat frequency (CBF), mucus rheology (viscoelasticity), epithelial barrier integrity, and the epidemiological prevalence of specific sinonasal disorders such as tropical allergic rhinitis and non-allergic vasomotor rhinitis. A rigorous systematic literature search was conducted in strict adherence to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The search spanned PubMed, Scopus, Web of Science, and the Cochrane Library for peer-reviewed articles published between January 1, 2000, and January 1, 2025. Inclusion criteria mandated studies evaluating human nasal mucosal physiology, MCC time, CBF, and humidity levels exceeding 70%. Risk of bias was assessed using the Newcastle-Ottawa Scale for observational studies and the Cochrane Risk of Bias tool for randomised trials. Out of 1,420 initially identified records, 45 studies met the stringent inclusion criteria for qualitative synthesis. The aggregated data indicate a complex, non-linear, U-shaped relationship between ambient RH and MCC efficiency: optimal MCC occurs between 40% and 60% RH, while chronic exposure to tropical humidity (>70% RH) combined with high ambient temperatures (>28°C) is associated with mucosal engorgement, altered mucus rheology characterised by decreased viscosity and elasticity, and a paradoxical slowing of ciliary beat frequency (CBF). Furthermore, the modern tropical lifestyle involves frequent, abrupt transitions between highly humid outdoor environments and cold, desiccating air-conditioned indoor spaces. This 'micro-climatic shock' may contribute to reactive turbinate hypertrophy, disrupted osmotic gradients, and increased susceptibility to perennial aeroallergens such as house dust mites and fungal spores. High ambient humidity in tropical climates is associated with altered nasal mucosal function, including impaired MCC and changes in mucus rheology. These findings highlight the need for climate-specific approaches to the management of sinonasal disorders. Keywords: Nasal mucosa; humidity; tropical climate; mucociliary clearance; rhinitis; air-conditioning.