Pulmonology · Medical Literature
Chronic Humidity and Nasal Mucosal Dysfunction in Tropical Climates: Implications for Sinonasal Health in Sub-Saharan Africa and the Developing World - A Systematic Review.
Abstract
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.
Clinical relevance
This systematic review addresses a critical evidence gap regarding the physiological impact of chronic high relative humidity (>70% RH) and elevated ambient temperatures (>28°C) on nasal mucosal function — a gap particularly consequential given that approximately 40% of the global population resides in tropical or equatorial climates. Sinonasal disorders represent a significant yet underappreciated component of the otolaryngological disease burden in sub-Saharan Africa, where region-specific clinical guidelines are largely absent. The rapid urbanisation-driven adoption of air-conditioning in tropical cities is further reshaping the epidemiology of chronic rhinitis, making this review immediately relevant to pulmonologists and otolaryngologists managing patients in these regions.
Key findings
1. Of 1,420 initially identified records, 45 studies met PRISMA-compliant inclusion criteria (studies evaluating human nasal mucosal physiology, MCC time, CBF, and RH >70%) for qualitative synthesis; no pooled meta-analytic effect sizes or confidence intervals were reported. 2. A non-linear, U-shaped relationship was identified between ambient RH and mucociliary clearance (MCC) efficiency: optimal MCC occurs at 40–60% RH, with significant impairment at both extremes. 3. Chronic exposure to tropical humidity (>70% RH) combined with ambient temperatures >28°C is associated with mucosal engorgement, paradoxical slowing of ciliary beat frequency (CBF), and altered mucus rheology characterised by decreased viscosity and elasticity — collectively impairing the primary upper airway defense mechanism. 4. Frequent abrupt transitions between high-humidity outdoor environments and cold, desiccating air-conditioned indoor spaces ('micro-climatic shock') are implicated in reactive turbinate hypertrophy and disrupted osmotic gradients across the nasal epithelium. 5. The same micro-climatic shock pattern appears to increase susceptibility to perennial aeroallergens — specifically house dust mites and fungal spores — potentially driving elevated rates of tropical allergic rhinitis and non-allergic vasomotor rhinitis. 6. Risk of bias was formally assessed using the Newcastle-Ottawa Scale for observational studies and the Cochrane Risk of Bias tool for randomised trials; however, specific bias scores and inter-study heterogeneity metrics were not quantitatively reported in the abstract.
Practice implications
Clinicians managing sinonasal disorders in tropical or high-humidity settings should recognise that standard temperate-climate guidelines may not be directly applicable, as chronic RH >70% independently impairs MCC and ciliary function even in the absence of the desiccation typically emphasised in existing protocols. Patient counselling should address the physiological hazard of repeated indoor-outdoor micro-climatic transitions — specifically, the use of air-conditioning set at extremely low temperatures — and where feasible, indoor humidity and temperature regulation targeting the 40–60% RH optimum range should be recommended as an adjunct to pharmacological management of chronic rhinitis and sinonasal disease. Given the identified epidemiological burden and absence of region-specific guidelines, there is a clear and urgent need for prospective, geographically targeted clinical trials in sub-Saharan African and tropical Asian populations to generate the quantitative effect-size data required to inform evidence-based, climate-adapted sinonasal management pathways.