How Indoor Air Quality Affects Respiratory Health in Winter and Summer

By Vijay Kumar MalesuReviewed by Benedette Cuffari, M.Sc.

Introduction
Understanding IAQ
Seasonal factors affecting IAQ
Health impacts of seasonal IAQ changes
Mechanisms linking IAQ to respiratory health
Mitigation strategies
Key takeaways
References
Further reading

From elevated winter levels of polycyclic aromatic hydrocarbons (PAHs) in schools to humidity-linked infection risk in older adults, this review reveals how seasonal changes in indoor air may influence respiratory health and what can be done to reduce the burden.

Image Credit: Sinegor / Shutterstock.com

Introduction

Seasonal variation in indoor air quality (IAQ) is increasingly recognized as an important determinant of respiratory health. Indoors, air composition varies significantly throughout the year due to seasonal differences in ventilation, occupancy, heating habits, and outdoor air infiltration.

School-based investigations in central Europe demonstrate seasonal variation in bacterial, fungal, and polycyclic aromatic hydrocarbon (PAH) concentrations, with winter characterized by elevated PAHs (particularly 4+ ring compounds with higher carcinogenic and mutagenic equivalents) and ventilation-dependent microbial fluctuations.² It is important to note that these findings derive from limited seasonal sampling in five primary schools and were reported in a conference abstract format.² In residential settings among older adults in Hong Kong, lower indoor absolute humidity (AH) during cool seasons was prospectively associated with increased risk of acute respiratory illness (ARI).³

Collectively, these observations suggest that seasonal IAQ deterioration may contribute to cough, allergies, asthma exacerbations, and respiratory infections, most notably during the winter.^2,3 This article explains how seasonal changes in ventilation, humidity, and indoor pollutants are associated with cough, allergies, asthma flares, and respiratory infections.

Understanding IAQ

IAQ refers to the concentration and composition of pollutants within enclosed environments including particulate matter (PM), volatile organic compounds (VOCs), biological agents like bacteria and fungi, and key physical parameters such as temperature and humidity.

Poor IAQ has been linked to adverse respiratory outcomes in observational studies. In densely populated residential settings in Delhi, culturable bacterial concentrations ranging from 730–5,300 CFU/m³ and fungal concentrations ranging from 1,330–6,050 CFU/m³ have been reported, frequently exceeding recommended guideline values.¹ These findings were based on culture-dependent sampling in overcrowded households with limited ventilation.¹ Higher levels of PM, VOCs, and bioaerosols can irritate the airways and worsen asthma, allergies, and respiratory issues.^1,2

Seasonal factors affecting IAQ

Seasonal changes contribute to IAQ through their effects on temperature, humidity, ventilation, and human activity. During winter, colder outdoor air and the widespread use of heating systems reduce indoor humidity, creating dry indoor environments.

In primary schools in central Poland, winter sampling demonstrated increased PAH concentrations and stronger influence of outdoor air pollution on indoor air quality, particularly in the absence of efficient ventilation systems.² PAHs characteristic of coal and wood combustion were identified, suggesting outdoor seasonal emission sources contributed to indoor levels.²

There is also seasonal variation in ventilation patterns. Windows are often closed during colder seasons, reducing air exchange and facilitating pollutant accumulation. In school classrooms, bacterial levels were higher in poorly ventilated rooms and decreased with effective ventilation and after lessons.²

Seasonal microbial variation has also been documented in residential environments. In Delhi households, bacterial concentrations increased from winter to summer before declining in fall, whereas fungal concentrations progressively increased from winter to fall, with fine fungal particles predominating across seasons.¹ These seasonal trends were observed in cross-sectional sampling and were not designed to establish causal relationships with health outcomes.¹

Together, these findings indicate that seasonal IAQ changes are influenced by ventilation reduction, outdoor pollutant infiltration, occupancy-related emissions, and humidity-dependent microbial growth.^1,2

Health impacts of seasonal IAQ changes

Acute effects

Seasonal deterioration in IAQ has been associated with acute respiratory symptoms.

A prospective cohort study of 285 community-dwelling older adults in Hong Kong (2016–2019) reported 168 ARI episodes and demonstrated that lower indoor absolute humidity during cool seasons was associated with increased ARI risk, with a 6-day cumulative excess risk of −9.0% per unit increase in AH.³ No consistent association was observed during warm seasons, and associations with indoor temperature and relative humidity were less robust.³

These ARI episodes were defined by the onset of two or more symptoms including feverishness, cough, sore throat, headache, and myalgia.³ The study was observational in design and did not directly measure viral survival or transmission mechanisms.³

High levels of indoor bioaerosols in residential settings have been associated with headaches, allergic rhinitis, coughing, sneezing, and eye irritation in questionnaire-based surveys.¹ However, these associations were derived from self-reported symptoms rather than clinically verified diagnoses.¹

Chronic effects

Repeated or prolonged exposure to elevated indoor pollutants may exacerbate chronic respiratory conditions. Fungal bioaerosol concentrations exceeding WHO guideline values (500 CFU/m³) were observed in residential settings.¹ Fine fungal particles (>2.5 µm) capable of reaching bronchiolar and alveolar regions may promote airway inflammation and allergic sensitization.¹ Direct longitudinal evidence linking these measured concentrations to chronic disease progression, however, remains limited in the cited studies.

Mechanisms linking IAQ to respiratory health

Humidity and viral transmission

Indoor absolute humidity was statistically associated with ARI risk in cool seasons among older adults in Hong Kong.³ While laboratory studies outside the scope of these three papers have suggested humidity may influence viral stability, the cited cohort study demonstrated epidemiologic association rather than mechanistic confirmation.³

Bioaerosol particle size and deposition

Indoor bioaerosol size distribution influences respiratory deposition. Andersen impactor measurements demonstrated that fine fungal particles (2.1–3.3 µm and smaller fractions) were prevalent across seasons and are capable of depositing in lower bronchial and alveolar regions.¹ These measurements describe potential deposition patterns but do not directly quantify clinical infection risk.¹

Ventilation-dependent microbial load

Occupants represent a major source of indoor bacteria. In classroom settings, bacterial contamination decreased with improved ventilation and after occupancy periods ended.² These findings were based on short-term seasonal sampling and did not include direct health outcome measurements.²

Chemical pollutants (PAHs and VOCs)

Winter elevations in PAHs in schools were linked to outdoor combustion sources (e.g., coal/wood burning), demonstrating that external seasonal emissions influenced indoor chemical pollutant profiles.² The study did not directly evaluate respiratory health outcomes in the sampled schoolchildren.²

Mitigation strategies

Ventilation

Improving ventilation is one of the most effective strategies for mitigating poor IAQ. Evidence from school-based monitoring demonstrates that efficient ventilation reduces indoor bacterial contamination and moderates seasonal pollutant accumulation.²

Humidity control

Maintaining optimal indoor humidity may be important for respiratory health. Prospective evidence indicates that increasing indoor absolute humidity during cool seasons was associated with reduced ARI risk among older adults.³ Whether active humidification interventions would produce similar protective effects was not directly tested in the study.³

Air filtration

Given the persistence of fine fungal and bacterial aerosols in poorly ventilated indoor environments, filtration-based interventions may help reduce exposure to respirable microbial particles.^1,2 Direct interventional trials evaluating clinical outcomes were not included in the cited studies.

Key takeaways

Seasonal IAQ variation is supported by evidence from schools, residential households, and prospective cohort studies. Winter conditions were associated with elevated PAHs and ventilation-dependent microbial shifts in schools, while low indoor absolute humidity during cool seasons was associated with increased ARI risk in older adults in Hong Kong.^2,3

Residential studies in Delhi further demonstrated that bacterial and fungal bioaerosol concentrations frequently exceeded guideline levels, with fine particles capable of deep lung deposition.¹ However, these findings originate from geographically and demographically specific settings, and extrapolation to other populations should be made cautiously. Improving ventilation and maintaining adequate humidity during high-risk seasons may help reduce seasonal respiratory morbidity.^2,3

References

1. Kumar, P., & Singh, R. (2025). Microbial indoor air pollution in Delhi Metropolitan City is attributable to severe respiratory and general health effects among residents. Frontiers in Public Health 13. DOI: 10.3389/fpubh.2025.1626827. https://www.frontiersin.org/journals/public-health/articles/10.3389/fpubh.2025.1626827/full
2. Badyda, A., Muszyński, A., Affek, K., et al. (2024). Schools and Indoor Air Quality: seasonal variation. European Journal of Public Health 34 (3). DOI: 10.1093/eurpub/ckae144.797. https://academic.oup.com/eurpub/article/34/Supplement_3/ckae144.797/7844715
3. Han, L., Ran, J., Chan, K., et al. Indoor Environmental Factors and Acute Respiratory Illness in a Prospective Cohort of Community-Dwelling Older Adults. The Journal of Infectious Diseases 222 (6); 967-978. DOI: 10.1093/infdis/jiaa188. https://academic.oup.com/jid/article/222/6/967/5820877#google_vignette

Further Reading

• All Air Pollution Content
• Air Pollution and Lung Health
• Air Pollution and Eye Health
• Air pollution damages brains, as well as hearts and lungs
• Anti-Pollution Skincare: Protecting Skin in Urban Environments

Last Updated: Feb 19, 2026