The transition from an outdoor summer to an indoor classroom environment – Exposure and immunity
Children are back at school in late August/early September, happy to see their schoolmates again and eager to absorb more knowledge. But in just a couple of days, they are back at home, infected and sick. Why does this happen every year? Many factors explain why transitioning from fresh outdoor summer air to high-density classrooms exposes children to a hidden dual challenge: late-season pollen and the season’s first viruses.
Untold aspects of schools and children's exposure
During late August/early September, children transition from low-density outdoor spaces to the opposite in enclosed indoor spaces. That is when a dual-exposure event happens: children do not face viruses or allergens in isolation; they face a simultaneous surge of both.
In summer, children play outside in fresh outdoor air, yet invisible pollen particles still float around during the late-summer pollen season, with peaks of weed pollens, grasses and fungal spores. In early autumn, within 1-3 weeks of school reopening, children spend their time in enclosed indoor spaces, and sudden spikes in respiratory viruses follow.
The research connects all the dots
A hidden link exists between pollen–virus interactions, classroom aerosol dynamics and indoor environmental quality (IEQ) that impacts children. Pollen not only affects already allergic children, but all children exposed to high ambient pollen outdoors can also become immunocompromised against viral loads indoors. Regardless of allergic predisposition, exposure to airborne pollen actively weakens local children's antiviral mucosal immunity. The surge in pollen and virus transmission at the reopening of schools in autumn is driven primarily by a massive jump in cumulative contact hours and shared indoor air.
School and close-contact
- Social contacts in a school increase the spread of infection. Opening schools increases high-density, close-contact interactions among children by up to 3 to 4 times compared to summer holiday levels, accelerating the reproductive number (R0) of seasonal respiratory viruses within 7 to 14 days of school start.
- High student density + inadequate air exchange = exponentially higher cumulative exposure. When air changes per hour (ACH) are low, viral particles accumulate in the breathing zone, ensuring nearly 100% exposure for susceptible children.
- Children are not small adults. Children have smaller airways, higher breathing rates per kilogram of body weight, and less developed mucosal defences than adults. Thus, they inhale a higher proportional dose of airborne viruses and pollen, making them far more vulnerable to pollen and virus exposure.
Pollen and virus versus immunity
- Pollen and virus interaction suppresses local immunity. Exposure to high airborne pollen concentrations suppresses the nasal epithelium's innate antiviral defence, regardless of allergy status.
- Pollen exposure weakens innate defence against respiratory viruses. Pre-exposure of children's airway epithelium to pollen reduces the antiviral interferon response to the common cold. Crucially, this effect was observed in both allergic and non-allergic individuals.
Symptoms and exposure
- Differences in children with and without allergy. Pollen exposure for children with allergies or asthma causes chronic baseline airway inflammation. These inflamed tissues, especially in mucosal areas, allow respiratory viruses to enter and replicate much more easily.
- Symptoms can overlap, thus leading to misdiagnosis. Pollen allergies and viral colds share similar symptoms (stuffy nose, coughing, fatigue, etc). Children suffering from unmanaged pollen exposure rub their eyes and noses frequently, increasing hand-to-face transmission and secondary viral inoculation.
Rebreathing
- Rebreathing indoor air directly raises children's risk of inhaling infectious viral aerosols. Elevated CO2 in classrooms directly correlates with an increased probability of inhaling infectious viral doses, exhaled by children and their classmates. This is based on the classic Wells-Riley airborne transmission model, using the rebreathed CO2 fraction as a direct proxy for viral infection risk in shared indoor spaces like classrooms.
Dynamics of indoor air and ventilation
- Mechanical ventilation rates and high-efficiency filtration break the transmission chain. Maintaining ventilation rates of 5 to 6 air changes per hour (ACH) in classrooms, combined with particulate filtration (e.g., F7/F9 or ePM1 filters), reduces airborne viral infection risk by over 80–90% compared to natural or baseline ventilation (< 2 ACH).
- Health aspects are directly interlinked with learning performance. Higher outdoor air supply rates that keep CO2 levels below 900 ppm increase academic performance (speed by 12%, accuracy by 2%) while also reducing cross-infection absenteeism.
The science behind it
The primary cause of the immediate sick wave in late August/early September is not a sudden collapse of children's individual immune systems or weakened immunity, but a combination of the many factors mentioned, increase in contact rates and airborne viral concentration, immune suppression, pre-exposure and predisposition, differences for children with and without allergies, symptom overlap and misdiagnosis, rebreathing indoor air, mechanical ventilation with effective air change and filtration, and direct correlation between health to learning performance.
Based on the research
Buonanno, G., Stabile, L., & Morawska, L. (2020). Estimation of airborne viral emission rates for SARS-CoV-2: Implications for airborne transmission risk assessment. Environment International.
Damialis, A., Gilles, S., Sofiev, M., et al. (2021). Higher airborne pollen concentrations correlated with increased SARS-CoV-2 infection rates, as evidenced from 31 countries across the globe. Proceedings of the National Academy of Sciences (PNAS).
Eames, K. T., Tilston, N. L., Brooks-Pollock, E., & Edmunds, W. J. (2012). Measured social contacts in a school: Impact on the spread of infection. Philosophical Transactions of the Royal Society B: Biological Sciences.
Gilles, S., Blume, C., Wimmer, M., et al. (2020). Pollen exposure weakens innate defence against respiratory viruses. Journal of Allergy and Clinical Immunology.
Landrigan, P. J., Garg, A., & Dzurik, A. J. (2004). Children’s health and the environment: Public health issues and solutions. World Health Organisation (WHO).
Wargocki, P., & Wyon, D. P. (2021). Research-based recommendations for achieving high indoor environmental quality in classrooms to promote learning. International Centre for Indoor Environment and Energy.
Wells, W. F. (1955). Airborne Contagion and Air Hygiene: An Ecological Study of Droplet Infections. Harvard University Press.
Riley, E. C., Murphy, G., & Riley, R. L. (1978). Air hygiene of an epidemic school outbreak of measles. American Journal of Epidemiology.
Rudnick, S. N., & Milton, D. K. (2003). Risk of indoor airborne infection transmission estimated from carbon dioxide concentration. Indoor Air.
