Preventing aerosol transmission of respiratory diseases in schools and childcare centers

Since the beginning of the COVID-19 pandemic, uncertainty about how the virus is spread—and how to control that spread—has led many school districts to turn to remote learning and many childcare centers to close.

While the benefits of reducing the spread of COVID-19 are clear, the impacts of continued virtual learning and a lack of reliable childcare for families is significant, particularly due to the disproportionate impact on children and caregivers living in under-resourced communities. As our understanding of COVID-19 and its transmission has grown, we are better positioned to create safe learning environments for all children.

Historical perspective

The transmission of diseases through the air has puzzled humanity since the times of Hippocrates (400 B.C.), when people believed that bad smells (or miasma as it was later called) carried diseases. This route of transmission was the prevailing hypothesis for the cholera outbreak that John Snow investigated back in 1854. His investigations led to the conclusion that cholera was not transmitted through the air but through drinking water from a public pump which was polluted with human waste.

While the miasma theory was correctly debunked by Snow in the transmission of cholera, scientists pursued the theory as an explanation for the transmission of other diseases, leading to the development of the field of aerobiology in the 20th century and the discovery of airborne diseases like measles, anthrax, and tuberculosis.

The COVID-19 pandemic reignited the debate about how respiratory diseases are transmitted. This debate brought clarity to our understanding of the role of aerosols—low-weight particles which can remain suspended in the air—in transmitting pathogens. The overall body of scientific evidence based on principles of physics, microbiology and epidemiology suggests that respiratory diseases like COVID-19, influenza, and measles, are mainly transmitted through inhalation of respiratory aerosols.

Practical recommendations for schools and childcare centers

Considering the primary role of aerosols in the transmission of respiratory diseases, the main measures to reduce this risk in schools and childcare centers are:

• Ventilating classrooms and indoor spaces with fresh air. Or in case of bad weather or poor air quality, using portable air filters (e.g., those that use HEPA filters). These interventions are particularly important indoors where people share the air, such as in schools, day care centers, and public transportation (that’s why airplanes use HEPA filters). The idea is to dilute the pathogen-containing aerosols or remove them from the air through mechanical filtration such that their concentration is so low that the probability of widespread airborne transmission is unlikely. Currently, we should avoid aerosolizing disinfectants or using electrostatic filters and ionizers since these tools generate chemicals that hurt our respiratory system when inhaled and have not been studied extensively.
• Wearing high-efficiency face-fitting masks (N95, KN95, KF94) in shared indoor spaces. Although cloth and surgical masks are effective in reducing transmission, the COVID-19 Omicron variant is much more contagious. Better masks further reduce transmission through two improved mechanisms: 1) capturing virus-containing aerosols from the contagious people, and 2) filtering out aerosols from the air we breathe.
• Maintaining >1 meter distance from others in public spaces. This measure is effective because respiratory aerosols are more concentrated near contagious people, and they dilute with distance just like cigarette smoke does, resulting in a lower number of respiratory aerosols inhaled.

Considerations in the built environment

It is important to recognize that older school buildings (and likely daycare centers) historically have poorer ventilation rates. The cost of upgrading electrical systems in these buildings to support better HVAC systems has been estimated to be as high as $30,000 per classroom. While portable air filters are readily available and require less energy than HVAC units, the number of classrooms that would require an electrical system upgrade and the amount of portable air filters needed is highly variable.

These are important considerations that must be discussed by policy makers in dialogue with schools’ and daycare centers’ administrators for an equitable allocation of resources. This step towards better engineering controls is key as society transitions into the endemic phase of COVID-19 where, for example, masking ceases to be required if vaccination rates are high and disease prevalence is low. In-room air filtration devices are likely to be the 21st century equivalent of drinking fountains—a resource that provides safe air for everyone, reducing the chance of respiratory disease outbreaks.

How to check for proper ventilation indoors

As indoor air filtration becomes more common, opening windows remains a practical risk-reduction strategy when weather and outdoor air quality allow. One of the easiest ways to check if we are in a space with sufficient fresh air is to monitor the concentration of carbon dioxide (CO2) in the air. Because we exhale CO2, it will accumulate in rooms without sufficient ventilation.

The concentration of CO2 in fresh air is around 400 parts per million; a high concentration of CO2 indoors indicates that we are breathing little fresh air and that we are breathing air that has been exhaled by others. If the weather or outdoor air quality are not ideal to open the windows, mechanical filtration in-room with DIY MERV13 filters or higher (known as Corsi-Rosenthal boxes) and portable HEPA filters will reduce the concentration of respiratory aerosols indoors—but the CO2 concentration will remain higher than 400 ppm.

Key scientific support for these methods of COVID-19 prevention

Five principal scientific reasons that support aerosol inhalation as the primary route of transmission of COVID-19 are:

1. Super-spreading events can be best explained through aerosol transmission. The theory of transmission through ballistic droplets or through contaminated surfaces cannot adequately explain how a single person could infect people far away from them, which is a pattern frequently observed in these events.
2. Infections have been reported between guests in adjacent hotel rooms who were never in the same room. This is only possible if the rooms share unfiltered air.
3. Studies have shown that humans emit thousands of aerosols compared to only a few heavy droplets when speaking. Recent studies have also found that fine aerosols have higher viral RNA loads (<5 microns in size) than bigger ones (>5 microns in size). Together, this evidence implicates fine aerosols as the dominant transport vehicles for the virus that causes COVID-19.
4. Infectious virus has been detected in air samples in hospital rooms and a car with a contagious person inside in the absence of what in medicine are known as “aerosol-generating procedures” such as the intubation of patients. Although the measles virus or the tuberculosis bacteria have not been detected in the same way, they are universally accepted as aerosol transmitted pathogens because they are very contagious.
5. Finally, schools with increased ventilation with fresh air and better air filtration have experienced 35% to 48% fewer cases of COVID-19 than schools without these preventative measures. Ventilation and filtration are specifically designed to reduce the number of respiratory aerosols from the air we breathe and have no direct effect on large droplets that fall to the floor. This is “real world” evidence that ventilation with clean air and air filtration noticeably decrease the probability of infection in indoor spaces.

General public health tools

Vaccination remains the key in preventing serious illness and mortality, and thus it is the fundamental tool to keep COVID-19 and other diseases under control. Washing our hands should also remain a common practice, even though contact with contaminated surfaces has been determined to be a minor transmission route for COVID-19 based on epidemiological and environmental sampling studies. Good hygiene is still key to preventing enteric and other infectious diseases, as we have learned from improvements in food, water, and sanitation management since the times of John Snow and the cholera outbreak in 1854.

Avoiding future outbreaks

It’s important to acknowledge that the objective of these preventative actions is to reduce the number of cases to the point that they are the exception and not the norm in our schools and childcare centers. Infections are always possible due to the inherent variability in human behavior and environmental transport of infectious aerosols, but these prevention measures reduce the likelihood of their occurrence.

In this next decade and beyond, we could avoid the cold and flu seasons that interrupt our lives by raising the standard of air quality in public indoor spaces, starting with the buildings where our children spend most of their daytime. We can start now to ensure a future in which the air they breathe is as safe as the water they drink.