COVID-19 has brought increased attention to the quality of indoor air and the effect that ventilation has on reducing disease transmission in indoor spaces.
A recent infrastructure survey reported that, of the nearly 100,000 operating public school buildings in the U.S., more than a third have an immediate need for improvements to ventilation systems that help control indoor-air quality and the spread of “aerosols.”
Aerosol is the term used to describe the millions of microscopic particles that float in air – both indoors and out. People constantly inhale and exhale aerosols, some of which include allergens, particles from automobile exhaust, wildfire ash and microbes.
An environmental-engineer team at the University of Colorado has been studying the microbiological components of indoor air, called “bioaerosols”, for more than 25 years. We have surveyed the ventilation systems of hundreds of K-12 classrooms, health-care centers and restaurants. And we have provided building managers with affordable plans to improve indoor air quality.
Our own work as well as others’ has found that many classrooms are unfortunately poorly ventilated, and that better ventilation can reduce student absences resulting from illnesses – both during a pandemic and more normal times.
After surveying the installation of air-filtration systems in the past year, we found that they can significantly improve air quality in classrooms by lowering aerosol levels, which in turn can lower the risk of COVID-19 transmission But effective installation is essential.
A new age of filtration
As the pandemic continues to draw attention to the need for better ventilation and indoor-air quality, many academic institutions, government agencies, nongovernmental organizations and professional building science societies have been promoting better building-management practices to improve school ventilation.
Some building scientists have called for bringing the ventilation conditions in schools up to the levels prescribed for medical clinics. Unfortunately, the infrastructure investments required for that sort of improvement are well out of practical reach for many public buildings: Between 2008 and 2017 alone, state capital funding for schools was cut by $20 billion, or 31%.
In the absence of money for building improvements, simple in-room filtration technologies have been installed in some schools to improve ventilation in classrooms where many students spend their days in close quarters. However, these filters have only been deployed in a small fraction of public schools in the country.
This technology, called high-efficiency particulate-air, or HEPA, filtration, was born in the aerospace industry more than 50 years ago. HEPA filtration has been proved to efficiently remove microscopic airborne particles – including respiratory viruses – from air in high-occupancy spaces like classrooms.
Over the past few years, a new generation of HEPA filters have emerged from the U.S. commercial sector. These filters are more compatible with educational settings and less intrusive than their research-grade counterparts that are commonly used in the aerospace and pharmaceutical industries, for which “clean rooms” are needed. These latest models include improvements like multidirectional intake, reduced noise, lower power requirements, better durability and relatively small footprints.
HEPA filters have also become more widely used over the past couple of decades in homes in response to the recognition of rising asthma rates among children. But until the COVID-19 pandemic, they were rarely used in public school settings.
Bringing fresher air to classrooms
In the 2021 spring academic semester, our team installed hundreds of HEPA filters in public elementary classrooms in Denver, Colorado, the largest metropolitan school district in the Mountain West. These improvements were possible because of recent cooperation between the University of Colorado, the Intel foundation and the Carrier Corporation, a multinational ventilation-equipment company. Together, these organizations contributed more than $500,000 for large-scale ventilation assessments, HEPA filter installations and other air-quality improvements for Denver-area schools.
A yet-unpublished poll of teachers in many of those classrooms overwhelmingly found that this new generation of HEPA filters were welcome and easy to accommodate in classrooms.
But like all engineered products, air filters won’t be effective without proper installation. Our team’s field studies demonstrate that a simple “plug-and-play” approach will not be enough for the complicated reality of aerosol exposures in classrooms. In many situations, we found HEPA filters that were undersized and placed inappropriately – some, for instance, were facing a wall or were in a remote corner – and some that were not even turned on.
Networks of HEPA filters need to be thoughtfully installed, and the process must take account of other considerations such as the performance of existing ventilation systems, ceiling heights, desk layouts and the presence or absence of ceiling fans. HEPA filters can only work up to their full potential if schools have the right number of them, they are of the appropriate size and are placed in optimal positions.
The best HEPA filter installations take into account details like student seating charts, high-traffic areas and other variables related to student behavior. Fortunately, building managers and custodial staff can be trained, in little time, to install, operate and maintain HEPA filters in classrooms.
Air quality improvements are an investment in health and schools
A 2020 review of indoor-air quality estimates that an individual HEPA filter, tailored to elementary school classrooms with average energy use, will cost about $361. This finding jibes with our team’s experience in the Denver Public Schools system, where we typically installed at least two units per classroom at a cost of less than $800 per room. We estimate that this is roughly equal to the cost of one extra textbook per student over an academic year. In our opinion, this is exceedingly worthwhile.
In-room HEPA filtration is a long-term investment that will supplement existing ventilation systems. And although COVID-19 was the impetus for the installation of many HEPA filters, they are effective for far more than just reducing exposures to airborne viruses. Well-maintained and properly functioning filtration systems can also reduce exposure to wildfire ash, as well as allergens and other unwanted particles like automobile exhaust, tire detritus and construction dust.
But even the best indoor HEPA filtration cannot guarantee protection from airborne respiratory threats in schools. HEPA filters are effective only as part of a larger set of precautions. Ultimately, masks, distancing and limiting the number of students that will be packed into tight spaces will determine how well students are protected from COVID-19.
HEPA filters are the modern analogy of “seat-belts” for indoor-air quality in the age of COVID-19. If fitted correctly, they can only help lower the exposures to COVID-19 and other aerosols that students experience during their school days.