Do Air Filters in HVAC Systems Offer Protection Against Coronavirus Indoors? It Depends

There are air filters that can catch particles laden with SARS-CoV-2. But whether or not the filtration happens depends on other factors.

By Leslie Nemo
Jul 17, 2020 4:45 PMNov 3, 2020 4:48 PM
Air Filter Installation - Shutterstock
(Credit: MBLifestyle/Shutterstock)


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Heating, ventilation and air conditioning (HVAC) systems have never been the hottest conference or cocktail hour topic. “I’ve never gotten more than 15 people in a room that wanted to talk about ventilation,” says Theresa Pistochini, the engineering manager at the Western Cooling Efficiency Center at the University of California, Davis. But during a pandemic, her webinars draw hundreds of viewers. 

The sudden ventilation fascination comes from businesses and schools trying to operate while keeping indoor air as virus-free as possible. The American Society of Heating, Refrigerating and Air-Conditioning Engineers, or ASHRAE, weighed in on this issue by saying that air filtration systems can reduce how much of the coronavirus is indoors. You can browse a range of new guides to the best and worst air purifiers on the market. But when it comes to a new filter actually catching viral particles, a lot more needs to happen besides swapping a dirty screen for a clean one.

New Understanding Means New Interventions

Interest in HVAC systems is due in part to changing ideas about how the virus reaches new people. If the coronavirus was only dispersed by big spit droplets, no one would be talking about the efficacy of ventilation systems, says Brent Stephens, an indoor air pollution and filtration researcher at the Illinois Institute of Technology. Those globules would hit the ground long before a fan would suck them into a filter. But more scientists are agreeing that the virus moves through smaller particles, too — ones that float through the air and can get trapped by some filtration systems. 

The question of how the virus spreads is complicated by conflicting definitions of “droplet” in the research community. When aerosol scientists talk about droplets, they mean pretty big globs. “Those are like, ballistic droplets that land in your eye,” Stephens says. The particle size the WHO and CDC calls a droplet — a fleck 5 micrometers across — is small enough that Stephens and his colleagues consider those specks capable of floating through the air. Though the WHO has yet to agree with the hundreds of other scientists that say the coronavirus spreads via smaller particles, what the organization considers a “droplet” already qualifies as an airborne fleck in the eyes of other professionals.

Read next: Are Ultraviolet Sanitizing Lights Safe for Humans?

The good news is that ventilation, which means bringing fresh outdoor air indoors, can dilute concentrations of virus particles. There are also filters that trap some of the tiniest virus-carrying spit bits. One variety called a MERV-13 filter takes on the majority of particles between 0.3 and 1 micrometers in size. A more restrictive option, the HEPA filter, catches 99.97 percent of 0.3 micrometer particles. Offices, schools and restaurants may opt to install these filters in ventilation systems. 

The Filter Is Only Half The Battle

Unfortunately, many buildings struggle to provide adequate ventilation and filtration.

Pistochini saw this while studying ventilation in California public school classrooms. She and her team inspected the recently-updated HVAC systems in 104 classrooms across the state and found that 51 percent of the classrooms had too little ventilation. Per industry recommendations, state regulations say that every second, seven liters of outdoor air needs to flow into the room per student. The team calculated that the average classroom only moved about 70 percent of the air it should. “We were really surprised we saw the prevalence of problems that we did,” Pistochini says.

Some of these issues might be due to insufficient expertise and oversight. Though the industry association ASHRAE has recommendations on how building ventilation should be maintained, individual state protocols decide how that happens, Pistochini says. In California, the public schools are expected to do their own policing of their HVAC functionality. Installation and maintenance of HVAC systems is also a technical job. Organizations issue certifications to qualified repair people, and there are specific tools required. “Districts need to do this with certified technicians in order to really get it right,” Pistochini says.

She also thinks each classroom should have a carbon dioxide detector installed. Levels of the chemical — which we all exhale — serve as a proxy for how much fresh air moves into the room. If CO2 concentration rises above what state-specified airflow would maintain, then the school building knows it’s time to inspect the HVAC system. 

At the end of June, the California legislature introduced a bill that would provide funding for classroom CO2 detectors and inspections of school HVAC systems before reopening. The text is very similar to what Pistochini and her colleagues put on their program website.

Filtering Solo

Buildings too old to keep up with modern air filtration infrastructure might need stand-alone, plug-in units. This could be the case in, say, decades-old and historic college campus classrooms, Stephens says. 

Before installing one of these filters, there are a few things to look for on the box. One is that the machine uses a HEPA filter, the more aggressive of the two filter options. The device also needs a Clean Air Delivery Rate. This value shows how much air the system filters per second, depending on the particle size you’re targeting — again, for HEPA filters, that’s 0.3 micrometers. The number also proves a third party vetted the filter, a necessary qualification. "The air cleaner industry is fraught with people selling technologies that don’t really work," he says. Finally, the filter ought to say what square footage room it can handle. 

Freestanding devices can be useful even in environments with an HVAC system, Stephens says. Those building-wide units often cut the fan once the room is at the right temperature — and constant airflow is crucial to the whole filtration concept. 

Though Stephens thinks improved air filtration should be a line of defense below social distancing and mask-wearing, he’s helping his campus prepare for improved air purification. And Pistochini adds that improved filtration doesn’t mean schools should open. There are other factors to consider.

But once the pandemic is over, there are still benefits to gain from proper classroom ventilation. Research has shown that attendance records and academic performance drop in poorly-ventilated schools. And if your office has bad airflow, any of its accompanying mental slog might follow you too. “A lot of important decisions are made in board rooms and conference rooms where you have a dense number of people and expect good decisions to be made,” Pistochini says. 

Ultimately, Pistochini hopes the need to minimize coronavirus exposure will motivate these school HVAC changes in California. “If this isn’t enough, I don’t know what is.”

Editor's note: This story was updated to clarify that ventilation specifically refers to fresh air entering a room.

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