Why ventilation and filtration won’t COVID-disinfect your indoor air

In the White House’s National COVID-19 Preparedness Plan plan, within the “Clean Air in Buildings Checklist,” lurks a risky landmine: “ventilation and filtration.” That approach, the latest proposed “Operation Warp Speed,” is incompatible with most public building ventilation systems, would be expensive, noisy, windy, and worst of all, won’t adequately disinfect the air.  

Far superior, cheaper, safe and effective proven solutions are available. Here’s some simple arithmetic to understand why. 

First, the scientific reasoning: Filtering air after it leaves the room does zero to prevent transmission to people sitting in a room with someone repeatedly exhaling viruses. That’s why many experts favor in-room air disinfection.  

What about air filtration? High-efficiency particulate air filters — HEPA filters (also called MERV-17) can remove 99.97 percent of particles larger than 0.3 microns. That’s great for tuberculosis (1-5 microns), and while coronavirus and the flu virus are both 0.2 microns or smaller, HEPA filters can efficiently remove them also in laboratory conditions, floating dry or attached to water, via diffusion and interception. 

But in real-world conditions, for a space just 20 feet by 30 feet, CDC scientists reported that one or two HEPA air cleaners could spread an infectious aerosol around the room and could increase particle exposure depending upon the locations of the infectious source (usually unknown), the HEPA units, and the possible patients-to-be. Overall, the HEPA plug-ins provided a 6 percent exposure reduction per increased effective air change per hour. Masking alone beat or equaled even two HEPA units with no masking in every scenario. Moreover, the studies only test the best HEPA portables, marketed units that don’t keep cleaning the same air. 

Yet ventilation and filtration advocates are now promoting even less efficient filters, (MERV-13) that are only 66 percent effective for that size particle. That is because our heating/ventilation/air conditioning (HVAC) system blowers aren’t able to move air through those very tight HEPA filters. For a 100-200 cubic feet room with a ceiling less than 7.5 feet, whose air you can’t easily exhaust outside, a wall-plug HEPA air purifier is far better than nothing. But for disinfecting larger, taller rooms, forget about HVAC filtration alone as a cure.

Understanding why the ventilation cure fails requires multiplication. The American Society of Heating, Air Conditioning and Refrigeration Engineers (ASHRAE) sets standards (62.1) that most states and Centers for Disease Control and Prevention rely upon. A 9,000 cubic feet school classroom requires 10 cubic feet per minute of fresh outside air per occupant. That’s 300 cubic feet per minute for 29 students and a teacher, 18,000 cubic feet per hour — two air changes per hour to adequately remove odors and carbon dioxide.  

Since many HVAC systems are designed to provide 17 to 33 percent outside air — it’s too energy-inefficient and expensive to condition more of that cold or hot and humid outside air — at least 900 cubic feet per minute of fresh air is needed to meet the outside air requirement. Recalling CDC’s recommended six to 12 air changes per hour for minimal disinfection, multiply that 900 cubic feet per minute by three to reach the minimum of six air changes per hour and arrive at 2,700 cubic feet per minute of fresh air needed per classroom.  

If our classroom’s ductwork and registers (vents) were optimally built to have about 3 tons of cooling capacity delivered by a maximum of 1,500 cubic feet per minute, existing ductwork, registers and HVAC blowers cannot possibly move close to double that air volume required for minimal disinfection. Also, the noisy wind tunnel that the classroom would become would be intolerable — so the magical but inadequate filtration solution is invoked.  

Exhaust fan ventilation to the outside is a good idea for rooms where short stays won’t make the temperature intolerable and in bathrooms, where sick people go for private respite. Blow thick cigar smoke underneath those fans to test if they truly exhaust — defying existing codes, some noisy bathroom fans just blow into the attic.  

Disinfecting indoor air is crucial. For a ceiling of at least 7.5 feet, the least expensive, highly effective, and safe solution is a low-dose upper-room ultraviolet germicidal irradiation system. The 9,000 cubic feet classroom would be covered for $850 with a ready-made, plug-and-play system with nearly 1 watt of ultraviolet C output, or for $220 if someone spends an hour putting parts together for a bare-bulb unit with a small sheet of simple reflective aluminum below, tilted slightly up, so room occupants can’t directly view the bulb itself. 

Just two seconds of exposure 3 feet away or 60 seconds of exposure 30 feet away, providing total radiant energy of 600 microJoules/sq cm, will inactivate 90 percent of coronaviruses, tuberculosis and influenza. A ceiling fan or a $25 floor fan tilted along a wall, running at slow quiet speed, completes the installation, assuring exhaled air moves up into the disinfection zone. A school with 50 classroom equivalents would cost $42,500 (off-the-shelf) or $11,000 (self-assembly), plus the cost for fans. The 30W-40W ultraviolet germicidal irradiation bulbs consume minimal electricity and require replacement after 9,000 hours (one year.) Maintenance? Turn them off and get on a ladder with a gentle feather duster every 3 months.  

The Government Accountability Office reported in June 2020 that 40 percent of school districts have 50 percent of schools needing basic ventilation overhauls. Ventilation installers and consultants are eager to sell new filters, bigger blowers, consulting services and even gimmicky unproven ionizers as infrastructure money becomes available.  

Science-informed policy will follow the arithmetic and won’t be fast-talked into some new version of the Segway bust. Adequate and updated ventilation is essential for deficient indoor spaces but is not the path to viral air disinfection. 

Bruce L. Davidson, MD, MPH, a pulmonary and public health physician and member of the American Society of Heating, Refrigeration and Air Conditioning Engineers, is a clinical professor of Medicine at Washington State University College of Medicine and was president of the National Tuberculosis Controllers Association.