Why ventilation and filtration won’t make indoor air COVID-free
In the White House’s National COVID-19 Preparedness Plan, in the “Checklist for Clean Air in Buildings”, hides a risky landmine: “Ventilation and Filtration”. This approach, the latest proposed “Operation Warp Speed”, is incompatible with most ventilation systems in public buildings, would be expensive, noisy, windy and, even worse, will not adequately sanitize the air.
Far superior, cheaper, safe and effective proven solutions are available. Here are some simple calculations to understand why.
First, the scientific reasoning: filtering the air after it has left the room does nothing to prevent transmission to people sitting in a room with someone who is repeatedly exhaling viruses. This is why many experts favor disinfection of the ambient air.
What about air filtration? High Efficiency Particulate Air Filters – HEPA (also known as MERV-17) filters can remove 99.7% of particles larger than 0.3 microns. This is fine for TB (1-5 microns) but unfortunately the coronavirus and influenza virus are both 0.2 microns or less. HEPA will remove viruses attached to large water droplets, but nature does not impose this requirement on every virus particle; dry viruses also swirl around in the surrounding air.
Yet proponents of ventilation and filtration are now promoting even less efficient filters (MERV-13) which are only 66% efficient for this particle size. This is because our heating/ventilation/air conditioning (HVAC) system blowers are not capable of passing air through these very tight HEPA filters. For a room of 100 to 200 cubic feet with a ceiling less than 7.5 feet, from which you cannot easily exhaust the air outside, a wall-plug HEPA air purifier is much better than nothing, but to sanitize larger rooms, forget about HVAC filtration as a cure.
Understanding why the vent 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 on. A 9,000 cubic foot classroom requires 10 cubic feet per minute of fresh outside air per occupant. That’s 300 cubic feet per minute for 29 students and one 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 17-33% outdoor air – it is too energy intensive and expensive to condition more of this cold or warm and humid outdoor air – at least 900 cubic feet per minute of fresh air are needed to meet the needs. need outside air. Recalling the six to 12 air changes recommended by the CDC 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 the ducts and registers (vents) in our classroom were optimally constructed to have approximately 3 tons of cooling capacity provided by up to 1,500 cubic feet per minute, the ducts, registers, and HVAC fans existing ones cannot move nearly double the volume of air required for minimal disinfection. Moreover, 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 under these fans to test if they really exhaust – defying existing codes, some noisy bathroom fans simply blow into the attic.
Disinfection of indoor air is crucial. For a ceiling of at least 7.5 feet, the cheapest, most effective and safest solution is a low-dose ultraviolet germicidal irradiation system in the upper room. The 9,000 cubic foot classroom would be covered for $850 with an out-of-the-box plug-and-play system with nearly 1 watt of ultraviolet C output, or $220 if someone spends an hour assembling. parts for a bare bulb unit with a small, simple reflective aluminum sheet underneath, angled slightly upwards, so that the occupants of the room cannot directly see the bulb ioneself.
Just two seconds of exposure at 3 feet or 60 seconds of exposure at 30 feet, delivering a total radiant energy of 600 microJoules/cm², will inactivate 90% of coronaviruses, tuberculosis and influenza. A $25 ceiling fan or floor fan tilted along a wall, running at a low, quiet speed, completes the setup, ensuring exhaled air rises into the disinfection zone. A school with 50 class equivalents would cost $42,500 (ready-made) or $11,000 (build-it-yourself), plus the cost for fans. 30W to 40W ultraviolet germicidal irradiation bulbs consume minimal electricity and should be replaced after 9,000 hours (one year). Maintenance ? Turn them off and climb a ladder with a soft feather duster every 3 months.
The Government Accountability Office reported in June 2020 that 40% of school districts had 50% of schools requiring basic ventilation reviews. Ventilation installers and consultants are eager to sell new filters, bigger blowers, consulting services, and even fancy unproven ionizers as infrastructure money becomes available.
A science-based policy will follow arithmetic and will not be quickly introduced into a new version of the Segway Bust. Adequate, updated ventilation is essential for deficient indoor spaces, but is not the route to viral air disinfection.
bruce I davidsonMD, 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 past president of the National Tuberculosis Controllers Association.