How to Ventilate for COVID-19

TB guidance gets us started

Drug-resistant TB has been causing problems in hospitals for many decades — it can infect 80% of the people who spend a day in the same room.

Air Changes per Hour (ACH)

If there’s one thing to take away from this note, it is to pick a number of “air changes per hour” (ACH) and make it part of your checklist for each room in a building. There is no “proven” number for preventing the spread of SARS-CoV-2, but I will explain how we arrived at the numbers we have used.

  • Since SARS-CoV-2 is slightly less infectious than TB in most cases (it doesn’t infect everyone in one day) it seems prudent to look in the “lower” range of 6–12 ACH used in hospitals. Still, in places where people talk a lot (or sing), even more air movement may be required.
  • While portable HEPA filters are a big benefit, they do not filter all air equally in the room, so we should consider over-sizing these units (>10 ACH) to compensate.
  • Mirroring this concern about portable units, EPA warns that portable air cleaners might not be effective enough:

Preliminary recommendations

  1. Choose a number of “air changes per hour” between 6–12.
  2. For each technology considered, write down how they would sum up to meet this number for your room.

First-level options (less expensive)

  • MERV-13 particle filters should be added to all HVAC returns — this is sufficient to filter the virus according to several sources. Most HVAC systems give relatively low air changes/hour, but sometimes they can be run in a way that gives more airflow.
  • Portable HEPA filters should be specified considering sound level, airflow direction (vertical is better), and CADR (CFM). Consider over-sizing portable HEPA — we chose 150% of the desired ACH, but this is only a first approximation.
  • (Especially if you already have some): use exhaust fans in ceilings and windows when possible. Negative pressure (blowing to outdoors, filtered when possible)
  • Doors and windows always open

Next-level, more expensive options:

  • In higher-risk areas like bathrooms and nurses’ stations, or rooms with little fresh air available (like basements), add upper-room UV-C lamps. This requires that ceilings are above 8 feet tall so the lamps don’t shine into people’s eyes. Properly sized, UV-C is equivalent to 25 ACH, so it is the gold standard for risky spaces.
  • Whenever you use upper-room UV-C make sure you have “air mixing” such as from a low-speed ceiling fan
  • If cost allows, consider adding UV-C lamps to the air handlers in your HVAC — especially if your air handler has trouble with the extra pressure due to filters

Sizing Portable HEPA filters

Don’t trust manufacturer “square footage”

  • 5 air changes with 8 foot ceilings: CADR Required = 0.66 * SF
  • 10 ACH with 8 foot ceilings: CADR = 1.33 * SF
  • 5 ACH with 10 foot ceilings: CADR = 0.83 * SF
  • 10 ACH with 10 foot ceilings: CADR = 1.66 * SF

Considerations about air movement direction

  • When thinking about the Guangzhou Restaurant diagram: people were infected in the direction of flow from what may be a sidewall or mini-split that blows air horizontally (carrying virus across the room)
  • Takeaway: use caution with horizontal air movement, try to move air vertically when possible
  • Try to re-position sidewall supply ducts and mini-splits so they do not carry virus from one seat to another

Safety of HVAC, overall

After the Ghangzhou restaurant incident, there was some concern that all HVAC was somehow harmful. ASHRAE spoke up and suggested that proper use of HVAC equipment could be used to filter viral particles and should not always be considered a risk.



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Michael Herf

Michael Herf

co-founder of f.lux, finding the connections between circadian rhythms, sleep, healthy buildings, and light. (Previously made Picasa.)