For anyone who lands here in 2023: ASHRAE 241 is now published, and has better advice than this article. We should view ACH (“air changes per hour”) as a good way to ventilate unoccupied spaces. But a room with one person should be ventilated differently than one with 100, so the new advice provides a certain amount of clean air per person over time (basically CFM per person). The main idea is to dilute bad air, and fresh air and filtration add up to do this. You can read about this distinction in this article by Joey Fox, and if you need to use ACH you can use a converter like this one.
Since the pandemic started, lots of people have been infected indoors. Good ventilation helps reduce the number of viruses you breathe in, and it can make spaces a little bit safer during the COVID-19 pandemic.
First, you should read Zeynep Tufekci’s article on the issues associated with “short-range aerosol” transmission. COVID-19 is not measles, but a number of events (a cruise ship, a choir practice, a restaurant) and, relatively speaking, the lack of “super-spreader events” like this outdoors, are making us realize that you can get infected through the air.
If that’s possible, how should we fix it?
We Need to Talk About Ventilation
How is it that six months into a respiratory pandemic, we are still doing so little to mitigate airborne transmission…
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.
We don’t have to look very far to find ventilation guidance from the CDC, since moving air around has been an essential part of healthcare facilities for several decades. In places with infectious TB patients, hospitals use a metric called “air changes per hour” (ACH). They try to replace the room air with clean air 12 times per hour (12 ACH), and they use “negative pressure” so that bad air is vented outside and cleaned as its goes. In less infectious areas, >6 ACH are used. This is a lot more than most offices and classrooms.
We know these ACH values help prevent the spread of TB from experiments in actual guinea pigs, where one set of cages are exposed to “filtered” air and another group are exposed to infectious air. The levels recommended by CDC reduce infections for these animals by 80%.
SARS-CoV-2 is not a bacterial infection, but generally it rides along on particles that are big enough that the TB research is relevant. Another way to say it is this: TB ventilation research tells us how air moves through buildings and how you can dilute an infectious microbe. For COVID-19, it’s the best we’ve got.
Guidelines for Environmental Infection Control in Health-Care Facilities
Please note: An erratum has been published for this article. To view the erratum, please clickhere. Persons using…
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: https://www.epa.gov/coronavirus/will-air-cleaner-or-air-purifier-help-protect-me-and-my-family-covid-19-my-home
- Choose a number of “air changes per hour” between 6–12.
- 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
Generally we found these options cost below $1.00/SF.
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
Generally, we found solutions like UV-C lamps to cost $5.00/SF and up.
Sizing Portable HEPA filters
Don’t trust manufacturer “square footage”
Portable HEPA filters are rated by how much clean air they make using a number called “CADR” (clean air delivery rate) which is measured in CFM (cubic feet per minute).
Some manufacturers will tell you to buy enough filter to change the air just twice per hour. Here we are looking in the 5–10 ACH range, so we cannot trust manufacturer guidelines for square footage, and instead we should look at CADR.
You can easily convert CFM (CADR) to ACH. For 8 foot ceilings in a room with square footage “SF”:
- 5 air changes with 8 foot ceilings: CADR Required = 0.66 * SF
- 10 ACH with 8 foot ceilings: CADR = 1.33 * SF
For 10-foot ceilings:
- 5 ACH with 10 foot ceilings: CADR = 0.83 * SF
- 10 ACH with 10 foot ceilings: CADR = 1.66 * SF
Even at 10 ACH, we were able to find solutions below $1/SF.
Since these filters are running at a high capacity, they will be LOUD, and so shopping by sound and price is not a bad idea. The Wirecutter is a useful resource for balancing cost with sound and ACH:
The Best Air Purifier
Collapse all Tim Heffernan has been at Wirecutter since 2015 and has overseen this guide since then. Since 2017, he has…
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.
ASHRAE statement here and presentation are here: