Air Purifiers, Winter, and COVID-19

Brrr. It’s getting cold outside. As the northern hemisphere heads into winter, we are seeing a huge uptick in COVID cases from people moving their activities indoors. The United States is seeing more than 2x the daily rates of the last peak in August.

We’ve long known that the key to having safe social interactions is to have them Masked, Outside, and Distanced (MODified), but Outside is by far the biggest factor - 20x reduction of transmission risk compared to 4-8x reduction for Masked and 2-4x reduction for Distanced. In freezing temperatures, though, walks and picnics are a hard sell. So how are we going to make it through the winter?

Enter this study on air purifiers. Researchers found that running air purifiers in a classroom decreased the aerosol density in the room by 90%, which they predict would reduce the transmission of COVID by 4-6x. It’s not as good as being outside, but if you must have people over inside, an air purifier make the interaction safer.

We’ve added air purifiers as an option in the “Environment” question on the calculator.

If you decide to buy an air purifier for COVID purposes, here’s some things to keep in mind:

• Make sure the purifier has a HEPA filter - these are rated to remove 99.97% of airborne particles.
• Watch out for lesser filters - products labeled as "HEPA-type," "HEPA-like," "HEPA-style" or "99% HEPA" do not satisfy the HEPA standard!
• Central heat / AC systems don't work for this. These typically use MERV-rated filters. These are significantly less effective at removing small particles than HEPA-rated filters (the best MERV rating, MERV-16, merely removes 75% of particles.
• Air flow matters! In the paper, they used a flow rate equivalent to cleaning the whole room worth of air 5 times an hour. Purifiers will typically rate their flow as CADR (Clean Air Delivery Rate) in CFM (cubic feet per minute) or m³/h (cubic meters per hour). For a room with 8ft tall ceilings, this works out to a CFM rating of 2/3 your room’s square footage (length * width in feet).
• For example, my apartment’s living room is 22ft x 12ft = 264 ft3 . So I need 2/3 * 264 = 176CFM. This purifier advertises a flow rate of 141 CFM, so I would need more than one for my space.
• Of course, this is microCOVID, so we've made a little calculator to do this math for you:

And remember, all the wisdom from the summer still applies:

• Limit how many people you interact with indoors. Avoid gatherings with more than a few people.
• Wear a mask when you are out of the house and especially when you are indoors with strangers.
• If you are going to spend time indoors with someone, talk to them first about what their COVID practices are — encourage them to wear a mask and minimize their number of contacts in the 10 days before the event.
• If someone you’ve been in contact with starts to have COVID-like symptoms, isolate yourself until you can get tested.

We’ll make it through this together!