One effective way to reduce viral load of indoor areas is to exchange air with the outdoors, by opening the windows or running window-mounted AC or other HVAC that vents indoor air and brings in replacement air.

Another is to run a HEPA filter that is situated in, and sized for, any areas where people are breathing.

If you are located in an area with poor air quality (e.g. a city; especially an Asian megacity), opening a window will vent possible virus — but then you need the HEPA anyway in order to remove pollution from the outside air. We do this in China; it uses up filters faster, but it’s the only way to live in air that’s low in both CO2 and pollution.

Choosing a filter. For virus removal, look for a model with a HEPA filter, that removes PM2.5 particles. Viruses are smaller than 2.5mm, but consumer-market HEPA construction is such that a filter that is effective against 2.5mm particles is also effective at removing virus particles.

Sizing the purifier. Divide the CADR volume (defined below) by the volume of the room, to find the air exchange rate, in Air Changes per Hour (ACH) . At e.g. six exchanges per minute, half the particulates are removed every ten minutes. The target for hospital isolation rooms is twelve exchanges per hour; given the incredible infectiousness of the later omicron variants, this seems like a good lower limit for public spaces and residential use too.

Noise. The purifiers that I recommend are quiet. They have big fans (that can run move a lot of air while moving slowly and quietly) and big filters (in some cases, several big filters).

Automatic setting — good for pollution; no good for avoiding infection. The air purifiers listed below having an “automatic” setting, that dials up or down the fan speed (and therefore noise level) based on the measured particulate level. The sensors measure PM2.5 particles, not virus, so this setting is not appropriate for clearing the air of virus. It is appropriate for removing pollution and allergens from imported outside air, if you have the windows open.

Positioning the purifier. The air that is clean is that that is blown out from the purifier. Like “buy low sell high”, this sounds obvious, but can be difficult to implement in practice, where it is tempting to focus on how much air the purifier is “inhaling”, instead of where it is “exhaling”.

These notes and model recommendations are for retail purifiers for residential spaces. Office spaces, classrooms, etc., have their own concerns.

My recommendations:

The Coway purifiers are less available in the U.K. Levoit ships in the UK, and has good reviews. I bought a Levoit LV-H132. of these for a one-room apartment there:

Again, Coway purifiers. They sell different models in China, but they seem pretty good. These have huge slow (therefore, quiet) fans.

One each:

Previously I bought a BlueAir. It was so loud we didn’t keep it turned on much. After a couple of years its detector broke, and then its motor broke. We didn’t bother fixing it, and replaced it with a couple of Coway’s instead.

I love my Dyson vacuum cleaner. I love the aesthetics of their air purifiers. They don’t rate highly on purification efficiency. They’re basically fans; not enough air moves through their tiny filters.

CADR: Clean Air Delivery Rate — effective air flow (CFM).

P-CADR: particular CADR. This is what protects you from particulate pollution, and from virus.

F-CDR: formaldehyde CADR (VOC).

CO2 level above 1000 PPM is common in indoor spaces, and might make you drowsy or low-efficiency. (It probably does not have any long-term consequences.)

You can also measure the CO2 level in order to tell how much human exhalation is building up, and not exchanged into larger spaces such as the outdoors.

This makes CO2 an excellent (and frequently-used) proxy measure for possible airborne viral load.

In order to know how to interpret the indoor CO2 level, you need to know the outdoor CO2 level. This is the lowest possible value for your indoor level, absent extreme measures such as CO2 scrubbers that cost more than your house, or an impossibly high foliage density. In nature, 400 PPM is common (before the anthropocene era, it was lower). Depending on the weather, a city’s CO2 level can vary from 400~800 PPM. Your air quality app can give you an estimate.

I haven’t tested the calibration of these. But also, as a proxy for viral load, it doesn’t matter. Get a baseline from when their hasn’t been anyone in the house (or room) for a while.

Room solution: The Eve Room is HomeKit compatible.