What is air flow?

For most home appliance or electronics purchases, we know what specs are important. Is this TV high-def or 4K? What’s the capacity of this refrigerator? How much water does this washing machine use per load? For air purifiers, however, the facts and figures might not be as clear.

At the same time, having high-quality indoor air is important for both short- and long-term health, as well as managing conditions like asthma and allergies. So understanding what’s important in a purifier is just as — if not more — important than making sure that new fridge has adequate crisper drawer storage. Aside from choosing a model with a high-quality filter, a unit’s air flow stats are of vital importance to its performance. Here’s what you need to know:

Okay, you’ve got my attention. What is air flow?

Air flow is the rate at which air runs into a purifier in a given period of time. Generally, flow rate is measured in cubic feet per minute (CFM). The primary factor that determines a purifier’s air flow rate is the strength of its fans, which are responsible for pulling air into the unit and through the filters. The more “dirty” air a purifier can ingest, the better it will be at sending clean air back into the room.

Makes sense, but is air flow a spec I can look for on a box?

Not necessarily. Most commonly, what you’ll see advertised are a purifier’s coverage area (that is, how large of a space its fan, or fans, can reach) and air changes per hour (ACH). Think of ACH as the speed of an air purifier. Take the Airmega 400S as an example: Thanks to its dual fan system (there’s one on either side of the unit), it’s able to cover rooms up to 1,560 square feet, and can cycle through all the air in that space twice in one hour. In a room half the size, it can clean the air every 15 minutes.

Is that good?

In a word: Yes. At minimum, experts suggest purifiers have an ACH of four cycles per hour. Those suffering with serious allergies may need to up the ante to two cycles per hour, or invest doubly in purification to make up the difference.