How Many Air Purifiers Do I Need?

While concerns over pollution often center on outdoor air, recent studies from the EPA reveal that indoor air maybe two to five times more polluted than outdoors. 

Air purifiers clean stagnant indoor air by filtering out airborne particulates that are known to cause eye and lung irritation, trigger allergies, and other negative health impacts. Air purifiers can also reduce the spread of airborne disease, improve respiratory health in those with asthma, and neutralize unpleasant odors.

To work effectively, air purifiers must be sized correctly for the spaces they occupy. Choosing the right model for the right room type is essential.

Does Size Matter?

The simple answer is YES, size is extremely important. But bigger is not always better.

Appearance and price shouldn’t be the only factors you should consider when purchasing an air purifier. Your unit needs to be sized according to the volume of the room it is meant to purify, and to a high degree of accuracy. Choosing an air purifier too weak for a room will leave the air stale and unfiltered, while choosing an air purifier too powerful for a room will lead to energy waste and take up unnecessary room as you also need to consider where to put it.

Fortunately, there are a few easy steps consumers can take to ensure they find their Goldilocks purifier.

Manufacturers design different air purifiers for different room sizes, and rate purifiers using a number of metrics to help consumers choose the right one. Most portable air cleaners are rated by their Clean Air Delivery Rate (CADR), an industrial standard of measurement maintained by the Association of Home Appliance Manufacturers. 

The CADR measures in cubic feet the amount of a certain particulate – dust, pollen, or smoke – that is removed from the air, and is essentially a measure of how well an air purifier can clean a room. The larger the room, the higher CADR that is recommended. While some manufacturers do not use CADR – citing that the rate of purification is not necessarily an indication of efficacy – knowing the CADR is critical to calculating an air purifier’s sanitizing capacity.

Another measure of efficiency that consumers should be aware of is Air Change per Hour (ACH). If CADR is the potential cleaning power of a purifier, ACH is how well the purifier will work in your room. ACH is calculated using the CADR and the volume of a room, and is essentially the number of times the air in a room will be replaced with new air in one hour. An air purifier with an ACH of 4, for example, would turn over the air in a room 4 times within an hour. ACH fluctuates depending on purifier brand and room size, and can help you determine how many air purifiers you may need in your household. 

Pro Tip: If you suffer from bad allergies in the home, you will need to look for units with an ACH rate higher than what manufacturers recommend for the typical household.

Do I Need One for Every Room?

Air purifiers are designed to sanitize one room, not an entire household. In general, air purifiers should go in the busiest rooms, where occupants spend the most time. Other high-priority rooms should be places most affected by airborne pollutants, such as the living room, bathroom, or kitchen. The air quality of rooms with furniture that creates dust and other airborne irritants, or rooms used to store toxic chemicals, like basements and laundry rooms, will improve significantly with the addition of an air purifier. 

We can determine whether an air purifier is a good fit for a room by comparing its CADR to the size of the room it is meant to sanitize in two easy steps:

Find the volume of the room

The way manufacturers grade the efficiency of an air purifier is three-dimensional – cubic feet of particulate removed in a minute – so the measure of room size should be volumetric. The equation for determining the volume of a room, or the amount of air that needs to be purified, is:

Volume = room area (length x width) x ceiling height

A room with an area of 525 square feet and a ceiling height of 10 feet, for example, would have a volume of 5,250 cubic feet, because:

5,250 cubic feet = 525 square feet x 10 feet

Determine the minimum clean air delivery rate (CADR) for your room

While air ventilation standards can change depending on building or room type, it is generally recommended that a room in a residential home have at least 4 air changes per hour (ACH). Now that we know the volume of the air that needs purifying and how often the air needs to be cycled, we can determine the minimum clean air delivery rate (CADR) a purifier would need to properly vent the room.

Following general recommendations, a 525 square foot room with an air volume of 5,250 cubic feet would need that air replaced at least 4 times within an hour. In this case, the minimum CADR would be:

Minimum CADR = room area x 4 ACH (air changes per hour) x (1 hour/60 minutes) x ceiling height

Or:

350 CADR = 525 square feet x 4 ACH (air changes per hour) x (1 hour/60 minutes) x 10 feet

This table shows the minimum CADR an air purifier would need to maintain an ACH of 4 at a range of room areas (assuming a 10-foot ceiling):

As we can see from the table, as a rule of thumb the CADR number should be at least two-thirds of the square footage of the room the purifier is meant to occupy. Note, consumers with allergies may require a higher rate of Air Changes per Hour (ACH), and may need a unit with a higher CADR.

We reviewed the CADR of some of the most popular air purifier brands to help you determine what model is right for your ventilation needs:

Air purifier		CADR
Blueair Blue Pure 411		120 cubic feet per minute
Levoit PUR131		135 cubic feet per minute
Levoit Core 300		140 cubic feet per minute
Rabbit Air MinusA2		173 cubic feet per minute
Alen BreatheSmart 45i		245 cubic feet per minute
Levoit Core 400S		260 cubic feet per minute
IQAir Healthpro Plus		300 cubic feet per minute
Coway Airmega 300		306 cubic feet per minute
Honeywell HPA300		320 cubic feet per minute
Alen BreatheSmart 75i		347 cubic feet per minute
Medify MA-40		380 cubic feet per minute

Air purifiers have the potential to improve indoor air quality and quality of life, but consumers need to choose the right model for the right room to yield the maximum benefit. Matching the proper CADR to the volume of air it is meant to sanitize is one way to make sure shoppers get the most bang for their buck.

In addition to CADR, you may also want to draw distinctions on other product specifications. Air purifiers differ in noise level, and some of the more advanced models have special features like speed control, filter replacement indicators, and Wi-Fi connectivity for remote control. 

Happy hunting!