What’s The Difference Between ACH and CADR?

There are so many different options when it comes to air purifiers that it can be difficult to decide which one is right for your home. Fortunately, there are some measurements that can help you make the best choice. Two of the most common measurements used to compare purifiers are ACH and CADR.

What is ACH?

Air Changes per Hour (ACH) is the number of times all of the air in a room is replaced with completely new air, in one hour. 

This number is important because to properly clean the pollutants in any given area the entire volume of air must be filtered. It can take time for the air furthest from the purifier to be cleaned. 

The ASHRAE (The American Society of Heating, Refrigerating and Air-Conditioning Engineers) sets recommendations for the ACH of different facilities. 

Thee are the recommended ACH for some common spaces:

Homes .35-1

Hotel rooms 1-2

Offices 2-3

Schools 5-6

Restaurants 6-8

Air purifiers generally list the ACH on the packaging, but you can also calculate it yourself by multiplying the Cubic feet per minute (CFM) of your purifier by 60 and then dividing that number by the total volume (V) of air being exchanged or filtered.

To figure out the volume of air, multiple the length, width, and height of the room.

What is CADR?

The Clean Air Delivery Rate (CADR) measures how much air is filtered in an hour for a specifically-sized pollutant such as dust, pollen, or smoke. For instance, if you particularly wanted to remove pollen, you could compare the CADR for pollen on various air purifiers.  

Air purifiers with higher CADR ratings will be able to clean the air in a space more quickly than models with a lower CADR rating. However, making your choice purely based on the highest number could leave you with an air purifier that is far too large for the given space, wasting energy and money.

A good rule of thumb is to choose a CADR number that represents at least 3 times the total volume of your room.

For example, a room with an area of 120 square feet would ideally have a CADR of at least 80. 

How is CADR determined?

In order to get a CADR rating, purifiers must first be certified by The Association of Home Appliance Manufacturers (AHAM), an independent trade association that sets the standards for appliances.  

To test the CADR of different purifiers, AHAM follows a standard procedure:

The purifier is placed in a confined room which contains dust, pollen, and smoke contaminants. The amount of pollution is monitored as the air purifier runs at full speed.

The air purifier’s CADR rating is calculated based on how long it took the unit to remove these contaminants.

Are there limitations to CADR?

While CADR is a helpful measure of an air purifier’s efficiency, it does have drawbacks that should be taken into account. 

CADR is evaluated by testing in a small, confined room

Testing in a small room does not give an accurate assessment of larger rooms, where some of the air purifier models would not be able to reach every corner due to their low airspeed at the outtake of the machine.

CADR does not show the air purifier’s performance over time. 

CADR is usually evaluated for mint condition filters and not valid for the entire lifetime of the machine.

CADR doesn’t measure ozone

The effects of ozone can be just as dangerous as poor air quality.

Look for an air purifier that doesn’t use ozone.

CADR doesn’t measure gas. 

Gas pollutants can be very dangerous and a purifier’s effectiveness at reducing gas is completely unrelated to how well it reduces particles. 

CADR doesn’t measure noise.

In order to test CADR rating, purifiers are run at their highest possible setting, which usually generates a lot of unwanted noise that isn’t really practical for everyday use.

Can manufacturers manipulate CADR and AHC measurements to sell air purifiers?

Although CADR and AHC ratings are helpful in choosing purifiers, those numbers alone are not always enough. Both the CADR and the AHC ratings can be manipulated to make customers think they are getting more protection than they actually are.  

One misleading tactic is to use cubic meters per hour instead of the standard cubic feet per minute to make it look as though the CADR is higher than it actually is. Another trick is to test the purifier without using the filter, which further inflates the CADR number.  

A common way to distort ACH numbers is to not take into account ceiling size… meaning that rooms with higher ceilings may not be sufficiently protected. 

How can we decide which purifier to purchase?

The right purifier can make a big difference in your family’s health and wellbeing. While both CADR and ACH numbers can be useful in making a decision, neither one is perfect. It’s important to do extensive research before choosing an air purifier for your family.

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