Understanding CADR Rating: #1 Spec For Air Purifiers

Here’s a bit of the history of how CADR rating was invented:

In the 80s, all talk with air purifiers was about maximum airflow (measured in CFM) and filters. But nobody really could tell how effective this or that air purifier was at actually removing pollutants such as dust, smoke, mold spores from indoor air.

AHAM (Association of Home Appliances Manufacturers) working to connect air purifier manufacturers with end consumers developed a CADR test.

Based on the CADR test, every homeowner could how effective an air purifier actually is. Obviously, not all air purifiers are as strong as Blueair or Coway air purifiers.

CADR standards for Clear Air Delivery Rate. It is the most important metric you need to focus on when buying an air purifier. It tells us how quickly and effectively an air purifier will purify the indoor air.

It also helps us determine how powerful an air purifier we need, given certain square footage (or, better yet, volume) of a room/apartment/house.

Simplistic example: An air purifier has a maximum airflow of 300 CFM. Its CADR rating is 200 CFM. That means that an air purifier could process 300 cubic feet per minute. However, it will effectively clean only 200 CFM as per the CADR rating. That 100 CFM will not be cleaned in the 1st air purifier cycle.

Key metric before the 80s: 300 CFM maximum airflow.

Key metric after the 80s: 200 CFM CADR Rating.

Not every air purifier is ‘AHAM Verified’ and hence does not include CADR rating in the specification sheets. However, all of them include the old metric – the maximum airflow (measured in CFM).

The golden rule (simple to remember) is the ‘AHAM 2/3 rule’ that estimates a good CADR rating for an air purifier. We cover that in the 3rd chapter below.

What Is Clear Air Delivery Rate (CADR)?

According to AHAM, CADR rating is defined as “the rate of contaminant reduction in the test chamber when the unit is turned on, minus the rate of natural decay when the unit is not running, multiplied by the volume of the test chamber as measured in cubic feet.” in line with the ANSI/AHAM AC-1-2006 directive.

In simplified terms, CADR tells how effectively an air purifier can reduce:

  1. Smoke particles with sizes between 0.9 and 1.0 μm (Smoke CADR).
  2. Dust particles with sizes between 0.5 and 3 μm (Dust CADR). This was the most useful specification we used to rank the best air purifiers for dust removal here.
  3. Pollen particle with sizes between 5 and 11 (Pollen CADR).

In short, the higher the CADR rating, the faster an air purifier can clean the indoor air of pollutants. For smoke, dust, and pollen-specific CADR rating can be measured. Companies that include these metrics in their specification sheets usually produce better-than-average air purifiers. These include Coway, Honeywell, Levoit, Okaysou, Hathaspace, Medify, and Alen.

One such example is LEVOIT. Their small and affordable unit – which also scores high in the list of the best air purifiers you can find here – includes all the CADR ratings on their specification sheet:

air purifier cadr rating for smoke pollen and dust on specification sheet
Some air purifier producers include CADR rating (for smoke, dust, and pollen) on the specification sheet.

Expert tip about calculating ACH (air changes per hour) from CADR rating:

“The calculation of ACH is derived from the Airflow Rate (in cubic feet per minute, CFM), multiplied by 60, and divided by the room volume.

While both CFM and Clean Air Delivery Rate (CADR) are relevant metrics, CADR, which measures the rate of pollutant removal in CFM, is often used for specific contaminants like pollen, dust, and smoke.

To calculate ACH accurately, I incorporate the CADR values associated with the particular pollutants of concern. Tailoring the ACH to the unique requirements of the environment ensures effective air purification, with higher ACH recommended in areas with heightened contamination risks.

It is crucial to consult the manufacturer’s guidelines for precise recommendations based on the specifications of the air purifier in use.” (Reuven Adam, Managing Director, HVAC San Diego)

How Is Air Purifier CADR Rating Measured

To validly compare different air purifiers, CADR rating testing is standardized.

According to the ANSI/AHAM AC-1-2006 directive, all CADR testing must be conducted in a 1008 ft3 test chamber. This is a representation of a 12×12 room with a 7 ft high ceiling.

To conduct the CADR measurement, an air purifier is placed inside the test chamber. While it’s running at full speed, the amount of specific pollutants (dust, pollen, and smoke particles with sizes ranging from 0.10 to 0.11 microns) is measured.

The CADR rating is calculated based on how quickly the air purifier will remove those pollutants from the test chamber. The test will procure a total CADR rating, combining all 3 types of pollutants, as well as specific ‘Smoke CADR’, ‘Dust CADR’, and ‘Pollen CADR’ rating.

For most residential air purifiers, this test has a high degree of validity. The limitation presented by the size of the testing chamber volume include:

  1. Maximum Smoke CADR rating of 450 CFM.
  2. Maximum Pollen CADR rating of 450 CFM.
  3. Maximum Dust CADR rating of 400 CFM.

These limitations only cause problems when testing industrial air purifiers.

What Is A Good CADR Rating For Air Purifier? (AHAM’s 2/3 Rule)

Obviously, every informed homeowner who is comparing air purifiers would like to check each device’s CADR ratings. Sadly, the reality is that many air purifier manufacturers don’t conduct AHAM CADR testings. With those, we only find the maximum airflow (in CFM) on the specification sheet.

How to calculate the CADR rating from maximum airflow? In short, you can’t. CADR is not calculated; it’s measured in the test chamber. However, AHAM has since introduced it’s simple 2/3 rule, which helps us estimate CADR rating based on maximum airflow.

Here’s how it works: CADR rating is about 2/3 of the maximum airflow (in CFM).

Example: You’re looking at air purifier specification sheets. You can read that maximum airflow is 200 CFM, but there is no info about CADR rating. According to AHAM’s 2/3 rule, you can estimate that the CADR rating is 2/3 of 200 CFM. Hence, CADR would be around 133 CFM.

This also gives you an idea of what is a good CADR rating for an air purifier. If the CADR rating is the same or more than 2/3 of the maximum airflow, you know you have an air purifier with a superb filtration system. If, on the other hand, the CADR rating is less than 2/3 of maximum airflow, you know you should find a better air purifier.

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