What’s CADR and how is it different from airflow?

When buying a purifier, you’ll sometimes see companies use the word “airflow” (风量) to describe an air purifier’s performance and other companies the word CADR (clean air delivery rate) as seen in the picture below.

 

 

They’re both a measure of purifier performance, but are they the same thing? Can I compare one purifier’s CADR value with another purifier’s airflow value?

 

Short Answer

No. CADR and airflow are not the same thing. If you see somebody claiming they have the same CADR and airflow (like the screenshots attached below), you know something is up!

 

 

Long Answer

The real answer gets into some nerdy (but actually profound) details about how people measure CADR and airflow.

Here’s what you need to do for a CADR test:

 

 

(For total nerds, the government guidelines describe the full methods [in Chinese])

 

How People Test CADR

Step 1: Pollute a sealed room to crazy high pollution levels (normally by lighting cigarettes).

Step 2: Run your purifier.

Step 3: Take PM2.5 concentration readings every 2 minutes for 20 minutes.

Step 4: Use these numbers to work out the rate of purification of the room (the CADR).

The hard part about doing this test is that you need a lot of complex equipment, a large testing room, the time and patience to do the test, and some data analysis skills.

 

How People Test Airflow

To measure airflow, the only thing you need is a wind speed measure (fancy name “anemometer”):

How to Test Wind Speed

Step 1: Place the anemometer against the filter and look at the number.

Step 2: Multiply this speed by the area of your filter.

That’s it! So much easier.

This method works all right, but it has a problem. Since this method only measures wind speed, it only tells us how much air is coming out.

What doesn’t it tell us?

  1. How clean is that air coming out?
  2. Is any of that air actually coming in from a leak or the middle of the air vortex and then being pushed back out?
  3. How efficiently is the machine mixing that air in the room?
  4. How much of the air that comes out of the purifier gets sucked back into the machine quickly, rather than being pushed farther out into the room?

Air flow tests assume perfect values for all of these variables. For example, it assumes that all the air coming out is 100% clean. Because all the assumptions are rosy, air flow values tend to be much more impressive than CADR.

Bottom line: Air flow is less rigorous than CADR and will almost always be much higher than CADR. Do not directly compare the two numbers.

Couldn’t We Convert Airflow into CADR?

The nerd in me thought that if we just multiply the airflow with the efficiency of the filter (for example, our filters capture 99.9% of particles above and below 0.3 microns), then we can measure the amount of clean air coming of the filter. But this still leaves out one vital bit of information: this doesn’t consider real life conditions on how air recirculates and mixes in a room.

When you measure airflow, you’re assuming all the air passes the filter just once. This doesn’t happen in real life, and predicts higher effectiveness than happens in real-world tests. The air in a real room is constantly being mixed (like stirring sugar into our morning coffee), which means that already purified air will recirculate and pass through the purifier for second or even third time. Re-purifying already purified air is not optimal, but it is inevitable. And that leads to a lower than expected cleaning rate.

This explains why CADR values are lower than airflow values, and shows that comparing one purifier’s CADR with another purifier’s airflow value just won’t cut it.

How Much Lower Is CADR Than Airflow?

At Smart Air, we use airflow and CADR tests when testing purifiers like the Blast purifiers we just released. We use both because they both have advantages (airflow tests give a quick rough estimate of a purifier’s effectiveness; CADR is more precise). Because we do both types of tests, we can see how much smaller CADR values are on average.

 

Average across many different purifiers we’ve tested, we found that CADR tends to be around 60% of airflow results.

 

How to choose: When comparing purifiers, look for CADR over airflow (and pay attention to whether the company is reporting one or the other). And keep in mind that CADR tests are often done by third-party testing companies, whereas airflow is typically measured by the purifier manufacturer.

Thomas Talhelm

Thomas is a new Assistant Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.

4 thoughts on “What’s CADR and how is it different from airflow?

  1. I just moved to China and I’m trying to sort out what I need for my apartment (61.3 sq m, 2 bedroom + 1 hall) assuming I use one of your filters at low or medium for reduced noise. But I need some information I can’t find on your site.

    1. What is the CADR for your products (especially the Blast and mini Blast) at low and medium speed? I’m assuming the amount listed on your website is for high speed.

    2. What is the calculation to determine one’s CADR needs? I found the following on a post by Dr. Saint from myhealthwellbeing.com posts: sq meters X ceiling height X 5 (because air purifying requires 5 air changes/hr) = CADR required. But this seems off because it means the Blast at high (with a CADR of 890) would only work for a room that is 71.2 sq meters assuming normal ceiling height of 2.5 m (71.2 X 2.5 X 5 = 890) while your site says it is good for 130 sq m. Even if the air changes/hr was 4, the calculation is still off (it would mean a room size of 89 sq m). So where is my mistake?

    3. Can a single air filter be used for an entire apartment even if the bedroom doors are usually closed? (I’m assuming the total area is within the CADR of the filter).

    Ps: would be great if you posted this calculation on your site in the “shop” section and eventually had a simple CADR requirement calculator on the site or an excel one that could be downloaded to determine one’s needs and which of your products to get.

    1. Hi Ak, great questions. Looks like you’re being smart about working out your clean air needs! To answer your questions:

      1. I’ve updated the Blast and Blast Mini pages to include the CADR on the different speeds. Thanks for pointing this out!
      2. There are many different methods for calculating area based on CADR, all based on different assumptions and for different countries. For calculating the area, we use the China Government Standard, along with our own real life testing. The China standard says to take the CADR (in cbm/hr) and multiply by 0.07 – 0.12. So a CADR for the Blast of 890cbm/hr would give a room area between 62sqm – 109sqm. Based on our own tests in Beijing we found these to be pretty conservative, so we use a slightly looser rule which is why we claim the Blast to be good for 130sqm. Bottom line: if you want to be conservative take the CADR and multiply by 0.12 to get roomsize.
      3. From our tests, we’ve found that closing a door means not enough clean air does travels between rooms. So our advice is to have a purifier in each room. Of course, it all comes down to how a) leaky your windows are in the room without a purifier and b) how tight the seal is round the door. If you were to leave your door ajar then you should be fine.

      And regarding your P.S. point – that’s a great idea and something we’ve wanted to do for a while. We’ll work on getting something like this up!

  2. I love your site and all the data you’ve collected. Two things I’m curious about:
    1) How much does the flow rate change between with the filter and without?
    2) How much does the noise level change between with the filter and without?

    I’m asking because I’m curious about using a different fan with a known flow rate but a lower known noise level. If your data is correct, you’ve beaten the competition in CADR, price. Do you think it’s possible to maintain this performance while reducing the noise to a whisper? (I have faith)

    1. Hey Vincent, thanks for the kind words. Two good questions!

      1) We’ve not done tests on the airflow without a HEPA, but if we look at the numbers for the Cannon, which claims a max airflow of around 700cbm/hr, we can see that the airflow is reduced at least
      2) This is a much more difficult question to answer, since sound is much more a subjective thing. In terms of actual sound levels (measured in dB(A)) there is little difference between the fan with and without a HEPA, but in terms of what kind of sound you get (‘windy’ sound with just a fan or a kind of ‘whooshing sound’ with a HEPA – sorry if that doesn’t make much sense!) it is very different. Generally, with a HEPA it sounds more pleasing/calming.

      In terms of making a super quiet yet super effective purifier, we think we did a pretty good job with the Blast and Blast Mini! Ultimately to reduce noise you need a big HEPA and a big fan. This means the air can move more slowly through the whole machine, reduce the noise it makes as it passes through the HEPA filter. Check the Blasts out if you haven’t already!

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