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?
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!
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?
- How clean is that air coming out?
- Is any of that air actually coming in from a leak or the middle of the air vortex and then being pushed back out?
- How efficiently is the machine mixing that air in the room?
- 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 off 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 is an Associate 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.