When buying an air purifiers, the terms ‘airflow’ and CADR are often used interchangeably to describe an air purifier’s performance. But what’s the difference between CADR (clean air delivery rate) and airflow? Are they the same thing? Or which one – CADR vs. airflow – should be used when buying an air purifier?
Are CADR and Airflow the Same Thing – Short Answer
In short, CADR and airflow are not the same thing. They are slightly different measurements. That means CADR values from one air purifier should not be compared with airflow values from another air purifier. CADR is typically lower than airflow. We’ll get into that later.
However, if a company claiming that their air purifier has the same CADR and airflow values, then it’s likely this company isn’t providing the full picture.
What’s the Difference Between CADR and Airflow – Long Answer
The real answer gets into some nerdy (but actually profound) details about how CADR and airflow are measured.
Here’s what you need to do for a CADR test:
(For total nerds, the government guidelines describe the full methods [in Chinese])
Steps to Test an Air Purifier’s 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.
Testing Air Purifier Airflow
To measure airflow, the only thing you need is a wind speed measure (fancy name “anemometer”):
Steps to Test Air Purifier Airflow
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 Airflow Tests Don’t 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?
Airflow 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, airflow values tend to be much more impressive than CADR.
Can Airflow Values be Converted to CADR Values?
In short: no. Accurate CADR values can only be obtained from CADR tests.
With that said, it might seem logical to think that if we just multiply the airflow with the efficiency of the filter, then we get a CADR value. 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.
In real life, air circulating in a room doesn’t pass through the air purifier just once. Clean air that’s just left an air purifier is constantly mixed with the dirty air in the room (like stirring sugar into our morning coffee). This ‘mixed’ (dirty and clean) air will then re-pass through the purifier, leading to some of the already-clean air to pass through the purifier a second or even third time. Re-purifying already purified air is not optimal, but it is inevitable. And that leads to a CADR that’s lower than just airflow x efficiency.
How Much Lower Is CADR Than Airflow?
At Smart Air, we use airflow and CADR tests when testing purifiers like the Blast air purifiers. We use both because each has its own 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 CADR and airflow values are on average for a range of purifiers.
From this range of 5 air purifiers, we saw that tested CADR values were just 60% of tested airflow results. The drop was greater for the Blast and Blast Mini. This 60% value can be used as a handy rule of thumb where CADR values aren’t available. However it may not be applicable for all air purifiers. If in doubt, it’s recommended to obtain CADR values directly via testing.
Comparing Air Purifiers Before Making a Purchase
How to choose: When comparing purifiers, look for CADR over airflow (and pay attention to whether the company is reporting one or the other). CADR tests are often done by third-party testing companies, so may be more reputable. Whereas airflow is typically measured by the purifier manufacturer.