CADR

What is CADR? What does it mean? How is it used?

The truth is, it doesn’t matter.

CADR should be used for only one thing: to approximate CFM.

CFM is actually helpful.

CFM, of course, stands for Cubic Feet per Minute – it gives a standard way to describe how much air (cubic feet) is processed by an air purifier in a given period of time (a minute).

A 100 CFM unit can process 100 cubic feet of air per minute. A 200 CFM unit can process 200 cubic feet of air per minute. Thus, a 200 CFM unit processes twice as much air per minute as a 100 CFM unit.

Back to using CADR to approximate CFM:

Let’s take the Blue Pure 411 as an example. Three CADR numbers are listed:

The unit’s manufacturer specified CFM? 120 CFM. Thus, even if you were to only have access to its smoke CADR you would still be able to approximate its CFM as being right around 100 CFM – close enough to make comparisons to other units.

Another example, the Austin Air HealthMate:

The unit’s actual CFM? 250. Even if you didn’t know this you could still approximate the unit’s CFM as being close to 220 – good enough to make comparisons to other units on the market.

You can go through this same process – using CADR to approximate CFM – with pretty much any air purifier on the market.

This gives you

• Only one number (CFM) to compare – much easier than comparing three CADR numbers to three CADR numbers.
• One number that gives you a clear idea of how much air the air purifier can process per minute without any further research into what CADR is, its limitations, how to use it, etc. Using CFM keeps things simple.

If that’s a sufficient explanation for you – great. You’re now well equipped to compare the air processing ability of different air purifiers on the market. If not, we get into more detail regarding this topic (why we prefer using CFM over CADR) at the end of this guide.

Our guidelines for air purifier CFM

Now that you know how to approximate CFM, it’s time to determine what CFM is necessary for any particular size space.

The good news is that things get even easier from here. Our guidelines are simple:

1. We recommend only air purifiers with approx. 250 CFM of output. Anything more makes for a poor CFM to $$ paid ratio (this is one of many reasons we make this recommendation - for more see here). Anything less is problematic also (for reasons we outline here).

2. Use an approx. 250 CFM unit on lower settings in smaller spaces (below 150 sq. ft.), on higher settings in larger spaces (up to 300 sq. ft.) and use multiple units in extra-large spaces (well over 300 sq. ft.).

Other reasons we prefer using CFM instead of CADR to compare the air processing ability of different air purifiers on the market

1. CADR can be difficult to understand and makes the air purifier buying process more difficult than it needs to be for potential buyers.

You’ll need a full explanation (such as the one we have later on) to fully understand what it is. CFM is simple. It’s simply how many cubic feet of air is processed by the air purifier each minute.

2. CADR gives unnecessary information.

Almost all air purifiers on the market use HEPA filters and therefore clean air exactly the same way – they remove exactly the same particles, the same sized particles, the same types of particles exactly the same way. No additional metric (CADR) is necessary to show you how well they can do that (how well they can process smoke, pollen, etc.). A metric that simply shows how much air goes through the filter per unit time (CFM) is sufficient since the filter type is always given by the manufacturer.

3. CADR can be deceiving.

For those units that do not use standard particle filters (e.g. HEPA filters), CADR can be deceiving. For example, units that use ionization to remove particles from the air have extremely high CADR ratings. Does this mean that they clean the air better than units equipped with HEPA filters? Absolutely not.

Ionizers also have a higher CFM but CFM, unlike CADR, only shows how much air is processed. It doesn’t attempt to show how well air is processed, like CADR does.

4. CADR has set a standard for using confusing metrics to sell air purifiers.

Not all air purifiers come with a specified CADR. An air purifier has to be tested by AHAM to receive a CADR rating. Many air purifier manufacturers do not send their air purifiers to AHAM for testing. Thus, their air purifiers have no CADR rating.

CFM is much easier to determine and could easily be specified by all manufacturers for all models on the market. However, CFM is transparent – unlike CADR - which means that it exposes the low output of many air purifiers on the market with little to no confusion added. Thus, manufacturers are hesitant to reveal this data.

To make matters worse CADR is confusing which has set a precedence for using confusing metrics to sell air purifiers to consumers. Thus, metrics like square footage coverage, ACH, etc. – all confusing in their own right – are used instead of CFM (a much more transparent metric) to market most air purifiers when a CADR rating is unavailable.

We would much prefer CFM to be the standard metric used to compare the output of air purifiers. Again, it’s easy to understand and it’s easy to compare. That’s why we use CFM to compare different units and hope the whole air purifier industry adopts this standard in the future.

One extra caveat

CADR is based on the highest fan speed. Granted, CFM is as well.

Keep this in mind though if you’re using CADR to estimate maximum area of coverage according to other online tools.

Our room size recommendations above take into account the fact that you’re highly unlikely to set the air purifier to run at maximum fan speed at all times. Manufacturer specified CADR and online area of coverage tools do not take this into account.

For the curious – a full explanation of CADR

I. An easy explanation

CADR stands for Clean Air Delivery Rate. In other words, an air purifier’s CADR tells you the rate at which it delivers clean air.

Let’s look at the letters of this acronym one at a time in an attempt to clarify the meaning of this term:

Clean – what is clean air? Clean air is simply air that no longer contains particles. Which particles? CADR is always given as the CADR for smoke, dust, or pollen. Thus, “clean” air in the case of smoke CADR is air that no longer contains any smoke. Clean air in the case of dust CADR is air that no longer contains any dust. And finally, clean air in the case of pollen CADR is air that no longer contains any pollen.

Air – air is simple to understand – it’s a natural occurring gas that you breathe in. It’s about 80% nitrogen molecules and 20% oxygen molecules. It also contains other types of molecules but these are found in much lower quantities. An air purifier attempts to clean air – which means that it’s essentially trying to remove anything that’s not “air” from the air. Simple enough.

Delivery – clean air is delivered by the air purifier. The air purifier takes in dirty air, forces it through a filter, and outputs or delivers clean air.

Rate – here’s the most important word in the acronym. Rate has to do with time. If you run at a faster rate of speed you’re running a certain distance in a shorter time period. Naturally, larger air purifiers with bigger fans and/or fans that spin faster will be able to process more air in a shorter amount of time – at a higher rate.

And so, CADR largely has to do with the rate or speed at which an air purifier can process a particular quantity of air – how fast can it process that quantity of air. But remember, the air has to be clean. If the air purifier doesn’t clean the air it doesn’t matter how fast it’s delivering or outputting it.

Thankfully, as we showed earlier, most air purifiers clean air extremely efficiently. So the rate at which they process air (CFM) is very similar to the rate at which the clean air (CADR).

A more detailed explanation of why CADR and CFM numbers are similar

Why is an air purifier’s CFM so close to its CADR? The answer is simple – air purifiers do a very good job at what they do. Most units have a HEPA filter. HEPA filters remove 99.97% of the particles in the air. That means that a particular quantity of air has to go through a HEPA filter only once for 9997 of 10,000 particles to be removed from that quantity of air. The Blue Pure 411 doesn’t even have a HEPA filter – it has a proprietary particle filter that’s only 99% efficient. But at 99% efficiency it will still remove 99 of every 100 particles in a given quantity of air that’s processed.

It’s no wonder then that the Clean Air Delivery Rate for any particular air purifier is very close to the Delivery Rate, period – the CFM.

II. A more technical explanation

A more technical definition for CADR is this:

CADR is essentially a fraction.

The top part of the fraction (the numerator) is equal to the CFM of air in an approx. 1,000 cubic ft. from which all particles of a particular type (pollen, smoke, dust) have been removed (by the air purifier).

The bottom part of the fraction (the denominator) is equal to the rate at which those same particles would naturally fall out of the air under the same conditions.

Dividing this numerator by this denominator gives the CADR.