Most portable home air purifier manufacturers state several numbers used to measure effectiveness. The most common is the CADR, an acronym that stands for Clean Air Delivery Rate. You might also see references to the AHAM, or an AHAM rating, which stands for the Association of Home Appliance Manufacturers.
But what is the AHAM and what does CADR really mean? That is, how do they relate to what you – the home air purifier consumer – are really interested in: clean air?
CADR – Clean Air Delivery Rate
Like most fancy acronyms, this number is really a lot simpler than it first appears. But, at first blush the explanation looks complicated because to understand it requires a little bit of arithmetic, which intimidates some people. There’s no need to fear, though. It’s really basic. In words, it’s this simple: CADR is a measure of an air purifier’s ability to produce pollutant-free air.
That measurement is expressed as a certain number of cubic feet per minute. That is, the CADR essentially states the volume of clean air a portable air cleaner can produce in a given time. For example, suppose a particular model has a CADR of 250 for dust. That means the unit can reduce dust particle concentration equivalent to adding 250 cubic feet per minute of dust-free air.
This is a good time to note that most units actually state three CADR numbers: one for smoke, another for dust, and a final one for pollen.
The number means much the same thing in each case. It still refers to the ability to reduce that material’s concentration by a certain amount in a given time. It’s just that virtually all units can do that differently for the different kinds of home air pollutant, so the manufacturers state different ratings for each category. There are lots of reasons for that: particle size and weight, filter efficiency which differs from one type to another, and more.
AHAM – The Association of Home Appliance Manufacturers
One of the reasons home air purifier makers go to the trouble of measuring and reporting those three different numbers is that they are (almost) all members of the AHAM, the Association of Home Appliance Manufacturers.
That organization certifies that the numbers that air purifier makers report are accurate and honest. They use an independent, 3rd party lab to test the devices and verify the numbers reported.
Of course, “honest” is a little bit of a relative term here. No major manufacturer tries to deliberately misreport their numbers. That would threaten their membership in the AHAM and result in bad publicity, something none of them wants. But there are various ways they can report those numbers that can differ from one device to the next.
Ways of Reporting CADR
Two different home air purifier makers may have units with identical CADR numbers but still be quite different in their ultimate effects. Here’s how not to be fooled…
The key to using CADR numbers to compare models accurately lies in two things: (1) take them as approximate, (2) look closely at what’s behind them. The first is obvious, but what does (2) mean?
Simple, just get the context around that number. In particular, look for the number of air exchanges per hour, and the specific room characteristics assumed by the CADR. The AHAM gives manufacturers a little leeway here.
A stated CADR references a ‘standard’ room. Ensure that your room is ‘standard’. That is, the standard used for CADR’s assumes an 8-foot high ceiling because the ordinary home has (or had for many years in the U.S. after WWII) a ceiling that high.
As the years have rolled on, more and more home designs deviated from that, so adjust accordingly. If your home has a cathedral ceiling, an open area that leads up to a second floor, or other deviation from ‘the norm’ look for a home air purifier with a larger number to compensate.
In brief, don’t assume that because your floor area square footage is the same or even smaller than what the model specifies that the device will purify your room air totally. The device operates, after all, on the total volume, not just the air from your head on down.
It’s a good idea to follow the AHAM’s “2/3 Rule” but modified. What’s that?
The (Modified) 2/3 Rule
Suppose you have a room measuring 10′ x 12′ (120 square feet) whose air you want to purify and keep smoke-free. You should look for a home air purifier with a Smoke CADR number of at least 80. (120 x 2/3 = 80).
It’s important to keep in mind that CADR tests are carried out with the air purifier at the highest setting. Since your home unit will typically run at the middle speed/power most of the time, the CADR obtained by the 2/3 Rule is the bare minimum and you should add about 30% to get a ‘floor’ on the CADR number.
So start with 120 x 2/3 (the 2/3 rule) = 80. ADD 1/3 to that, so: 80 + (80 x 1/3) = 107 (approx).
That calculation assumes an 8-foot ceiling, so naturally if you have one that’s higher or not flat, open to another story, etc. multiply accordingly. It would be too complicated to detail here what is “accordingly” for the general case, so just approximate your situation.
For example, if you have a 12-foot sloping ceiling, add about another 30% to the number. So, for this case, that number becomes: 80 + (80 x 1/3) + (80 x 1/3) = 133 (approx).
CADR Helpful But Not The Only Factor to Consider
CADR numbers, helpful as they are as a starting point, are not the only important feature for ensuring a good air purifier model as the ratings don’t capture some very useful information.
For example, because the AHAM tests are short they don’t tell you how the air purifier will perform over the long haul. They also don’t encapsulate how well the air cleaner captures small particles, often the most damaging to health nor do they tell you how well a model removes VOCs (Volatile Organic Compounds), which can be unpleasant or even unhealthy.
So, the moral is: use the CADR as a starting point but look also to other features of any home air purifier you’re considering before you make a final choice.
The AHAM maintains a searchable Directory of Certified Room Air Cleaners that can help you get started on that process.