What is CADR of an Air Purifier and Why It is Important

In Brief: What is CADR?

Issued by Association of Home Appliance Manufacturers, CADR stands for Clean Air Delivery Rate. It is the total volume of filtered air thrown out by the air purifier in a certain time period.

CADR is issued by AHAM (Association of Home Appliance Manufacturers), USA.

It is meant to help one compare different air purifiers, since there are myriad claims by different brands. But, one must know what CADR tells you, and what it cannot.

Since AHAM CADR is an optional certification, many companies do not have, or give, a CADR number, especially in India.

Also, an air purifier can claim a CADR number without having AHAM certification, too.

How is CADR Measured?

CADR is measured in cubic foot per minute (as in the USA) or cubic meter per hour (as in Asia and Europe). So one has to be careful to check the units when talking about CADR.

1 cu m / hour
= 3.28 x 3.28 x 3.28 cu ft / hr
= 35.31 cu ft / hr
= (35.31 / 60) cu ft / min
= 0.5886 cu ft / min
In reverse, 1 cu ft / min = 1.699 cu m / hour

If an air purifier has CADR of 150 in the USA, it will be CADR of 254 in India, since it is measured in different units in the two countries.

CADR is not the rate of airflow rate out of an air purifier; it is the rate of purified airflow rate out of an air purifier.

CADR = Air flow rate x Air purification percentage

Thus, if the air purifier cleans only 90% of the particles from the air that goes into it, its CADR will be 90% of the air flow through it.

Since the air going into an air purifier will not disappear, the input air = output air = air flowing through the air purifier.

Since pollutants are of different sizes, no air purifier cleans all air pollutants to the same extent. Most air purifiers work well for larger particles but do a pathetic job removing smaller, and more medically dangerous, particles. Many cheap air purifiers don’t even clean very small particles.

An air purifier might remove pollen (10 – 1000 micron) far better than talcum powder dust (0.5 – 60 micron), since the former are bigger particles.

As a result, while the air flow rate through the air purifier might be the same for pollen and talcum powder, the CADR of the purifier will be higher for pollen than for talcum powder.

In an AHAM certification for CADR, there are 3 sizes of particles measured: Tobacco smoke, dust and pollen. Here are their sizes used in testing:

  • Small: Tobacco Smoke (0.09 – 1 micron)
  • Medium: Dust (0.5 – 3 microns)
  • Large: Pollen (5 – 11 microns)

Thus, a typical air purifier should mention three CADRs: Tobacco smoke CADR, Dust CADR, and Pollen CADR, or if they mention one, it should be the smallest of the three.

The Average CADR

what is cadr

Usually, the small particle CADR (tobacco smoke CADR) is the lowest of the three, since smaller particles are harder to clean.

Hence, if a single CADR is mentioned, it should be the Tobacco smoke CADR. But many brands just mention the average of the three.

The names of the three CADRs, viz. Tobacco smoke, Dust and Pollen are misnomers.

Tobacco smoke CADR of the air purifier does not really specify its tobacco smoke cleaning ability; just its ability to clean small (but not very small, less than 0.09 micron) particles.

Ideally, the names of the three CADRs should have been small particle CADR, medium particles CADR and large particle CADR, respectively.

Tobacco smoke has particles of sizes from 0.01 – 4 micron. In the testing, the laboratory uses only a subset of those particles, specially filtered to be within that range (0.09 – 1 micron).

Similar is the situation for dust particles, which range from 0.05 – 100 micron in real life, but for testing, dust particles of sizes from 0.5 – 3 microns is used.

CADR is only about physical or particulate removal; it does not talk about the removal of gases or odours.

Since gases are completely mixed with air, on the molecular level, physical filtration of gas particles cannot be done. That removal has to be done by chemical filters such as activated charcoal filters.

However, AHAM has no certification to measure that removal.

Thus, Tobacco smoke CADR will not include noxious tobacco odour removal, nor toxic gases in it such as formaldehyde, hydrogen cyanide, and benzene.

Here is a list of gaseous chemicals in tobacco smoke.

Why the Right CADR is important for Air Purification


In the image above, you can see that for the air purifier certified, Tobacco CADR is 48 cu ft / min, Dust CADR is 62 cu ft / min and Pollen CADR is 75 cu ft / min (since it is a USA certification, the units are assumed to be cu ft / min).

Dust CADR of 150 effectively means “the air purifier will reduce the dust particles to the same concentration that as would be achieved by adding 150 cu ft of clean air each minute”

CADR is measured at the highest fan speed of the air purifier.

Since air purifiers might be noisy at its highest speed, most consumers use air purifiers at lower fan speeds, resulting in lower effective CADR.

To calculate the room size that an air purifier can clean well, follow this rule of thumb:

  • To find the largest room size in sq ft that can be cleaned well, multiply the air purifier CADR by 1.5. So, an air purifier with CADR of 100 can be used for a room size of 150 sq ft.
  • To be more rigorous, AHAM suggests 1.55 times, not 1.5 times.

However, there are certain caveats to this rule.

Since Tobacco smoke CADR measures small particle cleaning ability, AHAM suggests that you consider Smoke CADR for calculating the room size that can be cleaned well by the air purifier.

The CADR mentioned by AHAM is the USA number. So it should be CADR in cu ft / min. If it is CADR in cu meter / hour as mentioned in India (Asia and Europe), multiply by 0.5886 to get the USA equivalent number, or multiply Indian CADR by 0.88 to get sq ft of room size.

How does AHAM suggest the room size for a CADR? It does this calculation using a standard room height of 8 feet and multiplies it by the room area (in sq foot).

A typical room is 9 – 10 feet in height. However, it may also have various fixtures, such as cupboards, sofas, beds, dressing tables and storage units. As a result, effective volume of air in a room will be 10 – 20% smaller than the actual volume of the room. So 8 feet height is a reasonable approximation for the room air.

For good cleaning, one should filter the full room’s air at least five times an hour. This is called ACH (Air Change per Hour).

Take the lowest of the three CADRs, which is usually the small particulate one – Tobacco Smoke CADR. Let us say, it is 150 (in the USA). So CADR = 150 cu ft / min

In an hour, it will clean 150 x 60 cu ft = 9000 cu ft of air. This should be five times the air in the room to get reasonably clean air in the room. So the room air volume should 9000 / 5 = 1800 cu ft.

Taking 8 feet as the effective height of the room, we get the ideal area of the room
= 1800 / 8
= 225 sq ft of room, which is 1.5 times the CADR.

In India, the rule would be:

Room size should be smaller than 0.88 times the CADR (in cu m per hour).

Hope this helps you select the right purifier for your office/home or car. Another important aspect to consider while buying an air purifier is ACH (Air Change per Hour).

Share your thoughts in the comments below.

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