Part Two: Did you Know that your Air Purifier is Lying to You?

How efficient is your air purifier when it comes to cleaning the air you’re breathing?

After discussing the flaws in air quality indicators that air purifiers generally have, its time to move the next marketing gimmick – Clean Air Delivery Rate!

The first step is to understand what Clean Air Delivery Rate or CADR actually is!

Clean Air Delivery Rate is the rate at which a specific volume of air passes through the Air Purifier.

It is measured in Cubic Feet per Minute.

This is where the marketing comes in – Cubic Feet per Minute is a unit of volume – a volume of air, not a volume of pollutants in air!

So when an air purifier has a CADR of 80, it means that it is moving 80 cubic feet of air per minute.

It does not mean that it is CLEANING 80 cubic feet of air per minute!

The ability to clean air depends entirely on the filter, and with use, the cleaning ability of a filter keeps decreasing!

Understanding CADR with an Example

Remember those math problems we had in school – The ones that had a tap draining a water tank at a specific rate (volume of water passing through the tap).

Our job was to either figure out how big the water tank was or how long it would take for the water tank to empty itself.

Well, this is kinda similar – instead of a water tank you have a room, instead of water, you have air, and instead of the tap, you have an air purifier fan pulling air through.

If the tap is bigger, more water will flow through!

This is where CADR becomes a marketing gimmick – for an air purifier, if the fan is bigger, the volume of air flowing through it will be high.

However, does that mean that the volume of air passing through the air purifier will be cleaned at the same rate as well?

Filters do not work on CADR

One of the primary mistakes consumers make is that they assume filtration rate and CADR to be the same!

Clean Air Delivery Rate has been worded very carefully to make “volume of air moved” seem like “volume of clean air moved” – which is millions of miles from being the same thing!

Time for another example: Let us assume that we have a 1 CADR air purifier with an HEPA filter that captures 10% of all particles that flow through it in each cubic feet of air.

Let us also assume that the HEPA filter has 100-filtration points where it captures these particles and the air has 400-pollutant particles per cubic feet of air (each occupying 1 filtration point when captured).

Now, if you run the air purifier for a minute, then 1 cubic feet of air (1 CADR) or 400-pollutant particles will pass through it.

Since the HEPA filter captures 10% of pollutant particles, 40-pollutant particles will get stuck in the filter (taking up 40-filtration points out of 100), and 360 particles will flow through uninterrupted.

The next time you run the air purifier for a minute, only 360-particles will flow through, but there are only 60-filtration points left on the HEPA filter.

So in the second pass, 36-pollutant particles (10% of 360) get stuck, meaning that a total of 76-filtration points are now used up on the HEPA filter.

Then, you have 324 pollutant particles remaining in that cubic feet of air, which means that on the third pass, the HEPA filter will capture 10% or 32 particles more, taking its total filtration point usage to 98.

Now, there are only 2-filtration points left on the filter so on the fourth pass, and no matter what the CADR might be the filter cannot hold more than 2-particles of pollution!

The only way it can capture more pollutants is if you clean it up and dispose off the captured pollutants safely and securely, not returning it to the outside environment in any way or form.

This simple example should help you understand that filters do not work at the same cleaning-rate all the time!

The cleaning rate depends on various factors – air speed, type of pollutant particles, quality of contact between pollutant particle and HEPA filter, efficiency level of HEPA filter, etc.

In fact, a very high CADR can indicate that the suction fan being used is quite powerful, which can work adversely in that the speed of air passing through the filters dislodges pollutant particles already stuck on it.

Look out for MERV Rating Instead

You know you are looking at a high quality HEPA air purifier when you have a MERV rating attached to it.

MERV stands for Maximum Efficiency Reporting Value – a rating scale that has been designed to measure the efficiency of HEPA filters.

This rating not only decides the size of the Particulate Matter pollutant that the HEPA filter can capture, but also the percentage of that pollutant’s content in air that it will capture each minute.

Here is a basic table of MERV ratings and what they mean:


Minimum Particle Size Type of Pollutant Captured Typical Application


PM10.0 or bigger

Pollen, dust mites, cockroach debris, sanding dust, spray paint dust, textile fibres, carpet fibres, etc. Residential Window AC Units


PM10.0 to PM3.0

Mold, spores, dust mite debris, cat and dog dander, hair spray, fabric protector, dusting aids, pudding mix, etc. High quality residential, industrial and office AC Units


PM3.0 to PM1.0

Legionella, Humidifier dust, Lead dust, Milled flour, Auto emission particulates, Nebulizer droplets Top quality residential and commercial units + Hospital Laboratories


PM1.0 to PM0.3

Bacteria, droplet nuclei (sneeze), cooking oil, most smoke and insecticide dust, most face powder, most paint pigments Hospital and General Surgery
17-20 PM0.3 or smaller Virus, carbon dust, sea salt, smoke

Electronics & pharmaceutical manufacturing cleanroom

Please Note: This rating does not apply to activated carbon filters as they are designed for chemical and toxin pollutant molecules which are 10,000 times smaller than the smallest physical pollutants.

That’s the thick and thin of it all – CADR is a marketing term used by companies who don’t have products with a strong MERV.

If you are looking to buy an HEPA filter-based air purifier, don’t worry too much about the CADR and focus only on the MERV – and you will never go wrong!

We hope you liked our articles on Air Purifiers’ Air Quality Indicators and Clean Air Delivery Rate.

We would love to hear from you, so if you have any questions or queries, please email us at

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