Because of my expertise in the air pollution and filtration field, people often ask me for advice about how to choose an air purifier. Since air pollution is a preventive health issue, I felt I should write a couple of articles explaining my thought process about air purification issues. You can read many good writeups about air pollution on this website.
In this article, I will explain how to decide on the capacity of air purifiers for your home or office rooms. I will discuss more advanced aspects, such as chemical (fumes) and biological (viruses) filtration, in some other articles.
Desired capacity for a given room area
Let us start with a typical question that I get:
What air purifier capacity do I need for a room of, say, 200 ft2 floor area?
Fair question. The right answer: insufficient information to make a decision. Based on the area given, there is no way one can give any suggestions on the capacity needed.
Now, some websites tell you rules–of–thumb such as “divide the room area by 1.55 to get air purifier capacity”. However, there are so many assumptions in such advice that you are more likely to go wrong following it.
And if you are wrong, either your purifier will be of much lower capacity, in which case, it will not protect the health of your family; or, it will be of much higher capacity, in which case, you would have paid a lot of extra money for no added benefit.
So, let me walk you through the process of decision–making.
The first issue you will encounter is about your country.
Since most of my readers are from two countries—the USA and India—I will use measurement units from both the countries.
There is a vast difference in the way air purifier capacities are quoted in the two countries. So, I will mention both the numbers. Most countries follow one of the two numbers.
The USA uses the Imperial system, which involves foot (ft) as the measure of distance.
In India and many countries around the world, the Metric system is used. The distance is measured in meters (m) in the metric system.
1 meter = 3.28 feet
So, 1 square meter = 1 m2 = 3.28 ft x 3.28 ft = 10.76 ft2
And, 1 cubic meter = 1 m3 = 35.31 ft3
As we will see later:
In the USA, the air purifier capacity is measured in ft3 per minute, or CFM (cubic foot per min), in short.
In India, the air purifier capacity is measured in m3 per hour (not per minute).
So, 1 m3 / hour = 35.31 ft3 / 60 min = 0.589 ft3 / min = 0.589 CFM
And the other way round, 1 CFM = 1.7 m3 / hour
In the USA, when they quote air purifier capacity as 100, it is actually 100 CFM, or 170 m3 / hour.
And, in India, when they quote air purifier capacity as 100, it is 100 m3 / hour, or 58.85 CFM.
In other words, if a USA–based air purifier has a capacity of 100, the same unit will have a capacity of 170 in Indian terms.
That is why you should always mention the units for air purifier capacity. Or, at least, specify the country you are referring to.
We will now look at what this capacity means and how it is calculated.
Room size, area and volume
If you know the area of the room, in which the purifier is to be used, that is not enough. You need to know the height of the room, too. After all, you want to know the total volume of air to be cleaned.
Let us say, the height of the ceiling is 10 ft. So, the volume of the room will be 200 ft2 x 10 ft = 2,000 ft3.
Now, usually, the height of a floor of a building is counted as 10 ft. And, out of that, 2 ft are left for building infrastructure, such as flooring and ducting. So, in general, the ceiling is considered to be 8 ft in height.
However, after 1995, the building developers in the USA have started using 9 ft as the height of the ceiling for most higher-end apartments, while 8 ft is still the norm for entry-level ones. So, in the USA, I suggest you take 9 ft for calculations.
The rule of thumb mentioned above uses 8 ft as the ceiling height. Right away, you will be 12.5% off in your calculations, if your ceiling is 9 ft high.
In India, too, the ceiling height is taken as 9 ft.
So, we will use 9 ft as the benchmark height for our calculations.
Measure the actual height, in case you are planning to get an air purifier for an attic (very low height) or a spacious villa (very high ceiling).
Thus, the inside volume of our room above will be 200 ft2 x 9 ft = 1,800 ft3.
There will be some furniture in our room. So, the volume of the air in the room will be less than 1,800 ft3.
If there are closets, wardrobes, and beds in the room, they will occupy certain volume. Thus, only the remaining volume will be that of the air that needs cleaning.
In general, the room air volume is taken as 80% of the total room volume. If your room is spartan, with minimalist furniture, you can take a different, higher number. For example, 90%.
And, if you have a long, empty corridor to be cleaned, you may take the number as 100%.
In our example, let us stick to 80%. So, the volume of the air to be cleaned is 80% x 1,800 ft3 = 1,440 ft3.
Air change per hour, or ACH
How fast an air purifier cleans the room depends on its capacity. However, it also depends on the room dynamics — room shape, placement of furniture, number of people inside, pollution generated in it, etc.
For example, a room with 200 ft2 floor area may be 20 ft long x 10 ft wide, or it could be a corridor 40 ft long x 5 ft wide. In such cases, the same air purifier will end up cleaning the two at different speeds.
Depending on whether the purifier is kept in one corner, or is kept under the centre table (as a showpiece 🙂 ), the room will clean slower or faster.
The placement of the furniture in the room will also decide this speed. Is there too much clutter? Is there a big bed or table in front of the purifier blocking air movement?
The number of people in the room, and what they are doing, will also affect the speed of cleaning. If the room is a bank office, a lot of people may be moving around. On the other hand, if the room is a library, most people will be sitting silently and reading. Such a room will clean faster.
It will also depend on the pollution generated in the room. In an extreme case, if there are people smoking in the room, you will surely need a much higher cleaning capacity. A doctor’s clinic, a workshop, a school classroom, and a kitchen will need a much higher cleaning capacity than a bedroom or a library would.
Also, no room will ever have 100% clean air (think about it!). The air purifier will bring the pollution down progressively. But, at what point should we consider the room to be clean enough?
All these decisions make it very complicated to calculate the exact capacity. There are just too many variables and complex equations involved.
Rules of thumb
So, the experts in the air control engineering field came up with a brilliant solution. They developed empirical rules — derived from numerous real-life observations — to calculate how any type of room can be cleaned.
“Empirically” means “by observation or experience rather than theory or pure logic”.
The engineers simply found that the capacity of the air purifier needs to be a certain multiple of the air volume in the room. This multiplier number is called Air Change per Hour, or ACH.
The multiplier changes with the type of room. A library might need an ACH of 3, and a kitchen may need an ACH of 7. Both these ACHs will ensure the respective rooms get ‘adequately’ cleaned in a ‘reasonable’ time.
Thus, a library having 1,440 ft3 air volume will need an air purifier that has a capacity of 1,440 ft3 x 3 = 4,320 ft3.
And, a kitchen having 1,440 ft3 air volume will need an air purifier that has a capacity of 1,440 ft3 x 7 = 10,080 ft3.
Now, I will come to it in a minute as to what this capacity means. However, note that these two rooms of exactly the same size need vastly different air purification capacities. So much for that rule of thumb: Divide by 1.55 to get…
Finding ACH values
You don’t calculate ACH for a room. You simply refer to a table that gives ACH values for various types of rooms, or spaces.
Various organisations, such as the American Society of Heating, Refrigerating and Air–Conditioning Engineers (ASHRAE), have developed standards and reference charts for that.
Table 1 below gives the rule–of–thumb ACH values for various types of rooms or spaces. I have added information to the table from multiple sources.
If a range is given for ACH, take the lower number as the minimum requirement for good quality air. If you calculate the capacity with an ACH number lesser than this, the room pollution will not be reduced enough. And, the higher number can be taken as the highest required ACH. Beyond this level, it will an overkill.Table 1. Type of room or space versus minimum and maximum values of ACH needed for removing air pollution
|Type of Room or Space||Min ACH||Max ACH|
|All spaces, in general||4|
|Churches, mosques, temples||8||15|
|Cleanroom, class 1,000 (ISO 6)||150||240|
|Cleanroom, class 10,000 (ISO 7)||60||90|
|Cleanroom, class 100 (ISO 5)||240||480|
|Cleanroom, class 100,000 (ISO 8)||5||48|
|Dining rooms (restaurants)||12|
|Factory buildings, ordinary||2||4|
|Factory buildings, with fumes or moisture||10||15|
|Kitchens, in homes||7||8|
|Kitchens, in restaurants||15||60|
|Mills, textile dye houses||15||20|
|Mills, textile general buildings||4|
|Photo dark rooms||10||15|
|Turbine rooms, electric||5||10|
|Waiting rooms, public||4|
If your room type is given in the table, use the lower ACH number. If your room type is not mentioned in the list, take an educated guess as to the closest room type it can match, and use the corresponding number.
Significance of ACH
Note that our air purifier cannot actually change the air. It simply recycles the air — sucks in impure air and throws out cleaned air, after purifying it. Just like your water filter does for water.
ACH encapsules almost all the characteristics of a room — how many people are typically inside it and what do they do, how much is the new pollution generated in the room, what kind of typical furniture it has, etc.
ACH does not account for where the air purifier is placed in the room, nor does it account for the design of the air purifier. So, ACH has nothing to do with the air purifier. It only refers to the room characteristics.
Thus, knowing the room dimensions (which tells us air volume), and the type of the room (which tells us ACH), you can calculate the air purifier capacity needed. Knowing only the room area, you cannot calculate the air purifier capacity.
Now you know why I can’t answer that innocent question: “What capacity of air purifier would I need for a 200 ft2 room?”
Clean air delivery rate, or CADR
The air purifier capacity is called its CADR.
CADR stands for Clean Air Delivery Rate. It is the number that we calculated above — the volume of air your purifier needs to clean (suck in, purify, and throw out) per hour.
Obviously, the units of CADR will be m3 / hour in India, and ft3 / min (CFM) in the USA.
Thus, in the above example, our library of 1,440 ft3 (oh, such a small library!) will need a CADR of 4,320 ft3 / hour and our kitchen of 1,440 ft3 will need a CADR of 10,080 ft3 / hour.
Of course, no country measures CADR in ft3 / hour. So, let us convert those numbers to the units used in India and the USA.
4,320 ft3 / hour = (4,320 / 60) ft3 / min = 72 ft3 / minute = 72 CFM.
4,320 ft3 / hour = (4,320 / 35.31) m3 / hour = 122.34 m3 / hour.
In other words, our 200 ft2 library will need a CADR of 72 in the USA. The units, CFM, should be mentioned but are often not mentioned by careless people.
The same library will need a CADR of 123 (rounding it off to a higher number) in India, the units being m3 / hour.
Similarly, our 200 ft2 kitchen would need a CADR of 168 CFM (in the USA) and 286 m3 / hour (in India).
The rule of thumb of ‘dividing room area by 1.55’, would have given us a CADR of 129 CFM for the library as well as the kitchen!
Examination Question 🙂
So, now that you are an expert on how to calculate required CADR capacity for a room, let me test you with a high–school question. Using table 1 above, answer this reverse question:
Here is the model answer, for full marks:
Since we are referring to India, CADR 220 means the air purifier cleans 220 m3 of air per hour.
Room areas that could be cleared by this air purifier will be different based on the types of rooms.
Using table 1, here are the areas you can clean for various types of rooms:
537 ft2: Bedrooms
269 ft2: Hospital rooms, offices, school classrooms, waiting rooms, banks
179 ft2: Living rooms, hair and beauty salons, shops
154 ft2: Kitchens in homes
133 ft2: Foyers, conference rooms, clinics, restaurants
89 ft2: Dining halls, club rooms
72 ft2: Restaurant kitchens, smoking rooms
53 ft2: Bars and discotheques
These are the room areas that an air purifier with CADR 220 m3/hr can clean to extremely pristine levels, with almost no pollution left.
Some experts recommend using such an air purifier for 25% lesser area than the calculated values above. This is to provide a margin of safety for errors in judging room dimensions and dynamics.
So, our purifier will clean an average room space of 268 ft2 to an excellent level.
These calculations assume the ceiling is 9 ft in height. If it is lower than this, you can increase the area covered proportionately.
This answer should fetch you full marks. 🙂
Obviously, the air purifier will not clean so well. But, what is ‘not so well’? Actually, two things will happen:
1. In the steady state that would be reached after some time, say an hour, the air in the room will be twice as polluted as it was intended originally. This may not be very bad if the purifier cleans the air to excellent levels as designed.
I have noticed many air purifiers cleaning to only a limited extent, in spite of claims of high CADR. Such air purifiers will struggle to clean, if they are made to clean double the area.
For example, I have seen a situation when the starting level of PM2.5 pollutants in the room was 50. When a famous brand name air purifier of properly calculated capacity was switched on, the PM2.5 came down to 20 after an hour. The highest annual average level of PM2.5 permissible is 10.
Ideally, PM2.5 should have come down to 1, the minimum possible number. It, indeed, did so when a high–end air purifier was used.
What that tells me is the famous brand name air purifier had either an incorrect CADR claimed, or it failed to function well in the high PM2.5 environment.
In such a situation, doubling the room area will reduce the indoor pollution control even more.
However, if the room area was doubled with the other high–end air purifier, it would not cause much harm to the PM2.5 levels in the room. Doubling the levels means PM2.5 of 2, which is still very low and safe.
2. The problem all air purifiers will get if you double the area is the room will take much longer, perhaps even twice as long to clean up, and to reach low steady-state pollution level.
Thus, if your budget is constrained, use a good air purifier for a larger area than recommended but be prepared to either wait to see the effect or start the machine an hour before you actually intend to use the space. A wifi-enabled air purifier might let you switch it on from your car using your mobile app before you reach home.
I personally feel that if money is the issue, it is better to buy a higher-end air purifier costing 2X and covering 2A area, than to buy two air purifiers costing X each and make them cover A area each.
So, now, you are an expert on calculating the air purifier capacity. 🙂 This is good enough for buying a simple, low–cost air purifier.
However, when you want no compromise on health issues, you will need to buy a high–end air purifier.
After all, an air purifier is your, and your family’s, spare lung. Whatever your air purifier does not filter, your own lungs will have to filter. And, your lungs are not disposable like the filters of your air purifier.
So, if you can afford it, always try to go for the highest quality air purifier. If you know what I know about air pollution and its newly discovered medical dangers, the cost of an air purifier should be the least of your worries.
If you can afford an air–conditioner, you should definitely consider an air purifier. That is my view.
The biggest danger with low–end air purifiers is that you don’t even know when they are not doing their job properly. You know when your air–conditioner or car is not working properly.
But, how would you know how much pollution your air purifier is letting pass unchecked to you?
When you don’t own an air purifier, you know that you are vulnerable to air pollution. But, when you install a cheap air purifier, you may get complacent thinking that you are protected. A false sense of security is worse than no security.
High–end air purifiers
High–end air purifiers clean far smaller particles than their cheaper counterparts. They clean a higher percentage, almost 100%, of them.
Their cleaning ability does not depend on the pollution levels. At higher pollution levels, low–end purifiers don’t clean as well.
Low–end purifiers work well when their filters are fresh and new. Once they start getting clogged with dirt, their efficacy starts dropping, often to below acceptable levels.
Low–end air purifiers, often, use outdated or harmful technologies. I will write about all these in a separate article in the future.
Removal of gases and chemicals
However, the biggest advantage of high–end air purifiers is the gaseous chemicals they remove from the air. Removing particles is far easier than removing gases, which are practically mixed to molecular sizes in the air.
The technologies are difficult to implement, especially if you try to clean multiple gases at the same time. The only solution is using thicker filters. But, that reduces the airflow through the filter.
Hold a handkerchief against your nostrils. Try inhaling through it. Now, replace the handkerchief with a thick towel. Check if you can inhale as easily. That is exactly what happens when the filter gets better and thicker.
So, you have a trade-off. Either, use thicker, more expensive filters, and let your CADR numbers drop. Or, live with less effective cleaning of gases, and boast of a high particle CADR.
The cheaper air–purifiers let you save costs by using thinner filters. The more expensive air purifiers use thicker, costlier filters. And they have to use bigger, more powerful motors to suck the air in harder, without causing additional noise. All this adds up to cost.
False sense of security
Unfortunately, most people don’t understand all these details, since this a very nascent field.
People have come to understand why some mobile phones and cars cost 5–10 times more than their cheaper counterparts. But, they don’t understand why a good air purifier would cost 2–3 times more than its cheaper counterpart.
A cheaper car or mobile does the same function as their expensive brothers do, albeit not so well.
That does not happen in the world of air purifiers. If an air–purifier does not clean as well, or as much, you are still exposed to the pollution.
And, very soon, I will release an article which shows the data on why the biggest risk of air pollution is not at the higher levels, but rather at much lower levels.
In layman’s language, let us say that if you inhale little pollution, your medical risk is X. If the pollution becomes 5 times, your medical risk does not become 5X. It will perhaps rise only to 2X.
So, to remove the risk, you need to remove all the pollution, down to the last bit. A little bit of pollution removal won’t help.
Considering it is a subject that you need to understand, I will write a separate article on it. Incidentally, I have not seen a single article written on it anywhere on the internet.
But, that is not surprising since the supporting medical research data is barely a year old. So, hang in till you get that eye-opener.
Various types of CADRs
Just to whet your appetite: an air purifier does not have a single CADR. It has many.
There are different CADRs for different particle sizes. That should be logical. As pollutant particles become smaller, the filtration ability of an air purifier drops. So its CADR for those small sizes drops.
This fall is drastic for low–end air purifiers since they use cheaper filtration technologies. High–end, costly air purifiers, are able to filter even the minute particles quite well. As a result, their CADR for smaller particle sizes does not drop.
Now, think about it: if you calculated the required CADR as above, should that not be the smallest CADR number of the air purifier? I know it is getting a bit difficult to grasp. So more of that later.
Back to chemical gases removal
Gases need to be chemically, and not physically, removed. Typically, for many polluting gases, the CADR is significantly lower. So, if your main concern is chemical pollution, a different set of CADR numbers need to be used. For example, Nicotine in tobacco smoke.
In tobacco smoke, nicotine is present as a gas. When you want an air purifier to remove cigarette smoke, you will check the air purifier’s CADR specifications for superfine particles. Incidentally, that number is called Smoke CADR of the air purifier.
However, nicotine is in the gaseous form when tobacco is smoked. So, you need to consider the air purifier’s CADR for nicotine gas, and not CADR for smoke particles.
Most air purifier companies will market their product showing the smoke CADR number, which has nothing to do with nicotine CADR number. How gullible can you get?
Due to technical challenges, the nicotine CADR is much smaller than the smoke CADR, for all air purifiers. It is not uncommon to have Smoke CADR 100 and Nicotine CADR 15 for an air purifier.
So, if removing nicotine from cigarette smoke is vital for you, you will need to check what your air purifier’s nicotine CADR is. Unfortunately, I have almost never seen an air purifier manufacturer mention nicotine CADR.
One needs to dig into various laboratory certifications for air purifiers to find this number. I have a suspicion that many low–end air purifier manufacturers do not even check their nicotine CADR, making do with just the smoke CADR.
In the absence of such information, air purifier field is a marketer’s paradise. Many innocuous-looking, but fake, claims abound: ‘our air purifier removes smoke’. Really?
For the time being, I suggest you ignore these complications and stick to simple air purifier calculations explained above. A step–by–step summary of how–to–calculate CADR is given below.
How to calculate air purifier capacity (CADR) for a room
- Measure the length and the width of the room.
Calculate: Area = width x length
- Measure the height of the ceiling in the room.
Assume 9 ft for most rooms, unless the ceiling is too low or too high.
Calculate: Room volume = area x ceiling height
- Estimate what percentage of room volume is its air volume.
Take 80%, if there is a normal amount of furniture.
Air volume = room volume x this percentage
- Decide on the required ACH for the room.
Refer to table 1 above to get suggested ACH for the type of your room. Most residential rooms have ACH = 4.
- Calculate required CADR in units of ft3/hour.
CADR = air volume x ACH
- Convert the CADR into the right units. This is the required air purifier capacity for your room.
For USA units, divide the number in point (5) by 60. For Indian units, divide the number in point (5) by 35.31.
First published on: 9th August 2019
Image credit: Deedee86 from Pixabay
Last updated on: 3rd May 2022