Buying an Air Purifier-Don’t Make a Mistake Like I Did

I have brought two useless air purifiers or “air cleaners” myself-probably $1500 of fairly useless plastic. The manufacturer’s advertising whilst truthful concealed what I now consider to be a hidden defect in the machines.

As a rule only consider air purifiers with a HEPA filter-ideally an H13 filter. Do not buy a machine that does not specify a clean air delivery rate (CADR). Even if it does specify a CADR which is large enough for your room, do not buy the machine if the noise the machine will produce running the fan at top speed to produce that CADR will be intolerable for you.

Please let me walk you through the problems so you do not buy a machine that either you cannot live with or does not clean you air properly. Remember, what you are looking for is simply a machine with a fan pushing enough air through a high quality HEPA filter. A carbon filter is useful for removing chemicals. Other technologies are less useful.

Should You Buy a Known Brand?

I am sure all of us would consider this very important as you will not know whether the machine is effectively cleaning your air and you may sit or sleep in a room with it running for years relying on it for your health. I can imagine that unfortunately there are 1000s of asthmatics like me around the world who think that they have tried an effective air purifier and so are convinced that their asthma does not respond to an air purifier-when the purifier was simply not supplying an adequate amount of clean air. So the lack of response of their asthma may have been due to the fact that the air was not truly purified air and had they had a better air purifier their asthma may have been much better.

So many people may wish to stick to designs from brands that are well known for their quality. Given an air purifier may affect your health and you cannot easily check on its function as the particles in the air are invisible, this seems a good strategy. A manufacturer with an established brand has a reputation to lose. So they have an incentive to be careful with the design and manufacture of their air purifier.

Unfortunately, this is no guarantee-there have been instances of-

  • Making claims that are not judged to be backed up by good data-as reported by the New York Times ” This Advertising Watchdog Just Rejected Basically All of Molekule’s Air Purifier Claims
  • Presenting data in a way which could easily mislead an unwary consumer. This happened ot me.
  • Omitting specification data which the consumer needs in deciding which machine to purchase-see the section on noise below. This is incredibly common probably 50% of manufacturers do this. It is customary for manufacturers to give the maximum noise that their air purifier makes. So I had to smile when I found a well known US manufacturer only gave the quietest noise for their air purifier-that on the lowest airflow. Unfortunately, this airflow was less than 1/6th of the maximum airflow. This increases the risk that the consumer will buy the air purifier for the maximum flow rate, find it too noisy, switch it down, have a higher particle count and their health will suffer.

So you cannot unquestioningly rely on manufacturers-not really a surprise. Still, because you are buying it to protect your health and you cannot see or smell of particles in the air and easily tell if it is not working, I do think it is worth buying a trusted brand and not an unknown import. Especially as there are some really good cheap air purifiers from US manufacturers-please see this article on air purifiers under $100.

Buying from a respected manufacturer should be a starting point for buying an air purifier. However, you should not completely rely on this as there are non-obvious problems with some air purifiers from respected manufacturers.

Buying an Air Purifier-Internet Reviews

I am a physician and I hope you forgive me for saying this, but I think that you already know it. I have seen so many dodgy reviews, wildly incorrect math, websites recommending models with seemingly earning commission as the overwhelming/only aim, little or no understanding of the science or health implications and frankly completely wrong recommendations. So you need to be very careful buying on the basis of internet reviews even from very respected websites. One crude test for this would be search the review for “ultrafine”, ultrafine particles enter our bloodstream and are from a health point of view probably the most damaging particles. If you do not find them even discussed in the review that you are reading, I would advise you to consider not to following the recommendations. They should give you information about the capability of the air purifiers you are thinking of buying to deal with ultrafine particles even if it is only to say there is no data for a particular air purifier but that another air purifier is specified to deal with them. Then you can make a choice to buy the air purifier that will definitely deal with ultrafine particles.

Reviews of Consumers-Usually Unintentionally Misleading Regarding Aspects Related to Health

The reviews of consumers are very misleading as regards health implications. The vast majority of reviewers simply comment on how much their air quality is improved without any data such as particle count to back up their assertion. It is impossible to tell whether a particle count is 10,000 per cubic liter or zero particles per cubic liter-an enormous difference! Most of the reviews, therefore, are simply reflecting observer bias where because the consumer has spent money on an air cleaner the consumer thinks that the indoor air is better. Even if they are right, they do not know how much better and whether a different air purifier would have given a better result in terms of particle reduction.

However, consumer reviews are useful for documenting mechanical or electrical failures of the air purifier, as these are easy to determine that the reports are probably accurate. They are also useful to let you know how easy an air purifier is to live with in terms of noise.

Buying an air purifier testing one model with an air quality meter
Testing an Air Purifier a near perfect reduction in particle count to zero

The Air Purifier Must Have a HEPA Filter

True HEPA filters are the core technology for reducing indoor air pollution. They provide highly effective filtration for airborne particles of sizes down to 0.1um as shown in the tables below. For particles smaller than this “ultrafine particles” there are some studies showing that they are very effective but others show only a 50% efficiency. This is important as 90% of the particles in the air are ultrafine particles and when we breathe them in these microscopic particles can enter our blood stream! So most of the health effects of air pollution are due to ultrafine particles.

There are only three manufacturers that certify their filters as being effective against fine particles and that is iQair for its “Hyper HEPA” filter, Oransi and Coway.

Be careful, do not buy air purifiers with “HEPA type”, “HEPA style”, “99% HEPA”….. these are not true HEPA filters. Neither should you buy an air purifier with washable HEPA filters or permanent HEPA filters unless you can see certified data showing that they really do function as true HEPA filters.

A HEPA air filter is excellent for removing harmful particles from the air including cigarette smoke, airborne allergens, and mold spores. Pet hair, house dust mite, pet dander and an occasional allergen are so heavy that gravity makes them fall to the floor and they never enter the purifier. Fragments of pet dander and house dust mite may be small enough to remain suspended in the air and enter the air cleaner.

The Institute of Environmental Sciences and Technology has categorized HEPA filters into 6 classifications A, B, C, D, E, or F, according to data provided by the National Air Filtration Association . In the EU they categorize HEPA filters as either E10, E11, E12, H13, H14, U15, U16, U17.

US classification-

HEPA Filter ClassPercentage of Particles 0.3um RemovedComment
A99.97%no ST
B99.97%no ST 2FLT
C99.99%ST Photometer
D99.999%ST Photometer
E99.97%ST Photometer 2FLT
F99.999%*ST Particle counter

ST=Scan test 2FLT=2 flow leak test * particle size 0.1-0.2 um

In the EU they classify HEPA filters into different names based on how many particles they remove from the air, shown in the table below:

HEPA Filter ClassPercentage of Particles of 0.3um Removed

These filters are excellent at reducing indoor air pollutants but have most difficulty reducing particle sizes of around 0.3um in diameter. Tobacco smoke produces particles in this size range and so one of the standardised tests is to assess their abiltity to deal with tobacco smoke.

You can still get an air purifier with a H13 True HEPA filter for under $100, so why settle for less?

The interval at which you will need a replacement filter varies with which HEPA air purifier that you buy-please see this article.

Carbon Filter for Reducing Chemicals/Odor

Carbon filters are not essential but good to have if possible. Most air purifiers now have an activated carbon filter for eliminating chemicals from the air. Most have a thin sheet filter covered in activated carbon with little adsorbing capacity. Actual carbon granules are very much more effective at removing these gaseous pollutants. So an air purifier with a filter with carbon granules should be chosen if the absorption of volatile organic compounds/smells is important in your situation. The amount of absorbing capacity can be estimated from the amount of activated carbon in the filter and the frequency at which filter replacement is needed. This can vary from as little as 6 months for the thin coating of carbon on material filter through to every 5 years for charcoal granules/solid charcoal. The best carbon filters are seen in the Austin and iQair air purifiers.

Factors that increase uptakes of volatile organic compounds-

  • Low temperature
  • low humidity
  • low air flow through filter
  • total surface area
  • removal efficiency of media in filter
  • higher concentration of chemical to be removed
An activated charcoal filter from an iQair HealthPro Plus-2.5kg. Carbon granules int he filter can be seen in the image on the right
An activated charcoal filter from an iQair HealthPro Plus-2.5kg. Carbon granules int he filter can be seen in the image on the right

Activated carbon is not very effective at removing sulfur oxides, hydrogen sulfide, low molecular weight aldehydes, for instance formaldehyde, ammonia and nitrogen oxide. So these filters are very good at reducing odor. The thicker the layer of carbon better the removal of volatile organic compounds. Some studies have used six-inch deep carbon filters, and even then there is a breakthrough of chemicals coming through the filter. The problem with thin filters is not only that they do not remove the gases on the first pass through the filter, but they quickly become saturated and will not adsorb further pollutant. They can even become a source of previously adsorbed pollutants. This type of filter does not generate any chemical byproducts. The carbon filter should come after a particle filter so that particles do not clog the pores in the filter.

Overall these filters can be very effective at reducing many chemicals from the air and so odor, provided that the filter has not become saturated with chemicals.

Certified Clean Air Delivery Rate (CADR)-That is adequate for your room size-the most important factor

Buying an air purifier that is not powerful enough for your room size is by far the most likely mistake to make. If you do make this mistake, then the air purifier is unfortunately useless for you. Or even worse, you may use it for years thinking that is purifying your air adequately and all the time breathing in more pollution than you had expected.

Although specifications of filters occupy our thoughts when buying an air purifier, provided we choose a HEPA filter the efficiency of removing fine and coarse particles in the air will be almost identical. Air flow through the filter.

Consider this thought experiment-

An ideal sealed room into which an air purfier is put. In the middle panel of the illustration the air purifier A has a filter that is 2.5% worse than that in air purifier B which is a large difference in terms of HEPA filters. The air purifier in the right hand panel has a flow rate that is 25% too low, this is actually a small difference in practice. Most people using air purifiers will have a much lower air flow rate than this because they buy the air purifier assuming they will use the highest airflow rate. People then get the air purifier home find it is far too loud on the highest setting and so switch it on to a lower setting.

Diagram of thought experiment to illustrate the importance of air flow

This makes some simplifications that do not apply in the real world but does illustrate the point of how important air flow is in choosing an air purifier.

When buying your air purifier you should have a particular room in mind and measure its dimensions. The bigger the room is, the bigger and more powerful the air purifier that you will need to deliver clean air fast enough. The clean air delivery rate (CADR) is the amount of clean air that the air purifier can produce in a given time. It is measured either in cubic feet per minute or meters cubed per hour. You will need the air in your room changing 5 times an hour to get a particle reduction of 80%. So if you multiply the volume of the air in your room by 5, the air purifier should be able to produce this volume of clean air every hour.

Choosing an air purifier that can provide an adequate air flow for your room size at a noise that you can tolerate is absolutely critical for good air purification

Some manufacturers do not specify the CADR of their air purifiers at all . This can occur when a corporation has an expensive air purifier with a low CADR. So if the manufacturer does not specify a CADR certified by AHAM you should not buy the air purifier as it could be a waste of money for you. I have managed to make this mistake myself when I was younger. There was a manufacturer who did not specify a CADR and it turned out the impressive flows of air coming from the air purifier were a mixture of clean and dirty air. Not all the air from the machine had been through the HEPA filter and clean air was only a small fraction of the total. If you get the AHAM approved CADR you have a measurement of the totally clean air that the machine can produce.

There are several ways to work out the CADR that you will need –

  1. An online calculator-there is one here
  2. It should be specified on the packaging-only accept the AHAM certified room size. Be aware that this calculation assumes you have an 8 foot high a standard height room. If your room is taller than this, you will need to increase the AHAM CADR value proportionately. You may find it easy to use a calculator. The reason for needing to be sure that it is an AHAM certified room size is this: some manufacturers will specify a room size which their air purifier can change the air twice an hour. This calculation leads to a much bigger room and makes their air purifier look wonderful. Please note CADR is not the same as the air purifiers fan power. The power of the fan is measured without a filter in the unit and so may look impressive but is irrelevant to you, you can think of it as the “dirty air delivery rate”.

Find Out How Noisy the Air Purifier is at the Speed You Will Need

Air Purifier Noise is an Important Issue During the Day

Noise is a problem because the high CADR which you buy the air purifier for is usually that when the fan speed is on maximum. This is because this is the only airflow rate that most manufacturers will give you. When you get the air purifier home, you will find operating it at that speed sounds like having a tornado in the room! So you may not be able to be in a room with the air purifier on its highest setting.

When you switch it onto a lower setting, the CADR may not be adequate for your room. You may only have a 20-50% reduction in particle count not the EPAs recommended minimum 80% reduction. So switching the air purifier onto a lower fan speed will mean inadequate air purification. Logically your health is likely to suffer, as there is no discernible lower limit to the effect of airborne particles on human health.

So because of the lack of information, you may find you have ended up buying an air purifier which is useless for you!

Ideally, you need to know what the sound level and CADR for each fan speed but very few manufactures give you this information. About 30% of customer services for manufacturers will give you this information if you contact them, the others will just say that they have not measured it. Alternatively, you could just choose from this website as if manufacturers will not provide this information we do not recommend their air purifiers.

So first before buying an air purifier you need to work out how loud an air purifier you would be able to live with. One way of testing for the maximum level of noise that you would be comfortable with would be to use your smartphone to record the number of decibels of sound in your room. Then put another phone or computer where the air purifier will be and play a file of white noise. You can then sit where you will sit for most of the time in the room and try putting the volume on the computer or smartphone up and down until it is as loud as you feel you could comfortably tolerate for hours at a time. Then with your smartphone note the noise level in decibels 6ft away from the computer/smartphone producing the sound. The 6ft distance is because this is the distance from which manufacturers seem to measure the sound that you read in the brochures or on their websites.

Personally, I would not want to be in a room long term where the noise from the air purifier was greater than 50db and I feel more relaxed when it is 41db. It is obviously fine for short periods to have it on a noisier setting to, for instance, clear particles from the air when cooking. So if the top fan setting is going to mean that the air purifier is going to be noisier than your noise tolerance level then you may want to ask the manufacturer what the noise level and CADR (airflow) are on the next level of fan speed down. I personally am only interested in settings at or below 50db.

You may have a higher noise tolerance In fact the next fan speed down in some air purifiers may be too noisy. What is too noisy for longterm use will vary from person to person. For me anything above 50db is too noisy.

So the most important part of choosing the air purifier is asking yourself the question “does it have enough airflow for my room at a noise level that I can tolerate”. So never buy an air purifier for its maximum air flow as you will likely find this fan speed too noisy, unless the noise at this speed is less than 50db (or 40db for a bedroom).

Alternatively, you may simply wish to try the air purifier in a store, but please remember the background noise in the store may make it seem much better than it will seem in your quieter home. Also, it will seem much better for a short testing period than it will be used for hour after hour in your home.

This may seem rather picky but to be exposed to more noise than is comfortable for you for hours on end becomes somewhat stressful, and turing the air purifier speed down could longterm lead to damage to your health.

Sleeping and Air Purifier Noise

Sleeping with an air purifier on and yet adequately purifying the bedroom air is difficult. Noise is obviously a particular problem in a bedroom-if it is even slightly too loud there will be an urge to switch the purifier down even if the air will not be adequately purified. I did this myself, and it was only when I brought an air quality meter that I realized that the air was not being adequately purified.

As a general rule you should be wary of putting an air purifier into “sleep mode”. This is because the clean air delivery rate is unlikely to be sufficient for your room to meet EPA minimum standards for air purification. The airflow in sleep mode is often 1/7th to 1/4 of that at peak airflow. It is unlikely that when buying your air purifier that you used this very low airflow in sizing it to your bedroom. So switching the air purifier into sleep mode will mean inadequate air purification. Logically your health is likely to suffer, as there is no discernible lower limit to the effect of airborne particles on human health.

So it is best to work out your noise tolerance when sleeping and then buy an air purifier that will deliver an adequate CADR for your room at this noise level or below. When sleeping my tolerance for noise is 41db-yours may of course be different. An interesting point is that a 10db drop in sound level sounds half as loud, so I need an air purifier for sleep that is half as loud for a bedroom as that for a living room You ideally should work out what your tolerable sound level is for sleeping using the method above to be sure.

Or you may want to use my ballpark estimate of the air purifier being 10db quieter for a bedroom. Your tolerance of sound may be quite different, but these are my working guidelines. It also depends how near you in the room the air purifier will be-sound falls off quickly with distance from the source of the sound-the inverse square of the distance from the purifier. For instance, doubling your distance from the air purifier will make it seem 4 times quieter. However, my guidelines do consider being at least 6 feet away from the air purifier.

There are likely millions of people who have brought an air purifier and that because of noise have switched the fan speed down and are running it on a fan speed that is too low to purify their air properly. These millions of people will have particle counts that are higher, and so health that is deteriorating at a faster rate than it would if the air was adequately purified.

My advice is if the air purifier manufacturer will not give you the CADR at the fan rate where the noise is slightly below 50db (or 40db for a bedroom) then do not buy that air cleaner. The reason is that if you buy one for its highest flow rate which is often noisier than 50db, you are likely to switch it down to a quieter fan setting. The airborne particle count will then be higher than it should be and your health will suffer.

Alternatively, just buy recommendations from this website, for bedrooms which are a special case please see the article on best air purifiers for bedrooms.

Do Not Judge an Air Purifier by Looks, Price or Fancy Technologies

Pleas not judge an air purifier by its looks some of the best looking best “image” air purifiers have surprisingly low airflows.

Also, please do not judge by how expensive an air purifier is. Some very expensive air purifiers have no clean air delivery rate (CADR) specified at all. This may make you wonder why a manufacturer would not specify the CADR, you may think that they would do so if the CADR was good. It also makes it impossible for you to size the air purifier according to the EPA’s room sizing guidance.

Conversely some cheap air purifiers have really good airflows for the price. For the air purifier with the best airflow per dollar please see this article.

As a general rule fancy technologies other than a fan and HEPA filter are disappointing for reducing particle count. Here is an example-

As you sized it for your room at the highest setting the next fan setting down will give you inadequate air purification, you may only have a 20% reduction in particle count. The inadequate reduction in particle count will mean that your health will deteriorate at a faster rate, and this will occur without you realising it until something goes wrong.

Do You Want an Air Purifier Certified to Remove Ultrafine Particles?

Ultrafine particles are damaging for human health, as outlined in this article. There are only 3 air purifier manufacturers that certify to reduce these, the Oransi, Coway and iQair. Only the iQair HealthPro Plus is certified to remove particles in the 3-10nm diameter range, as outlined in this article. It may be that other air purifiers reduce ultrafine particles but we cannot be sure as they are not specified to do this.

So if you want to remove ultrafine particles with your air purifier it really helps make the decision for you.

Technologies That Are Not Essential

UV light from a UV lamp, ion generators and photocatalytic converters are not essential. Ozone generators can be harmful for human health. An electrostatic precipitator is not essential for air purification.

The Purifier Should be “Smart”

It takes about 30-45 minutes for the particle count in the air to be brought down by the air purifier, which will need to be set to come on at least 30 minutes before you come into the room. So if you buy one that is not smart you will have to switch it on when you come into the room and have a period when you are breathing air when the particle count is high before the air purifier can reduce it.

Overall, I prefer to have an air purifier that will come on at the fan speed that it last operated at and running the purifier on a smart plug, connected to a smart home system. This way I can have a more complicated schedule for the purifier and save on electricity and filters.

Another great way of controlling an air purifier is to have one that connects to your home WIFI and then operating it via a mobile app, Alexa or Google home

Should the Air Purifier Have an Air Quality Sensor?

There are 7 problems with running an air purifier on the automatic setting using the inbuilt sensor-

  1. Not measuring at the right point in the room-The inbuilt dust sensors are usually cheap and too inaccurate to guide your use of the air purifier. They are also mounted on the air purifier itself, but what is important is what the air quality is like at your mouth where the air is entering your body, not at the inlet of the air purifier.
  2. They cannot detect ultrafine particles. When using an air purifier in automatic mode, this depends on the air quality sensor to detecting a rise in particles in the room and then increase the speed of the fan. The particle counters in air purifiers cannot even “see” about 90% of the particles in the air-the very smallest ones-“ultrafine particles”. These are the ones that have been found in human brains.Ultrafine particles can vary in concentration out of proportion to the fine and coarse particles that the sensors can see. So the air purifier sensor measurements are fundamentally flawed in terms of protecting human health.
  3. They use inaccurate technology for the particles that they are able to count. Not only are the particle sensors flawed in this sense but also because they are cheap and almost invariably use suboptimal measuring technology. They are often based on infrared technology rather than laser diffraction technology. Good, accurate particle counters start at $1000, so obviously these are not built into your air purifier. This is starting to improve with laser particle counters being integrated into some machines, but even these particle counters cannot see ultrafine particles at all.
  4. There are usually only 3-6 grades of air pollution indicated. Remember that the EPA recommends a PM 2.5 of 12ug/m3 or below as safe for health whereas recent studies suggest that even in the range of 0-12ug/m3 there are health effects. So we really need PM2.5 measurements to within the nearest 2ug/m3 and these sensor ranges may only be specified to the nearest 35ug/m3.
  5. Some manufacturer’s set ludicrously high levels for satisfactory levels of pollution. One manufacturer sets the upper limit of good air quality at a AQI of up to 100 equivalent to a PM2.5 of 34ug/m3, whereas the EPA set the limit at 12ug/m3 and current scientific evidence suggests that there is no lower limit so anything above 0 is potentially starting to impact long-term health. So at a PM2.5 of an air purifier by this manufacturer will sit there on its lowest setting letting you breathe the pollution in!
  6. Most manufacturers do not give specific levels for PM 2.5 for the various ranges that the sensor detects. So you have no idea what for instance that amber light indicating pollution level means when it is on. You are literally staking your life on these guys getting it right. These are the same guys that often/usually represent their machines as covering a 50% greater room area than the EPA specifications would say is reasonable!
  7. Some manufacturer’s allow the consumer to alter the sensitivity of the sensor. Once this is done you have no way of knowing what the coloured lights to indicate pollution levels mean. Other than by making your own measurements which makes the sensor redundant.

Actions may speak louder than words, top of the range air purifiers do not have inbuilt sensors eg iQair Healthpro Plus, Oransi, Coway. Also reputable companies such as Medify have removed inbuilt sensors from for instance their Medify MA-40 model.

So never run an air purifier in automatic mode! You should run it on the highest setting on which you can tolerate the noise.

As you are not going to run the air purifier according to the inbuilt sensor there is no need to buy one with an air quality sensor, but-

It is Essential to Buy an Air Quality Meter

You cannot tell if the particle count is good using your senses. So you need a meter to measure indoor air quality. The delicate HEPA filter may have been damaged in transit. You may have positioned the purifier in a place in the room where it does not join in well with airflows in the room. So it really is important to have some way of checking that it is purifying the air in the part of the room where you’re going to be breathing the air.


So when buying an air purifier you should look for one with a true HEPA filter and ideally an activated carbon filter. This combination of filters provides an excellent air filtration system for reducing airborne pollutants.

Air purifiers with this combination of filters are common but only a few are really good-I have a list of recommended air purifiers here, which you may find helpful. It is surprisingly difficult to lower the particle count in a room and so a good air purifier is a must. You should aim for a more than 80% reduction in particle count. To check that your air purifier is effective, you should buy an air quality meter. If you buy the right air purifier and test the air after it has been running for an hour you can be certain that you really are breathing much cleaner air.

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