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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 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 the air purifier.

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

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.

I have some helpful guidance on what to look for when buying an air purifier. I have asthma and I am the owner of 6 air purifiers and have learned the hard way what not to buy!

Reviews of Consumers-Usually Unintentionally Misleading

The reviews of consumers are very misleading. 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 count.

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.

Testing an air purifier
Testing an Air Purifier

It Must Have a HEPA Filter (E13 Class)

True HEPA filters are the core technology for reducing indoor air pollution. They provide highly effective filtration for airborne particles with 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 showing 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 is only one manufacturer that certifies its filter as being effective against fine particles and that is iQair for its “Hyper HEPA” filter.

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-Good to Have but Not Essential

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

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.

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.

Some manufacturers do not specify the CADR of their air purifiers because they know that they are not adequate for most people. So if the manufacturer does not specify a CADR certified by AHAM you should not buy the air purifier as it is likely to 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
  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”.
Buying an air purifier don

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

Noise is a problem because the high CADR which you buy the air purifier for is go by the manufacturer having the fan speed on maximum. When you get the air purifier home, operating it at that speed can sound like having a tornado in the room! You may not be able to sit in a room with the air purifier on its highest setting but on lower settings the CADR may not be adequate for your room. So because of the lack of information, you may end 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 almost no manufactures give you this information.

So before buying and 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 record the noise level in decibels right next to the computer producing the sound.

Personally, I would not want to be in a room long term where the noise from the air purifier was greater than 47db 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 maximum 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.

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 slightly stressful.

When sleeping my tolerance for noise is 41db-yours may of course be different. You could work out what yours is using the method above. If you are going to have an air purifier in your bedroom, you need to know if you can tolerate the noise when the fan is fast enough to produce an adequate CADR. 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.

Please do not consider running an air purifier at a setting below the specified CADR for your room long term because of noise, as it will not be adequately cleaning the air. Your health might suffer-better to put it in another room where noise is not such an issue and buy a better one for the 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.

Ideally, there would be a consumer boycott of manufacturers that do not provide sound levels and CADR for each fan speed so that the consumer is not at risk of buying a machine that you cannot live with at the highest setting because of the sound and at the next fan speed down the CADR is too low to adequately purify the air in your room.

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 2 air purifiers certified to reduce these, the Oransi EJ 120 and iQair HealthPro Plus. Only the iQair HealthPro Plus is certified to remove particles in the 3-20nm 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 have a Timer or be Run on a Smart Plug

It takes about 30-60 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 without a timer 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. Be careful, the degree of control that you have over the timer is also important. Some air purifiers have timers that specify a 2, 4, or 8-hour period for which the air purifier will operate- buy one that allows you to control to the nearest minute when it should come on and turn off.

Overall, I prefer an on-off switch and running the purifier on a smart plug. This way I can have a more complicated schedule for the purifier and save on electricity and filters.

Should the Air Purifier Have an Air Quality Sensor?

It is better to buy an air purifier without a sensor and have a separate sensor. This way the sensor can be used to test easily different locations, even in your car. This can be surprisingly useful.

Also, if the sensor is inbuilt, it will be impossible to check its accuracy if you feel it may have become inaccurate-please see this post on sensors. The cheapest reliable air quality meter is $35.

So I would suggest buying a cheaper air purifier without a sensor and a separate sensor.

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 you should buy a portable air purifier 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 will be breathing much cleaner air.

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