Air Purifiers: All-In-One Essential Guide And Overview

It’s about the air we breathe. If the promise of an air purifier were true, it would make an air purifier an essential appliance for every home. The reason is simple:

We care about the quality of air we breathe.

Adults make about 15 breaths every minute. That’s almost 1000 breaths per hour. If we’re fortunate enough to get 8 hours of sleep per night, our lungs inhale about 10,000 times during the night alone.

The quality of the air we breathe is important. Unfortunately, indoor air has an above can be 500% more polluted than outdoor air. Here’s the quote for the EPA’s Indoor Air Quality report:

“(We) spend approximately 90 percent of our time indoors, where the concentrations of some pollutants are often 2 to 5 times higher than typical outdoor concentrations.” (EPA Indoor Air Quality Report)

Air pollution has become the world’s single biggest environmental health risk, linked to around 7 million deaths in 2012 according to a recent WHO report.

Table of Contents

Our Ancestors Enjoyed Clean Outdoor Air

Our lungs were fully developed during our hunter-gatherer years. During that time, we were breathing mostly outdoor air, uncontaminated by modern pollutants such as industrial dust, pesticides, or other allergens.

When we built houses, schools, offices, we invariably started breathing indoor air.

As EPA found out, we spend more than 90% of the time breathing indoor air. 

While outdoor air is always moving around, with winds blowing left and right, indoor air is static and stale. It slowly yet steadily accumulates air pollutants such as dust, pollen, smoke, mold, pet dander, and presents a fruitful breeding ground for bacteria and even viruses. 

Here’s the thing:

Such air pollutants find an easy way in our lungs. Lungs are comprised of about 500 million alveoli; small pulmonary cavities that we expend with every breathe we take.

The diameter of the alveoli is 200 to 500 microns. Most air pollutants such as dust, pollen, smoke, and mold, are smaller than that. They can quite easily enter and settle in our lungs.

sketch of human lungs, alveoli and airways
Common air pollutants, like dust, pollen, and smoke, can easily enter our lungs and alveoli.

In short, our lungs were designed to breathe clean outdoor air. In the modern world, however, they are forced to breathe contaminated indoor air. This is not what they were designed to do, and may need some help.

This is the very pickle air purifiers are designed to solve. The promise of air purifiers is simple enough:

To improve indoor air quality by removing or capturing air pollutants. Dust, smoke, pollen, mold, even certain bacteria, and viruses are captured by a series of filters.

Really sounds simple enough, right? But are air purifiers actually capable to deliver on that promise, or is this all just a marketing hype?

To find out, LearnMetrics looked at scientific studies and answered 37 questions about air purifiers. In this article, you will find everything you need to know about air purifiers.

From HEPA filters to CADR ratings, the effect on allergies and cardiovascular system, to noise levels, ozone concerns, and if air purifiers are actually worth the money. 

We’ll start with the question most people have when contemplating about buying an air purifier, or three:

Do Air Purifiers Really Work?

Air pollutants are suspended in our indoor air. Every breath we take – even the one you’ve taken right now – is filled with air pollutants. Air purifiers promise to reduce the concentration of airborne air pollutants as much as possible.

But do air purifiers actually work?

This is a two-fold question. What we are really asking is this:

  1. Do air purifiers remove air pollutants, thereby increasing indoor air quality? 
  2. Will using an air purifier have a positive effect on the health of our family?

No one is really doubting that air purifiers are capable of capturing air pollutants such as dust, pollen, and smoke. As we’ll learn in the section about filters, HEPA filters alone are, by definition, capable of capturing more than 99.97% of even the smallest air pollutants that are only 0.3 microns in diameter.

The answer here is clear. If you measure the concentration of air pollutants, and thereby the quality of indoor air, every air quality sensor tells you the same thing:

Running an air purifier increases the quality of indoor air.

The second part of “Do air purifiers work?” is more tricky. It’s the important one; does increase the quality of the air we breath actually has any positive effect on health?

Several studies of how the use of air purifiers can affect specific health conditions were conducted during the last 20 years. Let’s look at some of them to see if air purifiers really do help in general and for certain conditions:

Do Air Purifiers Help With Allergies?

“The spread of respiratory allergies is increasing in parallel with the alarm of the scientific community,” is the first sentence in an Italian respiratory allergies overview study

Pretty everybody has an allergy to something. Sneezing, wheezing, itching, or shortness of breath are all symptoms of respiratory allergies, like asthma, allergic rhinitis, and other pulmonary allergies.

Dust mites in unfiltered indoor air are common source of allergies.
Dust mites in unfiltered indoor air are a common source of allergies.

Air purifiers do promise (and actually deliver on that promise) to capture airborne allergens, like dust, pollen, mold, smoke, or pet dander. Does that mean that air purifiers can help with allergies?

A Korean study about the effects of air purifiers on children with asthma has found that “…the indoor PM2.5 concentrations significantly decreased through the use of air purifier for 3 weeks” and concluded the following:

“We concluded that air purifiers can have a positive effect on the health of asthma patients by filtering fine dust and microbes from indoor air.” (Korean Asthma Study)

This study clearly suggests that air purifiers can help with asthma. Other studies like the 2016 asthma management study published in the American Academy of Pediatrics concludes that air purifiers are better at capturing allergens like smoke, but are less effective at removing other allergens, such as animal dander.

Despite studies showing a positive correlation between the use of air purifiers and the reduction in allergies, nothing is yet conclusive. It’s increasingly difficult to separate the increase in indoor air quality brought on by the actions of air purifiers are other factors, such as genetic and other environmental factors.

As such, it’s worth noting that are purifiers are not recognized as medical devices, according to the FDA guidelines.

Effect Of Air Purifiers On Cardiovascular Diseases

“Heart disease is the leading cause of death in the US,” according to the CDC. “1 in every 4 deaths” is the US is related to cardiovascular diseases. For example, 1 person suffers from a heart attack every 40 seconds.

Can air purifiers decrease the probability of being that 1 person that suffers a heart attack?

Some experts do suggest so. It’s easy to imagine the effect of air quality on our lungs. We do breathe with them after all. But how can indoor air quality impact our heart?

In an interview for Cleveland Clinic, a pulmonologist Rachel Taliercio said the following: “By filtering out fine particles, purifiers help clean the air you breathe and lessen the potential negative effects of pollution,” says Dr. Taliercio. “Purifiers have been shown to alter the blood chemistry in a way that may benefit heart health, too.”

Dr. Taliercio is not alone. “Despite improvements in air quality across the U.S. during the past few decades, more than 88,000 deaths per year occur in the U.S. due to fine particulate matter air pollution exposure,” said Robert Brook, M.D., a cardiovascular medicine specialist at the University of Michigan’s Frankel Cardiovascular Center in the interview on Michigan Medicine.

His arguments are underpinned by a 2018 US study published in JAMA Internal Medicine. The study focuses on the effect of air purifiers on the blood pressure of seniors. The study concluded that:

“The use of indoor portable air filtration for 3 days led to significant reductions in systolic blood pressure in elderly adults.” (2018 US study)

The researchers have specifically pointed out that air purifiers seem to be a very inexpensive method to reduce blood pressure. This does shed a light on the question if air purifiers are worth the money.

Another recent study – the New York University School Of Medicine 2020 study – came to a similar conclusion:

“Over a median 13.5 days, there was a significant reduction of mean systolic BP by ≈4 mm Hg.” (New York 2020 Study)

In short, there are scientific studies that air purifiers might help with cardiovascular diseases, especially with high blood pressure. Of course, the results of these studies are not yet enough for the FDA to recognize air purifiers as medical devices.

Other Diseases With Studies Showing High Air Quality Is An Advantage

Medical articles, published in the scientific community, provide insight into links between using an air purifier and certain other diseases as well.

According to the 2013 MIT study, “total combustion emissions in the U.S. account for about 200,000  premature deaths per year in the U.S. due to changes in PM2.5 concentrations.”

HEPA filters commonly used in air purifiers do help to reduce these PM2.5 concentrations (fine particles with a diameter of 2.5-microns or less).

There also seems to be a link between dementia and cognitive brain disorders, such as Parkinson’s disease and Alzheimer’s disease, and air pollution. 

A recent 2019 Australian systematic review about air pollution and dementia has concluded that:

“Evidence is emerging that greater exposure to airborne pollutants is associated with an increased risk of dementia.” (2019 Australian Review)

A similar link between air pollution and brain diseases was established by a previous 2010 Harvard study. Here’s the conclusion of the Harvard study:

“Ambient traffic-related air pollution was associated with decreased cognitive function in older men.” (2010 Harvard Study)

There is a saying in the medical community: What the liver can do in a day, the kidney can do in an hour, and the lungs can do in a minute.

With lungs being capable to bring such a quick and refreshing change to our body, it’s not surprising that we find studies that connect air quality, air purifiers, and certain diseases year after year.

Let’s look at what additional benefits air purifiers can bring:

What Air Purifier’s Also Do Well?

AHAM, the Association of Home Appliance Manufacturers, is an independent institution that uses laboratory testing in order to evaluate how well an air purifier performs. They have developed a standardized method of measuring CADR rating (Clean Air Delivery Rate) with regard to dust, pollen, and smoke enabled us to compare different portable air purifiers.

Smoke, Dust, Pollen

In essence, this is a lab-tested way of finding out how well air purifiers remove:

  • Smoke particles with a size of 0.09-1.0 micron.
  • Dust particles with a size of 0.5-3 microns.
  • Pollen with a size of 5-11 microns.

HEPA air purifiers are independently tested and proven to remove all 3 air pollutants. Air purifiers that undergo voluntary testing are awarded the AHAM Verifide certificate.

Odors, Smoke, And Wildfire Smoke

Air purifiers are also well equipped to deal with undesirable odors. Kitchen odors such as the smell of burning oil and cigarette smell are effectively removed via specialized filters.

These activated carbon filters selectively and unselectively adsorb gaseous air contaminants, thereby improving the smell in that specific room.

indoor smoke particle and smoke smell
Burning candles indoors does release pleasing odors but also increases the concentration of smoke particles.

Example: Wildfire smoke is a specific air contaminant, comprised of small solid smoke particles and wildfire smoke smell in gaseous form. HEPA filters in air purifiers capture small smoke particles, and activated carbon filter removes the smell of wildfire smoke.

Mold, Bacteria, And Viruses

Mold, mold spores, and mildew are another class of air pollutants air purifiers are designed to remove. Mold is big enough to be captured by HEPA filters almost all air purifiers have.

Air purifiers specifically designed for mold, however, may include an additional germicidal UV filter to not only capture but inactivate/destroy mold and mold spores.

The same is true for bacteria and viruses. HEPA filters are designed to capture every air pollutant that is bigger than 0.3 microns; which does include certain bacteria and viruses. The question if air purifiers work against bacteria and viruses effectively, however, is still an open one.

More innovative companies have decided to forgo HEPA-based filtration system in order to target airborne bacteria and viruses. Molekule Air, for example, uses advanced PECO technology (photo-based filter) instead of mechanical filtration (HEPA) to fight bacteria and viruses.

Modern Cleaning Products, Asbestos, Pesticides, And Chemicals

In the modern world, it seems that we’re unable to escape the airborne chemicals. Home products like paint and cleaning product might produce airborne solid particles as well as gaseous odors that are captured by HEPA filter and activated carbon filters, respectively. 

Asbestos is another story. We know that buildings from the 1940s to 1960s were regularly built with asbestos, not yet aware of its deadliness. Some air purifiers, especially air cleaners with medical-grade H13 HEPA filters, claim to capture even the small asbestos particles.

Other chemicals, such as pesticides, can easily enter our homes via an open window. All these particles and gases are minuscule, invisible to the human eye. Air purifiers are designed to keep them from entering our lungs, or at least trying to minimize that process.

Improving Sleeping Quality

The action of air purification might have an impact on how well we sleep. Alen, a company behind the sleep-friendly Alen BreatheSmart series of air purifiers, was studying the sleeping patterns of 40 individuals with poor sleep quality. With or without an air purifier in their bedrooms, that is.

They partnered with SleepScore Labs and managed to get sleeping patterns from 1,591 nights. This study seems to suggest the following conclusion:

“Participants reported falling asleep faster and felt more rested the next day when using Alen purifiers at night.” (SleepScore Labs Study)

It needs to be said that this is not an independent study. However, it might shed some light as to how the use of air purifiers might affect our sleep. Our lungs inhaling a less polluted bedroom air might positively affect how well we sleep.

smoke air purifier noise levels from 30 decibel to 60 decibel
Air purifiers have different noise levels. At a low fan speed setting, they are a bit louder than a purring cat.

A well-balanced specification and the improvement of sleeping quality is also the reason why Alen BreatheSmart 75i is currently the best air purifier on the market.  

Given the available data, we see that there is an abundance of positive effects air purifiers seem to have. What is the other part of the equation? Let’s look at what air purifiers are not good at:

What Are the Disadvantages Of Air Purifiers?

An air purifier is another appliance that needs space, electricity, creates noise, needs proper maintenance (regular filter replacements), and can have quite a high price tag initially.

All these things might be considered the disadvantages of owning an air purifier. But they are the realities of owning pretty much any appliance. The benefits of air purifiers are well worth it for a large percentage of households.

What necessarily needs to be addressed is two specific things:

  • Ozone.
  • Radon.

The question ‘Are air purifiers bad for you?‘ is inevitably connected with ozone production. Some air purifiers use air ionizers; specific filters that destroy bacteria, viruses, and other air pollutants via the creation of ozone.

We think of ozone (O3) as a protective shield for our atmosphere. Ozone is a molecule composed of 3 oxygen atoms and, if inhaled, might be damaging to the lungs. 

In this EPA article about ozone, EPA warns about “Ozone generators that are sold as air cleaners”. Essentially, it’s better to steer clear of any air purifiers that use air ionizer filters and are not approved by the EPA.

Here’s the good part:

A great majority of air purifiers do not include air ionizer filters, and hence do not produce ozone. Those that do produce ozone, usually produce it at levels that are far below the legally permitted levels. 

In any case, it’s best to choose between a myriad of the best air purifiers that do not produce ozone. This is just a precautionary measure.

Radon is actually not a disadvantage or advantage for air purifiers. We don’t have enough data to suggest that air purifiers actually do anything regarding radon concentration in indoor air. 

Nothing that we currently know is capable of reducing radon. As far as we thus know it, and as the EPA has stated, “studies are inconclusive on air purifiers’ ability to tackle this dangerous gas”.

It’s also worth mentioning that air purifiers remove airborne pollutants only. If, for example, you have a dusty carpet, the dust is not airborne and thus cannot be filtered out by an air purifier.

With all that in mind, let’s look at how air purifier actually cleans indoor air:

How Do Air Purifiers Work?

Air purifiers come in all shapes and sizes but, at their origin, they are all basically the same. An air purifier is nothing more or nothing less than a combination of a fan and filtration system.

Here the two-step system of how all air purifiers work:

  1. A fan sucks in indoor air with air pollutants. It creates airflow throughout the device, which influenced the recommended coverage area and CADR rating.
  2. A filtration system, consisting of a series of filters, captures air pollutants like dust, pollen, and smoke.
inner working of air purifiers
Air purifiers inhale dirty air and exhale clean air. Filters are like lungs, the fan is like breathing muscles.

The resulting expelled air contains a considerably lower concentration of air pollutants. When we’re are choosing the best air purifier for our home, we might take into consideration the design of the air purifier, the reputation of the brand, and so on.

We at LearnMetrics look at the things that truly matter: the fan and, even more importantly, the filtration system. The resulting specification of air purifiers – including CADR rating, max. airflow, noise levels, ACH, coverage area, power requirements, and so on – all depend on just two things. Namely, the filtration system and the fan.

To truly familiarise yourself with air purifiers, it’s essential to know a thing or two about its most important part: the filtration system.

Types Of Filters

Air purifiers are basically just devices build around the air filters. These filters do the heavy lifting of capturing air pollutants and thereby increasing the quality of indoor air.

There are more than 6 types of filters that we find in air purifiers. Some filters like PECO and Ionic Wind filters are quite exotic. Others, like HEPA filters, are the most common and essential filter found in almost all air purifiers.

types of air filters
HEPA filters and other air purifier filters come in all shapes and sizes.

In fact, the classic filtration setup that even the best brands use consists of a pre-filter activated carbon filter, and HEPA filter.

This 3-stage filtration is, with or without additional filters, found in more than 80% of all air purifiers currently on the market.

3 stages of filtration on blueair pure blue air purifiers like 211 and 411 model
Classic 3-stage filtration system with HEPA filter in the center, as employed by small Blueair air purifiers.

Let’s start with these 3 essential filters before continuing to more innovative filtration systems. None is more important than the HEPA filter:

HEPA Filters (Capturing Small Solid Particles)

HEPA filters are the bread and butter of air purifiers. Air purifiers promise to capture even the smallest air pollutants, and the way that is done is by using a HEPA filter. Here is the definition of what HEPA filter is by the US Department Of Energy:

“(HEPA filters) remove at least 99.97% of airborne particles 0.3 micrometers (μm) in diameter.” (DOE)

HEPA (High-Efficiency Particulate Air) filters are made out of randomly ranged fiberglass and polypropylene fibers. These fibers create very small holes and capture small particles by diffusion, interception, inertial impaction, and electrostatic attraction, or a combination of these mechanisms.

Thus HEPA filters capture dust, pollen, smoke particles, mold, and certain bacteria remarkably well. When independently tested, they measure how many 0.3-micron particles these filters can remove. If they can remove more than 99.97% they can be called HEPA filters.

When looking at air purifier filtration systems, you will find different expressions for HEPA filters, including but not limited to ‘True HEPA’, ‘H13 HEPA’, ‘Medical-Grade HEPA’, and so on. It’s that there are differences between the quality of HEPA filters.

True HEPA is, however, just a marketing term for ordinary HEPA filters (H10-H12). H13 HEPA filters – also referred to as medical-grade filters due to use in hospitals and clinics – are better than standard air filters.

Air purifiers with H13 HEPA filters can capture particles that are 0.1-micron in diameter with more than 99% efficiency.

hepa vs h13 hepa filter comparison
Air pollutants captured by True HEPA filter vs advanced H13 HEPA filters.

Be advised: HEPA filters need to be replaced every 4-12 months, depending on the specific air purifier brand and use. Replacement HEPA filters can cost up to $100; the average price is about $50.

Of all the filters in air purifiers, the HEPA filter is the most important and the most precious one. However, if you would blast a HEPA filter with large air pollutants, it would reach its capacity in a matter of weeks.

Air purifiers use pre-filters to filter out these large air contaminants and protect the precious HEPA filter (as well as prolong its lifespan):

Pre-filters (Capturing Large Solid Air Pollutants)

All air purifiers – including the new-tech ones without HEPA filters – use pre-filters as the 1st stage in the air purification process. Pre-filters are used for two specific reasons:

  1. To protect HEPA filters and other filters …
  2. … by capturing large solid air pollutants.

Large solid air pollutants refer to human hair, pet dander, big dust particles, and so on. If those would directly hit a HEPA filter, it would stay on top of the filter, quickly dampening its ability to filter out small particles.

Some prefilters can be washable, which is also a bonus. It’s impossible to wash the HEPA filter, for example.

Both HEPA filters and pre-filters, however, do have a blind spot for odors. These mechanical filters capture solid particles. Odors are, on the other hand, in gaseous form. To complete the 3-stage filtration system, air purifiers use gas-adsorbing activated carbon filters:

Activated Carbon Filter (Adsorption Of Odors, Smells)

Cigarette smoke is comprised of both small solid particles as well as gaseous cigarette smell. HEPA filters are exceptional well at capturing solid smoke particles, but without the activated carbon filter, the smell of cigarettes would still linger in the room.

Activated carbon filter uses carbon filtering to remove undesirable odors. These filters are made out of charcoal or carbon substrates; both are very porous granular structures with an exceptionally high surface area. For example, 1 gram of activated carbon has an active surface area of more than 32,000 sq ft.

When air with gaseous odors is run through such a filter, the gases will be adsorbed onto that was surface area. This effectively eliminates odors like burning oil smell, gasoline smell, damp smells, cigarette smells, and many other odors.

Here’s a bit of a joke (but it’s also true): If you were to ‘relieve a gas’ into an activated carbon air purifier, you wouldn’t be able to smell that foul odor at the other end of the air purifier.

The reason is simple: Odor was absorbed by the activated carbon air purifier.

Replacement filters: Activated carbon filters need to be quite regularly replaced, every 3 to 6 months. They are less expensive than HEPA filters, with an average cost of about $40.

Activated carbon filters come in many sizes (weights). The heavier it is, the larger active surface area for adsorption it has, and it will do an increasingly better job. Example: If you check the best air purifiers for smoke, you’ll see they all have heavy activated carbon filters with high smoke CADR rating.

Germicidal UVGI, PCO, And PECO Filters (Against Mold, Bacteria, Viruses)

Air purifiers remove large and small air pollutants very well. However, indoor air is also potentially full of bacteria and viruses, and an air purifier seems to be the perfect device to remove them.

HEPA filters can capture them but live bacteria can grow on the HEPA filters and can be again released into our home via the air purifier’s airflow. That’s why several air purifier brands started adding germicidal filters to the established 3-stage filtration systems.

These filters have one common way of eradicating airborne bacteria and viruses: the UV-C light.

According to a Wikipedia entry “Ultraviolet germicidal irradiation (UVGI) is a disinfection method that uses short-wavelength ultraviolet (ultraviolet C or UV-C) light to kill or inactivate microorganisms by destroying nucleic acids and disrupting their DNA, leaving them unable to perform vital cellular functions. UVGI is used in a variety of applications, such as food, air, and water purification.”

Brans like GermGuardian use powerful UV-C light filters to try to destroy the bacteria and viruses. UV-C light is proven to destroy them; however, the effectiveness of these filters in air purifiers is questionable. This is primarily due to the limited exposure of airborne bacteria and viruses to the germicidal UV-C light.

The exposure lasts for seconds. It is deemed necessary for this exposure to last at least a few minutes for DNA disruption to be great enough to leave the bacteria and viruses inactive.

PCO shows a greater promise. PCO stands for Photo Catalytic Oxidation. In the Guide to Air Cleaners in the Home report, EPA states this about PCO:

“PCO cleaners use a UV lamp along with a substance, called a catalyst, that reacts with the light. They are intended to destroy gaseous pollutants by converting them into harmless products.” (EPA on PCO)

In essence, UV light is coupled with a catalyst to fight gaseous pollutants. Molekule, a well-known air purifier brand, for example, has perfected this technology and introduced its ground-breaking PECO filter

molekule air with peco filter and vegan handle
Molekule managed to create an advanced PECO filter which uses photoelectric catalyst oxidation to destroy indoor air pollutants.

Ozone Generators (Not 100% Safe, Or Even Useful)

You might even find some air purifiers that have air ionizer filters or complete ozone generations. These are devices that create ozone and try to use it in order to kill germs.

Ozone is problematic. Ozone is a known lung irritant. Ozone generator producers do ensure that the levels of ozone generated by these devices is not harmful to humans and are below publish health standards.

That may be true but, according to the EPA, at such low concentration, ozone is ineffective at removing most indoor air contaminants.

Here’s the bottom line on ozone generators:

You have plenty of ozone-free air purifiers to choose from. Considering several studies, ozone doesn’t really do much at low concentration, and it may be harmful to your lungs.

It might be best to steer clear of any air purifiers that do produce ozone, just in case.

Air Purifier Room Coverage (Fan Capability)

The filtration system is the most important part of any air purifier. The second most important part is the fan. It is the capability of the fan that determines air purifier:

  • Coverage area.
  • Maximum airflow.
  • (In part) CADR rating.

The bigger air purifiers have more capable fans. This, in turn, creates a bigger maximal airflow. Hence, the coverage area of the air purifier bigger.

Depending on room size a unit can cover, we differentiate between:

  • Very small personal or desktop air purifiers (coverage area up to 100 sq ft).
  • Small air purifiers (coverage area between 100 and 200 sq ft).
  • Medium-sized air purifier (coverage area between 200 and 400 sq ft).
  • Large room air purifiers (coverage area anywhere from 400 to 800 sq ft or even 1,000 sq ft).
  • Whole-house air purifier (coverage area above 1,000 sq ft).

Apart from the whole-house air purifiers which are installed in the HVAC duct system, all other air purifiers are portable.

A general rule is that you have 1 air purifier per room. That’s very true for smaller air purifiers. Bigger ones can cover 2 or even 3 rooms.

When planning to buy one or several air purifiers for our home, we need to estimate how big should these air purifiers be. For every air purifier, you will usually get a ‘recommended coverage area’, calculated by the producer.

size of room for certain air purifier
Measuring room size is essential to pick the right size of air purifier.

In order to understand what recommended coverage area actually means, we need to look into ACH:

Air Changes Per Hour (ACH) And Recommended Room Size

“Air purifier has up to 800 sq ft coverage area.”

This is a standard sentence in descriptions of air purifiers. However, it doesn’t tell us as much as this sentence (for the same model):

“Air purifier can clean up to 800 sq ft area every 60 minutes, up to 400 sq ft area every 30 minutes, up to 200 sq ft area every 15 minutes, and up to 160 sq ft area every 12 minutes.”

The number of times an air purifier can completely change the air in the entire room per hour matters. That number is referred to as ACH. You can read more about ACH and how it’s calculated here. Here’s how the ACH equation looks like:

calculator that calculates air changes per hour from cfm and ach formula

The dilemma consumers face is this:

If an air purifier cleans up to 800 sq ft area, that means it’s a large room air purifier, and we can put it in an 800 sq ft room, right?

But, if you look at the ‘up to 160 sq ft area every 12 minutes’, you would be right to think this is a small room air purifier, fit for a 160 sq ft room.

So which is it?

All these coverage areas depend on how many air changes per hour we use. Here are the options we have for the case above:

  • 1 ACH = 800 sq ft.
  • 2 ACH = 400 sq ft.
  • 4 ACH = 200 sq ft.
  • 5 ACH = 160 sq ft.

It’s generally recommended that we look at 5 ACH square footage, especially if we want a thorough air cleaning. For people with allergies, for example, it’s recommended that they put such an air purifier in a 160 sq ft room.

If we just want to increase indoor air quality, 2 ACH is enough. We can put such an air purifier in a 400 sq ft room.

These calculations and directives might be confusing. That’s why air purifier producers turn to AHAM to verify the air purifier room coverage, based on the measurement of the all-important CADR rating:

CADR Rating And AHAM Verified

Every air purifier producer can voluntarily decide to have their unit independently tested by AHAM. As a general rule, if you see the ‘AHAM Verified’ seal, you can trust the air cleaning specifications of that specific air purifier.

What does AHAM test for? CADR rating; the Clean Air Delivery Rate. This rating is the single most important specification for portable air purifiers. 

In most simple terms, it tells us how well the air purifier removes the most common air pollutants – dust, pollen, and smoke – from indoor air. We talk about:

  • Smoke CADR; measurement of how well air purifier removes 0.09-0.1 micron smoke particles.
  • Dust CADR; measurement of how well air purifier removes 0.5-3 micron dust particles.
  • Pollen CADR; measurement of how well air purifier removes 5-11 micron pollen particles.
AHAM independent testing of smoke CADR rating in a 1,008 cubic feet chamber.

In many ways, the CADR rating is the evaluation of the efficacy of the HEPA filter. It is the HEPA filter that captures these three air pollutants.

More importantly, CADR rating is one of the very few ways how we can compare air purifiers head-to-head. AHAM performs an independent standardized test in a 1,008 cubic feet chamber. Based on the CADR rating, we can easily see which air purifier is better at capturing dust particles, for example.


Additionally, based on the CADR rating, AHAM also issues recommended room coverage. 

AHAM issued coverage area, with AHAM Verified seal, is the most accurate assessment of the real coverage area of an air purifier.

Measurement of CADR rating is voluntary and the CADR rating does not apply to whole-house stationary air purifiers.

Now, let’s talk about another important specification:

Noise Levels

Air purifiers, in general, are quiet devices. Most of them run at about 50 dB noise levels; that’s the same as the sound of a fridge.

decibel illustration with air purifier between 40 and 60 db
Air purifiers produce roughly the same noise levels as fridges.

However, some air purifiers are louder and some are quieter than 50 dB. It’s important to check the noise levels, for example, if we’re looking for an air purifier for the bedroom.

Bigger air purifiers for large rooms tend to generate more noise. This is simply a side effect of a bigger airflow. If you were to put an air purifier in a larger master bedroom, you better check the noise rates beforehand.

It’s not unusual to see 60 dB max. noise level when it comes to bigger air purifiers. Air purifier producers do try to use new technologies to reduce the decibels, in order for the air purifier to be used in a master bedroom. Alen BreatheSmart, for example, is a series of the quietest air purifiers with max. noise levels below 50 dB which makes them perfect for bedrooms.

Here’s a customary line in air purifier specification sheets:

“Noise Levels: 25-53 dB”

What does that mean? Air purifiers have several fan speed settings. Every speed setting will generate air with different airflow, and will hence produce different noise levels.

In our case, the ’25 dB’ refers to the noise created by the lowest speed setting (usually Sleep Mode). The ’53 dB’, on the other hand, refers to the noise created by the highest speed setting.

Here’s how you can easily navigate these noise levels:

Since air purifiers are recommended to run all the time, you can run them on a high fan speed setting during the day (when noise levels are usually not a major concern), and switch it to a lower speed setting when you go to sleep.

Extra Features Air Purifiers Can Have

All air purifiers come with extra features that make your life easier. Some cheaper units include just the most basic ones (like ‘Timer), others include a whole array of extra features.

Here are the most useful features you should check for before buying an air purifier:

  • Set Timer. The ability to tell the air purifier to turn on or shut down after 2, 4, 8 hours, and so on. Perfect for when you want to come home and immediately enjoy high-quality indoor air.
  • Auto Mode. With this feature, air purifiers can automatically adjust fan speed settings in accordance with the quality of indoor air. Dirty air = high-speed setting, clean air = low-speed setting.
  • Air Quality Sensor, and Monitor. A requirement for Auto Mode is a sensor that measures the quality of indoor air. You usually get a monitor to see how indoor air quality is changing.
  • Wi-Fi Compatibility Apps. Apps that let you monitor air quality, and control the air purifier accordingly. Big brands like Dyson have their own apps. These smartphone applications make controlling the air purifiers seamless.
    dyson link app
    Newer Dyson models like HP02 and HP04 come with the Dyson Link app. The older Dyson HP01 model is controlled via remote control.

Before we look at which air purifier brands are the best, let’s clear up one question that is particularly important to people on the budget:

Are Air Purifiers Worth The Money?

As researchers in the 2018 US study, published in JAMA Internal Medicine, put it, “use of portable air filtration seems to be a very inexpensive method to reduce blood pressure”.

However, most homeowners are not buying an air purifier specifically of the documented cardiovascular benefits.

Air purifiers can cost from $100 to $900, and it’s customary to buy more than 1 for a standard home.

The unit cost is not the only expense. While air purifiers might be very inexpensive when it comes to electricity usage (look for Energy Star label for energy-efficiency), the cost of replacement filters can easily run more than $100 per unit.

The question ‘are air purifiers worth the money’ is quite an appropriate one. 

Pretty much every household has to decide for themselves if the investment in an air purifier makes sense for their budget, and if it’s within their means.

The benefits of air purifiers, however, are quite apparent and significant. We all have to inhale and exhale every 4 seconds or so. The quality of the air we continuously breathe today, tomorrow and for the next 50 or more years does matter.

Air purifiers are the best appliances that keep potentially harmful and irritant air pollutants from our lungs. Given that we spend about 90% of our time indoors, it does make sense to take measures to ensure we breathe high-quality air.

In addition to the numerous benefits breathing less polluted air brings, many also think about the interior design.

Air purifier’s aesthetics has come a long way. Some owners think of them as well-designed interior design pieces.

Given all the available information about air pollution and the importance of breathing high air quality, air purifiers are well worth the money.

They are, after all, the only affordable appliances that are documented to capture air pollutants, and have independent testing to back their claims. 

In the end, however, every household has its budget and its priorities. If healthy air is not a priority your budget would allow, make sure to spend it wisely on other priorities.

Best Air Purifier Brands

“First pick a brand, then pick a model.”

This is usually how buying an air purifier goes. Air purifiers from the same brand usually have the same filtration system, use the same built materials, have the same testing procedures, extra features, and so on.

Frankly, for many people, the design of a specific air purifier matters the most. All brands have a certain uniformity of design across their models.

The reputation of a brand, as well as the price of their individual units, also needs to be taken into account. It’s worth mentioning that big brands like Dyson or Honeywell are not necessarily better than air purifiers specific brands like Alen or Coway.

In fact, quite the opposite is true: companies that specialize in producing air purifiers only, instead of having a large portfolio of HVAC producing, usually produce the best air purifiers.

When you have your brand picked out, you can focus on the specific air purifier model, based on your needs (room size), budget, and aesthetic preferences.

To help you out and make an educated decision, LearnMetrics covered top models of several air purifier brands. You can check them all out, make your own assessment, and decide on the right model:

Alen BreatheSmart Air Purifiers Blueair Air Purifiers Coway Air Purifiers Dyson Air Purifiers Hathaspace Air Purifiers Honeywell Air Purifiers Levoit Air Purifiers Molekule Air Purifiers Okaysou Air Purifiers Winix Air Purifiers GermGuardian Air Purifiers Medify Air Purifiers


With air purifiers being so popular, it’s only natural that there are several unanswered questions about them. In the section below, LM addresses the most frequent questions about air purifiers:

Air Purifiers FAQ

These questions are categorized in no particular order. If you have any of your own questions, you may pose them in the comments below:

How Long Does It Take For An Air Purifier To Clean A Room?

Air purifiers start working – cleaning the air in the room – from the moment it’s turned on. Given the room size, the air purifier will clean all the air in the room in 12 minutes to 60 minutes.

Some air purifiers have air quality index sensors that can measure and monitor indoor room quality. You can see how long does it take for an air purifier to clean a room using that AQI monitor.

Generally, however, we leave the air purifier on 24/7. An air purifier might clean all the air in the room quickly but new air pollutants are being introduced into the air from the carpet, walls, and so on.

Can You Have 2 Air Purifiers In One Room?

Yes, you can have 2 purifiers in one room. What you need to be aware of are the individual room coverages of each of the two units in that one room.

In some cases, having two purifiers in a room could be overkill. A large air purifier, for example, is enough for a large room.

However, if you have a large room and two small room air purifiers, it does make sense to put them both in the large room. With combined air purification power (CADR ratings are added), they will capture air pollutants better than on their own, and substantially increase the indoor air quality in that room.

Do I Need An Air Purifier In Each Room?

A general rule is 1 air purifier per room. However, if you have one of the larger air purifiers with an extensive recommended coverage area, it can remove air pollutants in several rooms at once.

Example: Air purifier has a recommended coverage area of 400 sq ft. You’re looking to increase the quality of air in 2 200 sq ft rooms. If there is a door or air passage between these two rooms, you can turn an air purifier on in one room, and it will serve both rooms.

What you only need to be careful of is not to expect small air purifiers to be able to cover two small or even medium-sized rooms. It’s best to stick with the AHAM verified room coverage recommendations.

Can Air Purifier Be Too Big For A Room?

That can happen, of course. If you put a 400 sq ft recommended coverage area air purifier in a 200 sq ft room, that would be overkill.

Other than the higher price of the bigger air purifier, there are no negative effects of putting a too big air purifier in a too small a room. You will also likely to spend more electricity but the cost of electricity powering air purifiers is almost negligible to start with.

It’s smarter to properly size an air purifier. That would be the most optimum use of an air purifier.

Can I Have Too Many Air Purifiers?

Some people do like their air purifiers. In fact, it’s not all that unusual to find a home with 5 or even 10 air purifiers. 

Apart from the cost of the units themselves, the replacement filters, and electricity costs, there is nothing wrong with having too many air purifiers.

Obviously, it makes sense to properly size air purifier size and number according to your household needs. People with bigger houses that are concerned about allergies might have more air purifiers than those with small houses.

Does Air Purifier Remove Odor?

Every air purifier with an activated carbon filter does remove odor. The kitchen smells, cigarette smells, wildfire smells, or general undesirable odors are exactly what we want to get rid of with an air purifier.

The filter that removes gaseous odors is the activated carbon filter. It’s a porous material with an extensive activated surface area that is very efficient at absorbing odors. In fact, 1 gram of this porous material can have a surface area of more than 30,000 sq ft.

Some specific odors can be very resilient to adsorption to an activated carbon filter, however.

Do Air Purifiers Reduce Germs And Mold?

This is a heavily contested question. Air purifiers include HEPA filters with small holes. These holes are smaller than the diameter of many types of germs. Logically, HEPA filters should filter out certain types of germs.

However, the HEPA filter does not kill or inactivate germs or mold. The risk is that the captured germs might fester inside the air purifiers.

Have you ever wondered what does UV light do in an air purifier?

Advanced methods to remove germs are based on the germicidal nature of UV-C light. UV-C light filters are employed to destroy germs; however, the actual effect of this is disputed. UV-C light might not kill all or any germs within those few seconds of exposure.

PCO and PECO technologies were employed to remove germs. These, in addition to UV-C light, use metal catalysts and oxidation processes to effectively kill germs. How well these air purifiers reduce germs depends on the model and on the nature of the filter.

You can find several UV-C light filter air purifiers for mold here.

Is A HEPA Filter Worth It?

Yes, without question. Imagine trying to capture air pollutants so small and minuscule that you can hardly see them under the microscope. How do you start with capturing them?

HEPA filters, of course. HEPA filters present a neat solution of how to capture air pollutants that are as small as 0.3-micron in diameter. By definition, HEPA filters can only be called ‘HEPA’ if they can capture more than 99.97% of such particulates. 

Replacement HEPA filters cost from $20 to about $100. That’s not a high price to pay for the type of air filtration that would otherwise be almost unthinkable.

What Is Better Than A HEPA Filter?

HEPA filters are exceptionally good at filtering out particles that are about 0.3 microns in size. However, there is a myriad of air pollutants such as pesticides, asbestos, and fine dust particles that are a lot smaller than 0.3 microns.

H13 HEPA filters are better than HEPA filters. They capture more than 99% of air pollutants that are 0.1 microns in size. Those are 3 times smaller particulates than an ordinary True HEPA filter is capable of capturing.

Some air purifier brands like Medify specifically use these medical-grade HEPA filters in order to achieve a higher air quality index score.

Does An Air Purifier Use A Lot Of Electricity?

Not at all. Electricity-wise, air purifiers are very inexpensive appliances. Most air purifiers need a lot less than 100 Watts to run.

An average air purifier will, if let on all day at maximum speeds, spend about 1 kWh. The price of kWh depends on where you leave, the US average is about $0.1319. That means that running an air purifier for a week on the maximum speed will, on average, cost you only about 1$ in electricity costs.

Why Are Air Purifiers So Expensive?

For a mere combination of a fan and filtration system, air purifiers might at first seem unnecessarily expensive. Of course, if you consider the benefits – breathing clean air with high air quality index – the prices are quite fair.

Nonetheless, some air purifiers do cost $100, others $500, and some air purifier even come with a $1,000+ price tag.

The price depends on the size of the air purifier. Bigger air purifiers have higher prices. You will also find premium as well as budget brands.

The main cost of producing air purifiers is the filtration system and the other shell. Fan, for example, is quite inexpensive. Filtration systems – HEPA filter or even more advanced PECO filters, for example – can drive the cost up.

In general, expensive air purifiers have better air filtration systems. In the world of air purifiers, the filtration system is pretty much the most important part, and it does make sense to pay a bit more for better filters.

Can A Dirty Air Filter Make Your House Smell?

Yes. Every air purifier comes with a recommended lifespan of its individual filters. For example, you have to change activated carbon filters every 3 months and HEPA filters every 6 months, or so.

What happens if you don’t replace filters regularly?

Dirty air filters can make your house smell, obviously. When carbon filter adsorbs all the undesirable odors and HEPA filter captured all the small particles, they need to be replaced. If they aren’t, running an airflow through dirty filters will make your house smell.

What is more, certain germs can start to multiply in the filtration system, if it’s not properly maintained. Germs produce undesirable gaseous smells that can make your house smell as well.

Is It OK To Leave Air Purifier On All Night?

Of course. Air purifiers are designed to be run all the time, even during the night. In fact, we should put it ‘especially during the night’. During the night we inhale and exhale about 10,000 times. Having an air purifier delivering a higher quality of air to our lungs might be of some benefit.

Is sleeping with an air purifier bad? Not at all.

In order to not disturb us during sleep, the better air purifiers have a ‘Sleep Mode’ fan speed setting. This is a setting that usually produces less than 40 dB of noise and allows for undisturbed sleep.

What is more, Alen has conducted studies that seem to connect the use of air purifiers with waking up better rested. They recommend you should sleep with your air purifier on.

Where Should I Place My Air Purifier?

The bedroom is where most people have their air purifier. The advantage of portable air purifiers is that you can move them from room to room. 

You can read some tips about where specifically in the room to place air purifiers here. It’s important to follow these tips in order not to decrease the effect air purifier can give us.

In general, an air purifier can be put in pretty much any room. Primarily bedroom, and it’s also customary to have them in living rooms, kitchen, kids room, even bathroom.

Can I Put An Air Purifier On A Carpet?

Why not? A carpet is a good place as any for an air purifier. 

The only consideration when putting an air purifier on a carpet is that dust in the carpet might be lifted up in the air. That can happen with the down-up air purifier inflow airflow. This will force an air purifier to suck the dust out of your carpet and might shorten the lifespan of HEPA filters.

In general, however, it’s not dangerous or majorly counterproductive to place an air purifier on a carpet.

Should You Run An Air Purifier With Windows Open?

You can. According to EPA, the concentration of air pollutants in indoor air might be 2 to 5 times higher than in outdoor air. What is more, opening windows is thus recommended when trying to elevate the quality of indoor air.

The air purifier does pretty much the same job as opening the window; it’s just that it’s more thorough and it can run 24/7. Opening windows and running an air purifier go hand in hand.

Some sources of scientific studies quoted in the article:

(1) Molinari G, Colombo G, Celenza C. Respiratory allergies: a general overview of remedies, delivery systems, and the need to progress. ISRN Allergy. 2014;2014:326980. Published 2014 Mar 12. doi:10.1155/2014/326980

(2) Indoor Environmental Control Practices and Asthma Management Elizabeth C. Matsui, Stuart L. Abramson, Megan T. Sandel, SECTION ON ALLERGY AND IMMUNOLOGY, COUNCIL ON ENVIRONMENTAL HEALTH Pediatrics Nov 2016, 138 (5) e20162589; DOI: 10.1542/peds.2016-2589

(3) Morishita M, Adar SD, D’Souza J, et al. Effect of Portable Air Filtration Systems on Personal Exposure to Fine Particulate Matter and Blood Pressure Among Residents in a Low-Income Senior Facility: A Randomized Clinical Trial. JAMA Intern Med. 2018;178(10):1350–1357. doi:10.1001/jamainternmed.2018.3308

(4) Effects of Home Particulate Air Filtration on Blood Pressure: A Systematic Review. Dalia Walzer, Terry Gordon, Lorna Thorpe, George Thurston, Yuhe Xia, Hua Zhong, Timothy R. Roberts, Judith S. Hochman, Jonathan D. Newman Originally. Published 1 June 2020 Hypertension. 2020;76:44–50

(5) Fabio Caiazzo, Akshay Ashok, Ian A. Waitz, Steve H.L. Yim, Steven R.H. Barrett, Air pollution and early deaths in the United States. Part I: Quantifying the impact of major sectors in 2005, Atmospheric Environment, Volume 79, 2013, Pages 198-208, ISSN 1352-2310,

(6) Peters R, Ee N, Peters J, Booth A, Mudway I, Anstey KJ. Air Pollution and Dementia: A Systematic Review. J Alzheimers Dis. 2019;70(s1):S145-S163. doi:10.3233/JAD-180631

(7) Power MC, Weisskopf MG, Alexeeff SE, Coull BA, Spiro A 3rd, Schwartz J. Traffic-related air pollution and cognitive function in a cohort of older men. Environ Health Perspect. 2011;119(5):682-687. doi:10.1289/ehp.1002767

1 thought on “Air Purifiers: All-In-One Essential Guide And Overview”

  1. Hello there! It’s important to get quality air for breathing. Thank you for sharing some essential guides on air purifiers; their effects, advantages, disadvantages, type of filters, etc. It really matters to us. Keep posting!!


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