How Do Air Purifiers Work? (Basics Explained: Fan + Filters)

The basic concept of how air purifiers work is quite simple. We start with the need for every household to have a high indoor air quality.

According to the EPA’s Indoor Air Quality report:

“Americans, on average, spend approximately 90 percent of their time indoors, where the concentrations of some pollutants are often 2 to 5 times higher than typical outdoor concentrations.”

Air purifiers’ inner workings are designed to reduce or eliminate pollutants such as dust, smoke, pollen, mold spores, VOCs, and several other allergens that may present a health risk.

To effectively do their job, air purifiers need to perform 2 actions:

  1. Create airflow through the air purifier (measured in CFM). These devices need to gather all the unclean indoor air to have the ability to remove numerous pollutants.
  2. Use effective filters to purify the air. The filters are the heart of every air purifier; they capture the pollutants in the indoor air. The created clean air is expelled out of the air purifier by the continuous airflow.

In short, we can summarize that air purifiers are nothing more than:

Air Purifier = Airflow + Filters

There is no denying that the number and quality of filters (HEPA, Ionizer,  Activated Charcoal, UV-Light) is the most important part of how an air purifier works.

step by step photo of how air purifier work
The basic air purifier works by creating an airflow through its filtration system (with several filters that gather air pollutants).

However, before looking at how the 5 most common air filters work, let’s first focus on the basics.

Filters can’t work properly if there is no movement of air throughout the air purifier. Here is how an air purifier can gather all the indoor air to be purified:

Creating Air Flow Through The Air Purifier (CADR)

To ensure that air is sucked from every corner of a room, an air purifier comes with a powerful fan. The speed of the fan can be controlled; an average air purifier will have 3 or even 4 fan speed settings.

The fan represents an integral part of how an air purifier works. Simply put, it works by sucking in the air, running it through the filters, and expelling clean air.

How well and how quickly does air purifier increase the quality of indoor air depends on:

  1. Fan speed and therefore airflow. Airflow is measure in CFM (cubic feet per minute); the best and most powerful air purifiers can achieve the maximum airflow of over 300 CFM.
  2. Airflow-capacity of filters. Filters can handle only so much airflow. For example, if you create 500 CFM airflow through the HEPA filter, the filter will become saturated, and it won’t be capable of catching all the pollutants as effectively.
how airflow affect the air purifier operation
Some air purifiers can have up to 4 fan speed settings.

Therefore there is a compromise between the maximum airflow and airflow-capacity of filters air purifier is using. For understanding how air purifiers work, it is also important to acknowledge that 100% of input airflow is not 100% cleaned by the filters. Obviously, every filtration system has a maximum yield at a given airflow. Some special filters like the new Molekule filters or Airdog X5 filters have some special rules.

The amount of air an air purifier can clean is measured by CADR ratings. CADR stands for Clean Air Delivery Rate. Here is an example of how to look at CADR.

Let’s say we have an air purifier with 500 CFM maximum airflow. In a perfect world, we would get 500 CFM clean air airflow, right? However, no filtration system is perfect. Usually, only 300 CFM of that input airflow is 100% clean. That is the CADR rating – 300 CFM.

This also means running an air purifier on a maximum fan setting is not always the best choice. There are a lot of examples where lower fan speed can produce a higher CADR. Here are two cases:

  • An air purifier with 300 CFM airflow (fan speed 4) produces a CADR rating of 150 CFM.
  • The same air purifier at 250 CFM airflow (fan speed 3) can produce a CADR rating of 180 CFM.

The CADR rating is measured in testing by AHAM (Association of Home Appliance Manufacturers). Not every air purifier is tested, by the way. The ones which are will get an ‘AHAM Verified’ mark.

In short, the airflow is a necessary condition for the air purifier to work properly. However, the essence of every purifier is its air filtration system. Not only does it do most of the workload when it comes to removing pollutants from indoor air; it’s airflow-capacity also determines the CADR rating.

5 Common Types Of Air Purifier Filters

The whole point of how air purifiers work thus far was to get the unclean indoor air to the filters. Here is where the ‘magic’ happens.

The created airflow passes unclean air, which is clean air + pollutants, through the filters. Every filter has a very limited time (milliseconds) to:

  1. Pass through the clean air.
  2. Capture as many pollutants as possible.

Filters differ in the method of how they capture air pollutants. The most popular one – HEPA filters – are mechanical filters. They will not allow anything bigger than 0.3 μm to pass through (for comparison, human hair has a diameter of about 50 μm).

inner workings of hepa filtration system in air purifiers
The HEPA filter is the most commonly used filter in air purifiers. It captures pollutants that are 0.3 μm in diameter or bigger. In photo: Honeywell HPA300 air purifier.

The HEPA filter will catch air pollutants that are more than 100 times smaller than a human hair. However, if they are 200 times smaller than human hair, the HEPA filter won’t catch them.

What one filter can’t capture, another can. That’s why air purifier works by employing:

  • Several filters, one after the other.
  • Several types of filters. What HEPA filter can’t clean, Ionizer or Activated Charcoal filter might catch.

However, that doesn’t mean that an air purifier with a bigger number and types of filters will be the best. Some air purifiers, like Molecule, will employ only one filter – the PECO filter in this case – that will show incredible efficiency (it is a NASA-based tech, after all) at catching all the air pollutants.

Let’s have a look at how the most common air purifier filters work:

1. How Do HEPA Filters Work? (Removal Of Air Pollutant Based On Size – 99.7%)

Pretty much every low-end and middle-end air purifier uses the HEPA filter as the primary link in the filtration process.

HEPA filters are mechanic filters; they remove pollutants based on their size. The true HEPA filter will filter 99.7% of particles larger than 0.3 μm. Here is the diameter of some of the most common air pollutants (Source EngineeringToolBox):

  • Bacteria: 0.3 to 60 μm.
  • Dust mites: 100 to 300 μm.
  • Mold: 3 to 12 μm.
  • Mold spores: 10 to 30 μm
  • Oil smoke (odor): 0.03 to 1 μm.
  • Pesticides: 0.001 μm.
  • Pollen: 10 to 1.000 μm.
  • Cooking oil (odor): 0.03 to 0.9 μm.
  • Viruses: 0.005 to 0.3 μm.
  • Tobacco smoke: 0.01 to 4 μm.

As you can see, the HEPA filter works very well for capturing dust, mold, mold spores, even most bacteria. However, HEPA filters can be very ineffective when it comes to removing odor (oil or cooking oil smoke), pesticides, viruses, and tobacco smoke from the air.

Essentially, the HEPA filter does the lion’s share when it comes to bigger particles. For removing smaller particles, air purifiers come equipped with other specialized filters (especially activated charcoal filter). All the top air purifier brands like Coway, Dyson, Okaysou, Hathaspace, Honeywell, Levoit, Blueair, and even Alen BreatheSmart rely on these very filters to improve indoor air quality.

2. How Does An Air Ionizer Work? (Based On Electric Charge)

Air ionizers aren’t mechanical filters; they catch air pollutants based on their electric charge. Some pollutants are positively charged (positive ions), some are negatively charged (negative ions), and most of them aren’t charges at all.

The heart of an air ionizer is a small but powerful electric field. Such a field can accelerate electrons, which bump into pollutants without a charge, and effectively give it a charge.

Simultaneously, positively charged pollutants (in the electric field) are directed toward negatively charged metal plates, and vice versa.

This is a very quick and effective way to:

  1. Electrically charge all neutral pollutants (create ions).
  2. Attract created (and existing) ions to a charged metal plate.

The pollutants will be captured by the electric field leaving only the clean air through. The whole process of how an air ionizer filter works might sound complex, but it’s practically-speaking quite straight-forward. The only problem is that air ionizers don’t have a big airflow-capacity.

3. Activated Charcoal Filters (Adsorbent For Odor, Fumes, VOCs)

Many air purifiers work by including an activated charcoal filter in their filtration system. It goes especially well in tandem with the HEPA filter.

The way the activated charcoal filter works is by the process known as adsorption. This filter is kind of a labyrinth for pollutants that get stuck (adsorbed) in it due to both size and electric charge.

For example, the activated charcoal filters are best employed for removing odors, fumes, and chemicals. If you check the table of how big pollutants are under the HEPA filters chapter above, you will see these are the small pollutants HEPA filters cannot catch.

Most air purifiers work when it comes to combining these two filters by placing the first HEPA filter and then activated charcoal filter directly behind it. HEPA takes care of the biggest pollutants, and activated charcoal cleans all the smaller ones.

4. UV-Light Filters (Sterilizes Mold Spores, Bacteria, Viruses)

UV-light filters are handy when it comes to living organisms. We’re talking about:

  • Mold.
  • Mold spores.
  • Bacteria.
  • Viruses.

We must neutralize these living organisms because they can easily multiply, spread via the indoor air, and even cause health problems. As you can see, we used the word ‘neutralize’, not ‘remove’.

This is because you don’t have to remove the bacteria; it’s better to kill them. The UV-light filter does just that. It uses penetration UV-C light to damage bacteria’s DNA to such a degree that it will cause it to die.

UV-C light filters are especially useful if you’re fighting mold. We have gathered the best UV air purifiers for mold here; you can check them out to see how a UV filter is placed inside the air purifier.

5. Special Patented Filters (PECO, PlasmaWave, Vital Ion)

If you’re looking at some of the most hi-tech air purifiers, you might see filters like PECO, PlasmaWave, or Vital Ion.

These are usually cutting-edge tech filters developed by air purifier companies, are protected by a patent.

The best-known example is the PECO superfilter, developed by Molekule Air. PECO stands for Photo Electrochemical Oxidation. The filter incorporates NASA-level technology that uses nanoparticles to break down pollutants.

peco superfilter that makes air purifier work
Molekule Air purifier uses special PECO Superfilter instead of conventional HEPA and Activated Charcoal filters.

It needs to be powered by a whopping 85W power source to do that; most air purifiers draw about 40 or 50W of power.

These specialized filters usually don’t need additional HEPA or activated charcoal filters to assist them in cleaning the indoor air. In some cases, that can mean an increased CADR rating, but that’s not a rule.

Simple Inner Workings Of Air Filters

Purifiers are not complex devices. How air purifiers work is just by combining a fan and a series of filters. Compared to most HVAC devices, such as air conditioners and heaters, they are pretty straight-forward devices that don’t need an expert to handle them properly. You can check the list of air purifiers that work the best here.

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