Air purifiers are wonderful devices that use air filters to help improve our indoor air quality by making the air cleaner. They can remove odors, eliminate allergens, stop the spread of germs, cut back on smoke, and so much more. They even help users with conditions like asthma or allergies whose symptoms can worsen with poor indoor air. But how do these devices work? And what does “air purification” really mean?

What Is an Air Purifier?

At their most basic level, purifiers are essentially a fan with a filter. The fan sucks air in from around the room and blows it over a filter. In the process, toxins, particulates, and other allergens are trapped in the filter and removed from the air. The remaining air that passes through the filter is cleaned and transported back into the room. This allows the consumer to breathe easily and without the worry of inhaling airborne contaminants.

But, if a purifier is just a fan and a filter, what is the point of buying a fancy air purifier? Couldn’t a filter, some tape, and a box fan do the trick? While this has been demonstrated to be a semi-effective trick—and was a solution I heard about during the height of the pandemic—I wouldn’t recommend it as a long-term method for air purification. Firstly, box fan motors are not made to handle the added stress of pushing the air through a filter, and they could overheat. Secondly, this type of system is wildly inefficient when compared to purpose-built air purifiers with a tighter seal around filters and better filtration methods.

Types of Filtration

Despite what you may initially think, air purifiers don’t use a simple home air filter. In fact, most air purifiers don't use just one filter but instead use many layers of filtration. There are a variety of filter types available through the different purifier brands, but most purifiers have some version of the following filters:

1. Pre-Filter

A pre-filter is designed to stop downstream filters from clogging up with larger particulates. Often washable or easily cleanable, these types of filters are designed to grab larger, often visible, contaminants from the air and catch them in an easily accessible and cleanable location. This allows your later filters to work with greater efficiency and helps make cleaning easier for the user.

2. Medium Filter

Some air purifiers have a secondary pre-filter—also known as a medium filter—that is designed to catch contaminants down to one micron in size. These types of filters are great for catching less visible air pollutants such as dust, pollen, and pet dander. Although this stage of filtration is less common, it is a nice extra filter to have as it keeps your finer filters flowing more freely and makes them better able to catch smaller pollutants. The ability to capture these types of airborne particles is measured by the minimum efficiency reporting value or MERV rating.

3. HEPA Filter

Almost all higher-end air purifiers then have a high-efficiency particulate air (or arrestance) filter, also abbreviated as a HEPA filter. These are the most common type of filter found in air purifiers and are necessary for catching smaller airborne allergens or germs and viruses. When buying an air purifier, it is best to ensure that it includes a true-HEPA filter and not a HEPA-like or HEPA-type filter. A true HEPA filter can theoretically capture 99.97% of particulates down to 0.3 microns in size. With this level of efficiency, you can expect your filter to be able to capture many airborne allergens or other illness-causing viruses and bacteria. Even smaller viruses like Covid-19 are often bound in respiratory droplets and are easily captured in HEPA filters.

4. Activated Carbon Filter

Typically after the HEPA filter, but also sometimes before it, there is an activated carbon filter. Activated carbon filters work by adsorption and are able to remove volatile organic compounds (VOCs), odors, tobacco smoke, and other gases from your air. These types of filters are common in most purifiers. The more activated carbon that your purifier has, the better it will be at filtering these types of pollutants.

5. Ionizer

Some purifiers use ion generators—sometimes known as ionizers—to create charged particles known as negative ions that can latch onto pollutants in the room, and then become attracted to a collector within the purifier. Some purifiers do not have a collector and the charged pollutants attract to your floors, walls, or other objects in your room which removes them from the air until agitated. This additional filtration method gets scrutiny because of its creation of ozone. Not only is ozone-damaging to our environment, but it is known to be a harmful lung irritant at higher levels of concentration. Most purifier manufacturers claim their levels are well within the Environmental Protection Agency’s (EPA) guidelines on ozone output for medical devices. They are effective at adding extra filtration but they are not without some risk. Often, this stage of filtration can be turned off if the user doesn’t want additional protection from airborne contaminants or the ozone risk.

Specialty Filters

Some purifiers also have the option of attaching additional specialty filters. These filters are designed to provide additional protection against specific air pollutants such as pet dander, germs, toxins, or other odors. These are a great option for additional protection against the specific air pollutants you are dealing with.

Contaminate Destroying Technology

Some purifiers use additional technology to trap airborne contaminants and other indoor air pollution and also destroy them on contact to prevent them from spreading. This type of technology is helpful for viruses, mold spores, or other organisms that could otherwise spread or live on your filters. Often accomplished with ultraviolet light between 200 and 280 nm, this UV light (also called UV-C) destroying process is also sometimes referred to as ultraviolet germicidal irradiation.

Sensors and Smart Technology

Many air purifiers also have additional built-in sensors and smart technology that helps them run more efficiently and does so in a way that requires less user input. Some purifiers use smart tech that is isolated strictly to the unit, while others are able to pair with smartphones or other smart devices for a more seamless integrated experience.

Air Quality Sensors

It is common to see sensors that display the air quality in the room. These often show the current Air Quality Index (AQI) which is a standardized measurement of particular airborne contaminant levels at a given time. Some of these displays are a simple three-light configuration; a red light representing poor air quality, a yellow representing moderate air quality, and a green representing good air quality. Others offer a more in-depth look at the current air quality, either through a built-in screen or via a smartphone app, with the actual AQI number displayed. Some purifiers also can display the level of specific components that make up the AQI such as particulate matter smaller than 10 microns (PM10), particulate matter smaller than 2.5 microns (PM2.5), Nitrogen dioxide (NO2), or levels of Carbon Monoxide or Sulfur Dioxide. Some purifiers can also display levels of VOCs or Formaldehyde (HCHO) in the air, although these do not play into the AQI.

Auto Mode

Many air purifiers can auto-adjust fan speed to offer more or less filtration based on the amount required by the current air quality. These purifiers sync with the onboard smart sensor (or sometimes with multiple onboard sensors) to change the level of filtration throughout the day as needed. These auto sensors allow a more hands-off experience with their purifier and make maintaining good indoor air quality consistent and easy.

Night Mode

Some air purifiers offer a night mode. Otherwise known as sleep or quiet mode, this setting quiets the fan speed and allows the user to easily fall asleep with their purifier running. While some purifiers require the user to manually change the mode of the purifier, others use additional smart inputs, such as light or noise sensors, to adjust the purifier settings to be quieter and minimize light pollution during sleep.

Filter Change Sensor

Finally, some purifiers also alert the user when it is time to swap the filter. Although these are typically based on run-time rather than actual contaminate levels of the filter, these are still a handy feature to have to remind you when it is time to order filter replacements for maximum efficiency.

Purifier Ratings

MERV Rating

The minimum efficiency reporting value (MERV) rating of a filter helps the consumer better understand the filter's ability to capture molecules from 0.3 to 10 microns in size. Most dust, pollen, and other visible air contaminants fall into this category. Understanding the MERV rating of a purifier helps the consumer understand the level of filtration of non-HEPA filters within their purifier.

Energy Star Certification

Energy Star Certification is a joint program by the EPA and the Department of Energy (DOE) that adds certification to energy-efficient products. The goal of the program is to help consumers save money while also protecting the environment. Air purifiers with this rating allow you to filter your indoor air 24/7 without the worry of excess energy consumption.

CADR

The clean air delivery rate (CADR) of an air purifier lets the consumer know what sized room can be filtered from various types of pollutants. There are typically separate CADR numbers for smoke, dust, and pollen. The CADR rating of an air purifier is more helpful than the cubic feet per minute (CFM) because it combines airflow and particle removal efficiency. This allows the consumer to better distinguish between purifiers that are more efficient, but only filter small volumes of air, and purifiers that are not very efficient, but move higher quantities of air. When choosing an air purifier for an area with 8ft ceilings, it is best to choose a unit with a CADR of ⅔ the area of your room or higher.

AHAM

The Association of Home Appliance Manufacturers (AHAM) gives purifiers a certification based on room size and the CADR of a purifier as verified by a third-party lab. AHAM verification is based on rooms with 8ft ceilings and gives the size of the room that a purifier can filter 4.8x per hour. This amount of air changes per hour (ACPH) is considered best for users who are suffering from asthma, allergies, or other health issues due to poor indoor air quality.

Conclusion

Although when initially looking into air purifiers it may seem like they are just glorified box fans with a simple method of filtration, there is a lot more complexity built into standalone units. Added layers of filtration, built-in smart technology, and standardized figures of merit between air purifiers require years of research and design from leading brands in the industry. Hopefully, this article helped dispel some of the mystery behind air purification and provided more insight into what is contained within an air purifier. With so much specialization in products throughout the air purifier category, I highly suggest messaging one of our Filtration and Air Quality Experts here on Curated and explaining your specific needs to get matched to the best purifiers to fit you.