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What does an air purifier do?

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📓 The short answer

Air purifiers clean indoor air by using fans and filters to trap tiny airborne particles like dust, pollen, smoke, and viruses, and may also include activated carbon filters to remove odors and gases.

📚 The long answer

When I say "air pollution" you probably think of outdoor air getting contaminated by dirty car exhaust or factory smoke.

But the reality is indoor air quality can sometimes be worse than outdoor air quality. From pet dander to dust mites, cooking fumes to cleaning product chemicals, even a spotless home is susceptible to high concentrations of airborne contaminants that can irritate lungs and worsen allergies.

Some indoor air pollutants have actually increased in recent decades because of factors like energy-efficient buildings insulating better, which results in less air flow, and increased use of synthetic materials in buildings, furnishing, and personal care products. Additionally, climate change continues to cause more frequent wildfires, which result in poorer air quality both indoors and outdoors.

Indoor air quality is even more important to consider as we spend a tremendous amount of time indoors. The average American spends 85-90% of their time inside, and people who are most susceptible to air pollution (the elderly, people with cardiovascular or respiratory problems, etc.) often spend even more time indoors.

Fortunately, we’re not powerless against indoor pollution thanks to air purifiers.

What is an air purifier?

An air purifier is a relatively simple device made up of two components: a fan and a filter. The fan draws in air from the room, pushes it through one or more filters to trap airborne particles and gases, and sends it back out into the room squeaky clean.

How do air purifiers work?

A typical air purifier will use one or more specially designed filters to capture airborne contaminants, often referred to as particulate matter (PM). The particles that air purifier filters capture are incredibly small.

Dust, pollen, and mold are classified as PM10, meaning they have diameters smaller than 10 micrometers. Smoke, heavy metals, and organic compounds like bacteria and viruses fall under PM2.5, having a diameter smaller than 2.5 micrometers. PM2.5 particles are considered more dangerous than PM10 because they are small enough to penetrate deep into the lungs and enter the bloodstream. Ultra-fine dust and some viruses, including COVID-19, are classified as PM0.3.

Air purifiers use advanced filters made up of a mesh material designed to capture these teeny-tiny particles. They can range from basic filters to capture PM10 and larger particles to higher efficiency, thicker filters that are able to filter finer particles.

The gold standard is the HEPA (High Efficiency Particulate Air) filter. To be certified HEPA, these filters must trap 99.97% of PM0.3 matter (particles fewer than 0.3 micrometers in diameter).

HEPA filters capture air particles with three mechanisms: interception, impaction, and diffusion. Interception occurs when particles stick to the fibers of the filter. Impaction is when relatively larger particles embed themselves into the fibers. And diffusion is when ultra-small particles and gas molecules smaller than 0.1 microns collide with each other, slowing down enough to get captured with impaction or interception.

The longer a HEPA air filter is in use, the less effective it will be over time, which is why they need to be replaced every few months. Air purifiers will often include a pre-filter to capture more of the larger dust and hair, extending the lifespan of a HEPA.

How do air purifiers remove odors?

HEPA filters are powerful, but most gases will just pass through. That's why some air purifiers may also include an activated carbon filter to add additional odor-removal capabilities.

Activated carbon is a highly porous form of carbon with a ton of microscopic holes that can trap gases. Each gram of activated carbon can have a surface area of over 1,000 square meters.

Activated carbon works through adsorption, a process where gases will stick to the surface. Since odors are simply chemicals in the form of gas, they get trapped in activated carbon.

Eventually, the activated carbon becomes saturated and must be replaced for the purifier to continue effectively removing odors and airborne chemicals such as volatile organic compounds (VOCs).

How effective are air purifiers?

Because of the promising benefits of air purifiers, there's been a ton of scientific research conducted on them. Here's a variety of ways that air purifiers have been shown to effectively remove particulates and improve health outcomes:

• 🫁 Asthma: A 2011 study found that smoke-exposed asthmatic children required significantly fewer unplanned doctors visits when HEPA air purifiers were used in the home.
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• 🤧 Allergies: A 2020 study on adults with allergic rhinitis found that using HEPA air purifiers led to significantly reduced medication use and lower levels of allergens.
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• 🫀 Blood pressure: A 2025 study of adults living near highways with elevated systolic blood pressure (SBP) found that one month of running HEPA air purifiers led to a significantly lower SBP.
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• 🫁 COPD: A 2022 study of adults with chronic obstructive pulmonary disease (COPD) found that adding HEPA air purifiers in the home resulted in better respiratory symptom scores and overall well-being.
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• 🤒 Respiratory infections: A 2024 study found that running HEPA air purifiers in secondary school classrooms removed 77% of fine particulate matter and was associated with a drop in respiratory infection absences and coughing in the classroom.
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• 🔥 Wildfire smoke: A 2022 government report concluded that HEPA air purifiers can effectively reduce concentrations of indoor PM2.5 and lower the negative health risks of wildfire smoke exposure.

How do you shop for an air purifier?

Air purifiers are incredibly impactful devices, but their effectiveness depends on a lot of factors. You need to consider the types of airborne contaminants in your home, the size of the room you want to clean, the type and number of filters a purifier uses, the clean air delivery rate (CADR) of the air purifier, and more.

On top of that, some marketing claims may exaggerate an air purifier’s effectiveness. For example, an air purifier might be advertised for large living areas, but in reality they can only do one air exchange (ACH) per hour, far below the recommended 3-5 ACH to effectively remove particulate matter.

We all deserve to breathe clean air, but finding the right air purifier can be an overwhelming task. That's why I decided to build Find My Purifier, a tool to make it easier to shop for an air purifier. If you're interested in getting an air purifier for your home, try out the free, 10-question quiz and let me know what you think.

💨 Other interesting air purifier questions ​ Is an air purifier the same as a humidifier? No, an air purifier is designed to improve indoor air quality by removing pollutants with advanced filters. Humidifiers work by adding moisture to the air, but they do not remove harmful particulate matter. ​ Is an air purifier the same as a dehumidifier? No, a dehumidifier simply removes moisture from the air. An air purifier actually cleans the air by trapping airborne contaminants in its filters. ​ Do you need an air purifier if you have a central air filter? Central air filters are designed to primarily to protect your home's heating and cooling system, not necessarily your indoor air. A central air conditioner or furnace can actually circulate particulate matter, like pet dander, viruses, and bacteria, throughout your home and expose you further. Air purifiers differ in two key ways: they use finer filters to capture lung-irritating particles and they are designed to clean the air in a single room, not a whole home. ​ Do air purifiers help with mold? Yes, air purifiers can effectively remove mold particles from the air, however they can't do anything to tackle the actual source of the mold. To eliminate mold from your environment, you need to employ tactics like cleaning surfaces, fixing leaks, and improving ventilation. ​ Do air purifiers help with dust? Air purifiers can help trap and remove ultra-fine dust particles from the air, and people often report less noticeable dust in their homes. But dust particles are often are too heavy to capture for purifiers, so they fall to the ground before reaching the air purifier. To tackle dust, it's best to vacuum more often, ideally with a HEPA-filter vacuum cleaner. ​ Do air purifiers help with smells? Only air purifiers with an activated carbon filter can effectively tackle smells. Find the air purifier that's right for you ​ ​ Take the quiz

--- Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :) ​ All my best, Caitlin ​Click here to read five facts about Caitlin​

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Sources for this week's newsletter (I think I broke my record of most citations with this one!)

🌐 Wikipedia article of the week

Déjà vu

"Déjà vu is the phenomenon of feeling like one has lived through the present situation in the past. It is an illusion of memory whereby—despite a strong sense of recollection—the time, place, and context of the "previous" experience are uncertain or impossible....

Experiencing déjà vu has been correlated with higher socioeconomic status, better educational attainment, and lower ages. People who travel often, frequently watch films, or frequently remember their dreams are also more likely to report experiencing déjà vu than others."

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👀 Catch up on other curious questions

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