Written by Barb Lulay, Co-Owner and electrical engineer at USAirPurifiers — a family and veteran-owned U.S. business. The Lulay Team believes your health is truly our top priority, because everyone deserves to enjoy clean air everywhere.
Air purifiers can come with a lot of unfamiliar terms — HEPA, CADR, ACH, VOCs, and more. We put this page together so you can get a clear, friendly understanding of what these terms actually mean, without needing an engineering degree to follow along. Whether you're just starting to research or trying to make sense of a spec sheet, we hope this helps you feel confident about what you're reading.
Definition: The most common form of filtration technology found in air purifiers today. A true HEPA filter traps 99.97% of airborne particles as small as 0.3 microns, and captures 95–99% of particles even smaller than that. It's excellent for controlling particulate matter like dust, pollen, animal dander, dust mites, smoke, mold, and mildew. HEPA filters were originally developed to capture radioactive dust in atomic plants, and today they're a mainstay in hospitals, clean rooms, and homes alike. They're often paired with a carbon filter to handle odors, since HEPA alone won't do much for smells.
Definition: A step beyond HEPA. ULPA filters trap 99.999% of airborne particles 0.3 microns or smaller — including tobacco smoke, household dust, and pollen. If you want the highest level of particulate filtration available, this is it.
Definition: Activated carbon, also called activated charcoal, is carbon that's been processed to be extremely porous, giving it a large surface area for adsorbing gases and odors. It's the go-to technology for tackling smells and gaseous pollutants, and it's commonly paired with HEPA filtration for a complete solution. Some units blend in zeolite, a more affordable adsorbent material — it works, but it isn't nearly as effective as carbon on its own.
Definition: A technology that uses internal heat to incinerate airborne mold spores, bacteria, fungi, and viruses — usually without any filter at all. This makes it a type of filterless air purifier.
Definition: UV light is used in some air purifiers to destroy microorganisms like bacteria, mold, and viruses. It's typically a supporting layer of purification rather than the main filtering method, and works well alongside HEPA and carbon filtration. UVC specifically refers to the germicidal UV wavelength range (commonly around 254 nanometers) — a high-intensity lamp designed to neutralize airborne mold, bacteria, and viruses.
Definition: A technology that uses photon and UV energy to activate a catalyst, breaking down odors, germs, and VOCs into fresh, breathable air — without producing ozone. Rather than just trapping pollutants, PCO actually destroys them.
Definition: With this technology, air passes through an electrical field that gives particles a charge. Those charged particles are then drawn to a series of oppositely-charged plates, pulling allergens, fine dust, some bacteria and viruses, chemical fumes, and other ultra-fine particles out of the air. It's a filterless technology, and it usually produces a small amount of ozone as a byproduct.
Definition: An unstable, highly reactive form of oxygen (O3), formed naturally by electric discharge or UV exposure. Because it's such a strong oxidizing agent, it's sometimes used deliberately to deodorize air. It's worth knowing the difference between a device built to generate ozone on purpose, and the small, incidental amount that a technology like an ionic precipitator can produce as a side effect.
Definition: A filter that catches particles one micron in size and larger before air reaches your primary filter. Some pre-filters use activated carbon to get a head start on odors and gases, too. A good pre-filter helps your HEPA filter last longer by catching the bigger stuff first.
Definition: CADR tells you how much clean air a purifier actually delivers. You'll usually see three separate numbers — one for tobacco smoke, one for pollen, and one for dust. The higher the number, the faster that unit filters that particular pollutant. AHAM (the Association of Home Appliance Manufacturers) independently tests and certifies air cleaners on CADR, which makes it easier to match a unit's real-world performance to your room size.
Definition: The number of times per hour a purifier can filter the entire volume of air in a given room. For particulates like dust, pollen, and smoke, a higher ACH generally means faster relief, since HEPA filters work just as well whether air is moving quickly or slowly through them.
VOCs and odors work a little differently, though. Carbon filtration depends on contact time between the air and the carbon bed, so air moving through too fast can pass by before it's fully absorbed. That means a higher ACH doesn't automatically mean better odor control — it's part of why odor-focused units are often built with denser carbon and airflow designed for dwell time, rather than just maximizing speed.
Definition: The amount of air, in cubic feet, that moves through a space in one minute (roughly 2 liters per second). This is the airflow measurement that CADR is built from.
One thing worth knowing: not all CFM numbers are created equal. A spec sheet will sometimes list CFM for the bare fan or for the pre-filter stage — before the real filters are doing their job. What actually matters is delivered CFM (also called post-filter CFM), which is the true output once your filters are installed and running. This number will vary a bit depending on your filter setup — carbon, HEPA, or a combination — so it's always going to be something of an estimate, but it's the only number that reflects what's actually moving through your room. For more on how this plays into sizing a unit for your space, visit our Air Changes Per Hour calculator.
Definition: The room size a manufacturer recommends for a given unit, based on its CADR and ACH. It's worth checking whether that coverage number assumes the unit is running on its highest setting — a purifier sized generously for your room can often run quieter and still keep up.
Definition: A unit of length equal to 39 millionths of an inch — for reference, a grain of salt is about 60 microns, and the human eye can generally see particles down to about 25 microns.
| Size Range | Particle Type |
|---|---|
| 0.3 – 1 micron | Tobacco smoke, bacteria, metallic fumes |
| 1 – 5 microns | Bacteria and small dust particles |
| 5 – 10 microns | Mold, pollen, medium dust particles |
| 10 microns | Large dust particles |
| 25 microns | Debris and dust particles (visible to the naked eye) |
| 150 microns | Human hair |
Pet Dander: Material shed from animals' bodies — dried skin, hair, or feathers — and a frequent cause of allergies in humans.
Dust Mites: These tiny critters feed on things like shed human skin and thrive in the stable environment of your home. They're a common trigger for asthma and allergy symptoms, and even their waste can be strongly allergenic.
Asthma: A chronic condition involving airway inflammation and narrowing, affecting roughly 7% of the U.S. population and an estimated 300 million people worldwide. During an attack, the airways become inflamed and constricted, making it harder to breathe.
VOCs (Volatile Organic Compounds): Chemicals that vaporize easily at normal room temperature. They're common in everyday indoor life, and depending on the compound, some can be harmful — which is why many are regulated.
Off-Gassing: The slow release of VOCs and other gases that were trapped in a material — that "new car smell" is a familiar example.
MCS (Multiple Chemical Sensitivity): A chronic condition where a person has trouble tolerating everyday environmental substances, most often chemical odors from things like household products or car exhaust.
A few signs a purifier is likely to help:
An air purifier is a great companion to good habits, not a replacement for them — regular cleaning, dusting, and ventilating still matter and work right alongside it.
Your purifier is only as good as how well its filters are maintained. Here's something interesting: as a HEPA filter accumulates particles, it actually becomes even better at trapping smaller ones — the particles already captured help catch more as air passes through. But that's only beneficial up to a point. If a HEPA filter isn't replaced regularly, that buildup becomes excessive, restricting airflow and putting extra strain on the motor. It also brings down your unit's real-world ACH, since less air is moving through it in the first place — which means the purifier can't filter your room as efficiently as it's designed to.
Carbon filters reach a different kind of limit. Activated carbon works by adsorbing VOCs, chemicals, gases, and odors into tiny pores in the carbon pellets. Over time, those pores become filled, and once the carbon reaches that saturation point, there's no room left to hold onto new odors and gases — so it becomes far less effective at the job it was doing so well before.
A pre-filter helps by catching the larger particles before they reach your HEPA filter, extending its useful life. Most manufacturers recommend replacing HEPA filters every 6–12 months, and carbon filters more often if you're dealing with significant odor — many units even include a filter-life indicator to help you keep track.
A bigger unit isn't automatically a better one. What matters most is CADR and ACH relative to your room size, not the size of the box.
Ionizers and ozone generators don't clean the air the same way HEPA does. They work through entirely different mechanisms, and neither physically removes pollutants from the air the way a mechanical filter like a HEPA does.
Running it is just one part of the picture. Doors, windows, room layout, and filter condition all affect how well your purifier performs — the same unit will do very different things in a closed room versus an open floor plan.
Not all HEPA is created equal. True HEPA means 99.97% at 0.3 microns — a tested, specific standard. Labels like "HEPA-type" don't carry the same guarantee, and ULPA filters go even further at 99.999%.
This page is part of our Learning Center — we believe a well-informed customer makes the best decisions for their family and home, and we're always happy to help if you have questions.