An air purifier is a fan that pulls room air through filters. A pre-filter catches hair and lint, a True HEPA filter traps at least 99.97% of particles at 0.3 microns (pollen, dander, dust, mold spores), and an activated carbon layer soaks up gases and odors. It can't lower humidity or remove settled dust.
That's the whole mechanism. The differences between products come down to which filter types are included, how big the filters are, and how fast the fan pushes air through them.
A good example is the Coway Airmega AP-1512HH "Mighty", a long-time US bestseller. It stacks four of the stages below: a washable pre-filter, a carbon filter, an ionizer you can switch off, and a True HEPA filter. Here's what each one actually does.
The Pre-filter
The first stage in most purifiers is a washable mesh pre-filter that catches large particles: dust, pet hair, lint, bigger pollen grains. Its job is to protect the more expensive HEPA filter behind it from debris that would clog it quickly.
Clean the pre-filter every two to four weeks by vacuuming it or rinsing it under the faucet (if the manual says it's washable). This isn't optional. A clogged pre-filter restricts airflow through the whole unit, cuts filtration performance and shortens the life of the HEPA filter, which is the part that costs money to replace.
The HEPA Filter
HEPA stands for High Efficiency Particulate Air. In the US, a True HEPA filter has to capture at least 99.97% of particles at 0.3 micron in diameter, a definition set by the Department of Energy. That size sits close to the most penetrating particle size (MPPS). Particles both smaller and larger are actually easier to catch, so the test targets the hardest size.
In practice, a True HEPA filter will catch pollen (10-100 microns), most bacteria (roughly 0.5-5 microns), dust mite allergen (1-10 microns), mold spores (2-10 microns), fine dust (PM2.5, under 2.5 microns) and most combustion particles. It won't catch gas molecules, which are far smaller than anything a fiber filter can hold.
H-grade filters come from the European EN 1822 standard, and you'll see "H13" on plenty of US listings. E11 (once called H11) captures 95% at MPPS. E12 (once H12) captures 99.5%. H13 captures 99.95%. H14 captures 99.995%. For home use, True HEPA or H13 is the benchmark.
HEPA-type or HEPA-style are marketing terms with no standard behind them. They can mean almost anything. If a listing doesn't say True HEPA, quote an H-grade, or show an AHAM-verified CADR, assume the filtration is below True HEPA.
The Activated Carbon Filter
Activated carbon (also called activated charcoal) works by adsorption. Gas molecules stick to the huge surface area inside the carbon's porous structure. A single gram of activated carbon can have an internal surface area of roughly 5,000 to 16,000 square feet, depending on its grade.
The carbon layer handles what HEPA can't: gases, VOCs (volatile organic compounds), cooking smells, pet odors, cigarette smoke, off-gassing from new furniture and paint fumes. It doesn't capture particles. That's the HEPA filter's job.
The amount of carbon matters. The EPA's guide to air cleaners says carbon filters work best with a large amount of material, "the thicker the better." A thin carbon-coated mesh or sheet saturates fairly quickly and stops working. A granular or pellet carbon bed with real weight to it lasts longer and performs more consistently. Cheaper purifiers often include just enough carbon to put it on the spec sheet.
You can't regenerate a carbon filter at home. Once it's saturated, it needs replacing.
The Ionizer (or PlasmaWave, or Plasma Technology)
Many purifiers add an ionization stage. An ionizer releases charged ions that attach to airborne particles, making them clump together. The clumps then either drop onto nearby surfaces or get big enough for the HEPA filter to catch.
Some brands go further. Winix says its PlasmaWave technology creates hydroxyl radicals that break down airborne pollutants, including some VOCs, rather than just knocking particles to the floor. That's the manufacturer's description, so weigh it accordingly.
The ozone concern. Ionization can produce small amounts of ozone as a byproduct. The question is how much. In the US, the easiest check is CARB certification: California caps ozone emissions from air cleaners at 0.050 ppm, and every portable air cleaner sold there must be certified against that limit. The EPA's position on devices that make ozone on purpose is blunter: inhaled ozone can damage the lungs. Cheap ionizers from unknown brands vary more. If you keep pet birds, avoid ionizers entirely. Birds are far more sensitive to airborne irritants than people.
Most purifiers with an ionizer, including the Coway Mighty and Winix models, let you switch it off and keep the HEPA and carbon filtration running.
UV-C Light
Some purifiers include a UV-C bulb that claims to kill bacteria and viruses as air passes by. In principle it works, because UV-C light does inactivate germs. In practice, air moving at fan speed spends very little time near the bulb, which limits how much it can do. It's hard to credit it with much benefit beyond what the HEPA and carbon stages already provide.
It won't hurt, but it isn't a reason to pick one purifier over another.
What Air Purifiers Cannot Do
Remove settled allergens. Purifiers clean airborne particles. Dust mite allergen, pet dander and pollen settle into carpets, couches, mattresses and curtains, where a purifier can't reach them. Regular vacuuming with a HEPA-filtered vacuum and washing bedding in hot water (130°F or hotter) deal with what has settled.
Fix damp or mold. Air purifiers can catch mold spores floating in the air, but they do nothing about the moisture that lets mold grow. As the EPA puts it, the key to mold control is moisture control. If you have condensation on windows or mold on walls, you have a humidity or ventilation problem, and a purifier won't fix that. See our comparison of air purifiers vs dehumidifiers for what to use where.
Replace ventilation. Outdoor air usually has much lower levels of indoor pollutants (cooking fumes, VOCs, CO2) than recirculated room air, so when you can open a window, do. A purifier earns its place when you can't: high pollen counts, wildfire smoke, a busy road outside, or a cold night.
Reduce humidity. A purifier moves and filters air. It doesn't remove moisture. If your home is damp, you need a dehumidifier.
For specific models, see our full air purifier roundup. There's also our HEPA filter guide for more on filter grades, and our honest take on whether air purifiers are worth buying if you're still deciding.
