Engraving of a freestanding room air purifier

HEPA air purifiers

Works, on particles

Stasis Home ResearchEvery claim on this page is graded against peer-reviewed health research and linked to its primary source.Updated 2026-06

A true HEPA purifier is among the best-evidenced tools in home air quality. Sized correctly and run continuously, it cuts a room's fine-particle level by 40 to 80 percent in intervention trials. The catch: it filters particles only. Gases, including VOCs and the nitrogen dioxide a gas stove emits, pass straight through unless the unit also carries a real activated carbon stage.

How it works

A fan pulls room air through a dense mat of pleated fibers, and particles stick to the fibers on the way through. The certification test uses the particle size filters find hardest to catch, about a third of a micron; a true HEPA filter has to trap all but a few of every ten thousand particles at that size in a single pass. Larger and smaller particles catch at even higher rates, which is why dust, pollen, dander, mold spores, and smoke particles are all within reach of one device. The filter can only clean air that reaches it. The fan's clean-air delivery rate, the seal around the filter frame, and filter replacement discipline decide what happens in a real room, which is why a purifier is sized to the room it serves.

What it handles, honestly

What this approach does and does not take care of.
Concern	Handled?	The honest note
Fine particles (PM2.5), including wildfire smoke	Yes	99.97% single-pass capture at the hardest particle size (0.3 microns); 40-80% time-averaged room reduction in intervention trials when the unit's CADR is matched to room volume.
Coarse dust and pollen (PM10)	Yes	Coarse particles are easier to catch than the fine fraction; per-pass capture is essentially complete. During wind-dust events, closed windows plus a running unit do the real work.
Pet dander	Yes	Airborne cat and dog allergen fell 77-89% with portable HEPA units in a randomized trial (Maya-Manzano 2022). Allergen settled into bedding and carpet stays until it is cleaned out.
Dust mite allergen	Partly	Catches the airborne fraction (a 75% reduction for one major mite allergen in the same trial), but the reservoir lives in mattresses and carpet. Encasements and keeping humidity below 50% (Arlian 1999) do that part.
Mold spores	Yes	Whole spores (2-20 microns) sit at the easy end of the filter's range. Filtration lowers the airborne spore load; it does nothing about the moisture feeding the growth, which is the fix that matters.
VOCs and formaldehyde	No	HEPA captures particles only. Gas-phase removal needs an activated carbon stage of real weight, 4 pounds of media or more, not a carbon-dusted prefilter.
Smog gases (ozone and NO2)	No	Gases pass through the filter. Activated carbon is the media that works on them; check the spec sheet for pounds of carbon alongside the HEPA grade.
Stuffy air (CO2)	No	No filter removes carbon dioxide. Only outdoor air exchange brings it down: an open door, a cracked window, or mechanical ventilation.

Getting it right

Size by the room, not the marketing copy. The working formula: clean air delivery rate (CADR, in cubic feet per minute) should be at least room volume in cubic feet times 5 air changes per hour, divided by 60. A 12 by 12 foot bedroom with 9 foot ceilings works out to about 108 cfm minimum, and high ceilings break the floor-area shortcuts, so use the volume math. The bedroom is the highest-value placement, since it covers the third of your life you spend asleep, and the unit should run continuously rather than only when the air seems bad. Two spec-sheet checks before buying: certified true HEPA rather than 'HEPA-type', and, if gases are part of your problem, an activated carbon stage measured in pounds. A leaky frame seal or an overdue filter quietly erases the rated performance.

Common questions

Do HEPA purifiers remove VOCs and odors?

No. HEPA media captures particles only. Gas-phase pollutants like VOCs, formaldehyde, and cooking odors need an activated carbon stage, and the carbon has to be substantial, measured in pounds on the spec sheet, not a thin carbon-coated prefilter.

Will an air purifier fix gas stove pollution?

Partly. It captures the fine particles cooking throws off, but nitrogen dioxide from the burner is a gas and passes through the filter. The source fix is a range hood vented to the outside and used every time you cook; the purifier cleans up what the hood misses.

What does 'true HEPA' mean versus 'HEPA-type'?

True HEPA is a tested standard: the filter captures all but a few of every ten thousand particles at the hardest size to catch. 'HEPA-type', 'HEPA-like', and similar labels carry no such guarantee. Confirm the certification before comparing anything else.

What size purifier do I need?

Match the clean air delivery rate to room volume: CADR in cfm of at least room volume in cubic feet times 5, divided by 60. A typical bedroom lands around 100-150 cfm; open living areas need 250-400 cfm. Undersized units run forever and never catch up.

Do air purifiers help with allergies?

They lower the airborne fraction of what you react to. In a randomized trial, portable HEPA units cut airborne cat and dog allergen by 77 to 89 percent. The reservoir in bedding and carpet still needs encasements, cleaning, and humidity control; the purifier handles what daily life stirs back up.