How Do Air Purifiers Work — HEPA, Carbon, Ionizers and CADR Explained

The HEPA Filter — How It Captures Particles

HEPA (High Efficiency Particulate Air) filtration is the most effective technology for capturing airborne particles, used in medical facilities, cleanrooms and residential air purifiers. Understanding how it works explains why HEPA captures particles the human eye cannot see.

What HEPA Filters Are Made Of
True HEPA filters consist of a mat of borosilicate glass fibers, randomly arranged, compressed and pleated to increase surface area within a compact housing. The glass fibers are 0.5–2 microns in diameter — far smaller than a human hair (60–120 microns). The spaces between the fibers are not a simple sieve: the fiber arrangement creates capture zones for particles at multiple size scales through three distinct physical mechanisms.

Mechanism 1 — Inertial Impaction (Large Particles, >1 micron)
Large particles (>1 micron) have enough mass and momentum that they cannot follow the airstream as it curves around HEPA fibers. They continue in a straight line and collide with the fiber surface, sticking through van der Waals forces. Dust, pollen (10–100 microns), pet dander (5–10 microns) and mold spores (2–100 microns) are captured primarily by impaction.

Mechanism 2 — Direct Interception (Medium Particles, 0.1–1 micron)
Medium-size particles (0.1–1 micron) follow the airstream closely enough to approach fibers without impacting. When their trajectory brings them within one particle radius of a fiber surface, contact occurs and they stick. This mechanism operates across the most penetrating particle size range, where bacteria (1–3 microns) and fine PM2.5 particles reside.

Mechanism 3 — Diffusion (Very Small Particles, <0.1 micron)
Very small particles (<0.1 micron) are small enough that Brownian motion (random thermal collisions with air molecules) causes them to zigzag unpredictably through the airstream. This random path dramatically increases the probability of contacting a fiber. Particles below 0.05 microns are actually captured more efficiently by HEPA than slightly larger particles at 0.1–0.3 microns — counterintuitively, very small particles are easier for HEPA to capture than medium-size ones.

The Most Penetrating Particle Size (MPPS)
The particle size hardest to capture by HEPA is approximately 0.1–0.3 microns — large enough that diffusion is not very effective, but small enough that impaction and interception work less efficiently. True HEPA is rated to 99.97% capture at exactly 0.3 microns (the MPPS for standard US HEPA). H13 HEPA (European standard) is rated to 99.95% at 0.1–0.3 microns, testing at the actual most-penetrating size. See our HEPA filter grades guide for the full classification system.

Activated Carbon — How It Removes Gases and Odors

Activated carbon is the second major filtration technology in most multi-stage air purifiers. It addresses an entirely different class of indoor air pollutants: gases and volatile organic compounds (VOCs) that HEPA cannot capture.

What Activated Carbon Is
Activated carbon is made by heating carbon-rich material (coconut shells, coal, wood) in a low-oxygen atmosphere, then treating with steam or CO2. This process creates an extremely porous structure with 500–1,500 square meters of surface area per gram — the equivalent of a football field in a teaspoon of carbon. This vast internal surface area is where gas-phase pollutants adsorb.

How Adsorption Works
Adsorption is the adhesion of molecules to a surface (distinct from absorption, where molecules are absorbed into a material). VOC molecules in moving air contact the carbon surface and bind through weak intermolecular forces (van der Waals forces). The carbon holds them against the airflow while clean air continues through. This process is physical and reversible — carbon eventually saturates when all adsorption sites are occupied, at which point no further removal occurs and the carbon must be replaced.

What Carbon Removes
Activated carbon effectively removes: cooking odors and smoke; pet odors (ammonia, hydrogen sulfide); VOCs from paints, solvents and cleaning products; formaldehyde (moderately — specialized impregnated carbon performs better); tobacco and wood smoke organic compounds; off-gassing from new furniture, carpet and building materials. Carbon does not capture particles (PM2.5, pollen, dander) — HEPA handles those.

How Fan Speed and CADR Determine Room Coverage

The fan is the active component of an air purifier — it draws room air into the unit and through the filter stages. The fan's capacity, measured as CADR, is the primary determinant of how quickly a room can be cleaned.

What CADR Measures
CADR (Clean Air Delivery Rate) is the volume of air cleaned per minute, measured in cubic feet per minute (CFM) by AHAM's standardized test. AHAM tests three separate CADR values: smoke, dust and pollen — representing small, medium and large particles respectively. The smoke CADR is typically the lowest (hardest to capture) and the most-cited conservative measure. A CADR of 200 (smoke) means the purifier delivers 200 cubic feet per minute of air that has had smoke particles removed.

CADR to Room Size Formula
AHAM's formula: CADR × 1.55 = sq ft at 4.8 air changes per hour (4.8 ACH). A purifier with CADR 200 covers 200 × 1.55 = 310 sq ft at 4.8 ACH. 4.8 ACH means the entire air volume of a 310 sq ft room (with 8-foot ceilings = 2,480 cubic feet) passes through the filter 4.8 times per hour. At 4–5 ACH, particle concentration drops to meaningfully reduced levels within 30–60 minutes of continuous operation. See our CADR explained guide for the full calculation method.

Fan Speed Affects CADR
CADR is measured at maximum fan speed. At medium fan speed, CADR drops by approximately 40–60% on most units. Running at medium speed provides adequate purification in a correctly sized room (roughly 70% of maximum CADR is typically sufficient for 4+ ACH in the rated coverage area). Most users run at medium or auto mode, not maximum, for noise management. Auto mode adjusts fan speed based on an onboard sensor reading.

Other Air Purifier Technologies — Ionizers, UV-C and PCO

Beyond HEPA and carbon, several additional technologies appear in air purifiers. Here is what each does and what its limitations are.

Ionizers (Negative Ion Generators)
Ionizers emit negatively charged ions that attach to airborne particles, causing them to clump and fall to surfaces or be attracted to a collection plate inside the unit. This reduces airborne particle concentration but does not permanently capture them — particles on surfaces can be resuspended. Ionizers produce ozone as a byproduct. CARB-compliant ionizers limit ozone output to under 0.050 ppm. PlasmaWave (Winix) and HEPASilent (Blueair) are HEPA-combined ionization systems that include HEPA for permanent capture alongside ionization for enhanced efficiency.

UV-C (Ultraviolet Germicidal Irradiation)
UV-C at 254 nm wavelength inactivates microorganisms by damaging DNA. At sufficient dose (irradiance × time), it kills bacteria and inactivates viruses. In consumer air purifiers, the exposure time (seconds of a second) is insufficient for meaningful inactivation rates at standard airflow speeds. UV-C in residential purifiers provides a marginal supplemental germicidal function; the HEPA stage is the primary pathogen-capture mechanism through physical interception.

Photocatalytic Oxidation (PCO)
PCO directs UV-C light at a titanium dioxide (TiO2) catalyst, generating hydroxyl radicals that can oxidize VOCs. At sufficient contact time, PCO destroys VOCs rather than merely adsorbing them (as carbon does). The concern: incomplete oxidation can produce formaldehyde and acetaldehyde as byproducts. PCO with carbon backup capture avoids this issue. Independent residential testing shows variable PCO effectiveness — it is a supplemental stage, not a replacement for carbon.