Three capture mechanisms
HEPA filters don't work like a simple sieve. They use three physical mechanisms to trap particles across different size ranges:
Interception — Medium particles (0.3–1 µm) follow the airstream and touch a fiber as they pass, sticking to it. This is the primary mechanism for the most common indoor pollutants.
Impaction — Larger particles (1+ µm) have too much inertia to follow the airstream around fibers. They collide directly and are captured. This handles pollen, mold spores, and larger dust.
Diffusion — The smallest particles (under 0.1 µm) move erratically due to Brownian motion and collide with fibers randomly. This makes HEPA filters effective against ultrafine particles and some viruses.
The 0.3 µm rating
HEPA filters are rated at 0.3 µm because this is the Most Penetrating Particle Size (MPPS) — the size most likely to slip through. Particles both larger and smaller than 0.3 µm are actually captured more efficiently. An H13 filter capturing 99.95% at 0.3 µm captures even higher percentages at other sizes.
HEPA grades: H10 through H14
H10 (85%) and H11 (95%) are not considered true HEPA. H12 (99.5%) is borderline. H13 (99.95%) is the standard for consumer air purifiers. H14 (99.995%) is medical/cleanroom grade and rarely necessary for home use.
HEPASilent and hybrid approaches
Some manufacturers (notably Blueair) combine mechanical filtration with electrostatic charging. This allows higher airflow with lower pressure drop, resulting in quieter operation and lower energy consumption — but the filters still need regular replacement.
