Myth vs fact · Air
The claim: “HEPA filters cannot catch particles smaller than 0.3 microns”
Can HEPA filters catch particles smaller than 0.3 microns?
The ruling
False, and it inverts the actual physics. HEPA filters are rated at 0.3 µm because that is approximately the Most Penetrating Particle Size — the size a fibrous filter captures least efficiently. It is the worst case chosen deliberately as the test point. Below it, capture efficiency rises again, because smaller particles move erratically under Brownian motion and collide with fibres by diffusion. A filter rated 99.97% at 0.3 µm is typically better than that at 0.1 µm and better again at 0.05 µm.
How this was checked
Settled by the underlying physics
Fibrous filtration operates through several capture mechanisms whose efficiencies vary with particle size in opposite directions. Their combined minimum is the Most Penetrating Particle Size.
- 1Large particles are captured by impaction — too massive to follow the airstream around a fibre, they hit it. This mechanism strengthens as particles get larger.
- 2Mid-size particles are captured by interception — they follow the airstream but pass close enough to a fibre to touch it.
- 3Small particles are captured by diffusion — Brownian motion makes them wander erratically across streamlines and collide with fibres. This mechanism strengthens as particles get smaller, because smaller particles diffuse more.
- 4Around 0.1 to 0.3 µm, particles are too small for effective impaction and interception and too large for strong diffusion. Both mechanisms are at their weakest simultaneously. That is the Most Penetrating Particle Size.
- 5EN 1822 rates filters at MPPS precisely because it is the hardest test. H13 requires at least 99.95% at MPPS; H14 requires 99.995%.
- 6The US convention of quoting 99.97% at 0.3 µm is the same idea expressed at a fixed diameter close to MPPS for typical media.
What is true in the claim
Every durable myth has a grain of truth, and pretending otherwise teaches nobody anything.
What it means for your purchase
- Do not let this claim talk you out of mechanical filtration and into an ioniser. Ionisers deposit particles on surfaces rather than removing them.
- Look for a stated class — H13 or better under EN 1822 — rather than 'HEPA-type' or 'HEPA-like', which are not covered by either standard.
- Ultrafine particles from cooking and combustion are well within what HEPA handles. The limiting factor is airflow and room sealing, not the filter medium.
What this page does not establish
- This describes the filter medium's efficiency. It says nothing about whether air is bypassing the filter through a poor seal, which is a common real-world failure and one that no rating protects against.
- It is about particles. HEPA removes no gases at any size, and that is a genuine limitation frequently confused with this false one.
Frequently asked questions
What is the most penetrating particle size?
Does HEPA remove ultrafine particles from cooking?
Sources
Every limit and standard quoted on this page comes from one of the documents below. Where a figure is not in one of these, we have not published it.
EN 1822: High efficiency air filters (EPA, HEPA and ULPA)
European Committee for Standardization · checked 2026-08-01
Rates filters at the Most Penetrating Particle Size rather than at a fixed diameter — which is the fact this cluster turns on.
WHO Global Air Quality Guidelines
World Health Organization · published 2021-09-22 · checked 2026-08-01
These pages check claims against published standards, the underlying physics, or our own testing where it exists — and each page says which. We do not emit ClaimReview structured data, because that markup is reserved for accredited fact-checking organisations and we are a commercial publisher. Last reviewed 2026-08-08.