An H13 filter is a high-efficiency particle filter certified under EN 1822 (or the equivalent ISO 35 H class under ISO 29463) to capture at least 99.95% of particles at the size that is hardest for that filter to catch. It works through a dense mat of fibres, not a sieve. The class describes the filter media and its seal in a test rig — how well a complete air purifier performs also depends on housing leakage, airflow and maintenance.
What "HEPA" actually means
HEPA stands for high-efficiency particulate air. It is not a brand or a single product but a performance category. The name is used in two main testing traditions:
- Europe and most international markets use EN 1822 and the closely aligned ISO 29463 series. These define classes such as E11, H13 and H14 based on efficiency at the most penetrating particle size (MPPS).
- The United States commonly refers to a U.S. Department of Energy definition: at least 99.97% efficiency for 0.3 µm particles. Consumer products often call this "true HEPA".
Because the two traditions test at different particle sizes with different methods, "99.95%" (H13) and "99.97%" (U.S. HEPA) are not a like-for-like comparison. Both describe media that remove the vast majority of airborne particles passing through it in a single pass.
The EN 1822 class table
EN 1822 groups filters into E (efficient particulate air), H (HEPA) and U (ULPA, ultra-low penetration). The table shows the minimum overall efficiency at MPPS and the corresponding maximum penetration — the share of particles that gets through.
| Class | Group | Min. efficiency at MPPS | Max. penetration | Roughly… | ISO 29463 equivalent |
|---|---|---|---|---|---|
| E11 | EPA | 95% | 5% | 1 in 20 particles passes | ISO 15 E |
| E12 | EPA | 99.5% | 0.5% | 1 in 200 | ISO 25 E |
| H13 | HEPA | 99.95% | 0.05% | 1 in 2,000 | ISO 35 H |
| H14 | HEPA | 99.995% | 0.005% | 1 in 20,000 | ISO 45 H |
| U15 | ULPA | 99.9995% | 0.0005% | 1 in 200,000 | ISO 55 U |
Two practical points follow from the table. First, each step up the scale cuts penetration by a factor of ten. Second, the percentages are measured at MPPS — the worst case for that filter. Particles larger or smaller than MPPS are captured even more efficiently.
How a HEPA filter captures particles smaller than its gaps
A common assumption is that HEPA works like a very fine sieve. It does not. The spaces between fibres are much larger than many of the particles the filter removes. Capture happens because particles collide with fibres and stick to them, through three main mechanisms:
- Inertial impaction — larger, heavier particles cannot follow air as it bends around a fibre and hit it head-on.
- Interception — medium particles follow the air stream but pass close enough to touch the fibre.
- Diffusion — the smallest particles are jostled by air molecules (Brownian motion), wander across streamlines and eventually meet a fibre.
Some media also carry an electrostatic charge that adds a fourth mechanism. That charge can fade over time, which is one reason standards include tests on discharged media.
Why the "most penetrating particle size" matters
Impaction and interception get stronger as particles get bigger; diffusion gets stronger as they get smaller. Somewhere in between, both effects are relatively weak. That dip is the MPPS, and for typical HEPA glass-fibre media it falls roughly between 0.1 and 0.3 µm.
This is why "smaller particles are always harder to catch" is wrong. A 0.01 µm particle is usually captured more efficiently than a 0.2 µm one. It is also why EN 1822 tests at MPPS: rating a filter at its weakest point gives the most conservative number.
How H13 filters are tested
Under EN 1822, the flat media is first tested to find its MPPS. The finished filter element is then tested at that particle size and at its rated airflow for overall efficiency and for local leaks. Filters in the H and U groups are tested individually rather than by batch sampling, and a genuine H13 element should be traceable to a test result.
Two consequences are worth remembering:
- The rating applies at the rated airflow. Push much more air through the same filter and face velocity rises, which generally lowers efficiency and raises resistance.
- The rating applies to the filter element. It does not certify the air purifier housing the filter sits in.
Sealing: the weak point the label does not cover
Air follows the easiest path. If there is a gap between the filter frame and the housing, some air bypasses the media entirely. That leakage is not filtered at all, so it dominates real-world performance long before the difference between filter classes does.
When comparing products, look for a gasketed or compression-sealed filter seat, a filter that can only be installed one way, and test data for the complete device, such as a clean air delivery rate (see CADR explained).
Pressure drop, airflow and noise
Dense media resists airflow. The fan must create a pressure difference to push air through, and that pressure difference grows as the filter loads with dust. Designers balance this by using deep pleats and a large media area: more square metres of media means lower air velocity through each part of it, lower resistance, higher efficiency and less fan noise. This is why two "H13" purifiers with the same fan can deliver very different amounts of clean air.
"HEPA-type" and other marketing terms
Terms such as "HEPA-type", "HEPA-like", "99% HEPA" or "HEPA-grade" are not classes in any standard. They usually describe media that falls short of H13 or U.S. HEPA performance. That is not necessarily bad — E11 or E12 media in a well-sealed, high-airflow unit can deliver a lot of clean air — but the buyer should know what they are getting. Ask which standard the filter was tested to and which class it achieved.
What HEPA does not do
- It does not remove gases or odors in any meaningful amount. That needs a sorbent such as activated carbon.
- It does not reduce carbon dioxide. CO₂ is controlled by ventilation with outdoor air.
- It does not control humidity or mould growth on surfaces. It can capture airborne spores that reach it, but moisture problems need to be fixed at the source.
- It can only clean air that reaches it. Room volume, airflow and placement decide how much of the room's air passes through the filter each hour — see how to size an air purifier.
How this applies to CEAROX
The CEAROX CX-PRO-580 specification pairs a HEPA H13 stage for particles with a separate carbon filter module for gases and odors, in a ceiling-integrated housing designed for airtight, no-bypass flow. The principles in this guide apply to it exactly as to any other purifier: its real-world result depends on room volume, clean airflow, installation and filter maintenance. Ask us for the current technical documentation when comparing it with other systems.
Key takeaways
- H13 means at least 99.95% efficiency at the most penetrating particle size — at most 1 in 2,000 particles passes at the worst case.
- HEPA works by collision, not sieving; the hardest particles to catch are around 0.1–0.3 µm, not the smallest ones.
- The class certifies the filter element at its rated airflow, not the whole purifier. Leakage around the filter can matter far more than the class.
- For room performance, look at tested clean air delivery, sealing and maintenance — not the filter label alone.
Frequently asked questions
Is an H13 filter the same as a 'true HEPA' filter?
Not exactly. 'True HEPA' is a U.S. marketing term usually referring to the U.S. Department of Energy definition of at least 99.97% efficiency at 0.3 µm. H13 is a European class defined by EN 1822 as at least 99.95% efficiency at the most penetrating particle size. Both describe high-efficiency media; they are tested differently, so the percentages are not directly comparable.
Does a HEPA filter remove viruses?
Individual virus particles are very small, but airborne viruses travel inside respiratory droplets and aerosols that are typically larger, and HEPA media captures particles across that size range with high efficiency. Whether a purifier meaningfully reduces exposure in a room depends on its clean air delivery rate relative to the room volume, not on the filter class alone.
Can I wash and reuse a HEPA H13 filter?
Generally no. Washing or vacuuming the pleated glass-fibre media can damage fibres and seals, creating leaks that the original test no longer covers. Pre-filters are often washable; the HEPA stage is normally replaced according to the manufacturer's guidance.
Sources and further reading
- EN 1822-1 — High efficiency air filters (EPA, HEPA and ULPA): classification, performance testing, marking
- ISO 29463 series — High efficiency filters and filter media for removing particles in air
- U.S. EPA — Air Cleaners and Air Filters in the Home (incl. Residential Air Cleaners: A Technical Summary)
- U.S. EPA — Guide to Air Cleaners in the Home