HEPA (High-Efficiency Particulate Air) filters capture at least 99.97% of airborne particles 0.3 micrometres in diameter, making them the baseline for cleanrooms, hospitals, and semiconductor fabs. This guide explains HEPA efficiency classes (H10 to H14 under EN 1822 / ISO 29463), where each class is applied, and the maintenance interval that keeps pressure drop and filtration efficiency within specification.
What a HEPA Filter Is and How It Works
A HEPA filter is a deep-pleated panel of microglass or synthetic fibres mounted in a rigid frame and sealed to prevent bypass leakage. Air is forced through the media, and particles are removed by four physical mechanisms: inertial impaction for large particles above 1 micrometre, interception for mid-size particles, diffusion for sub-0.1 micrometre particles, and electrostatic attraction. The most penetrating particle size is around 0.3 micrometres, so the standard test fixes the rating at that point. A true HEPA must hold 99.97% at MPPS; anything lower is merely a medium-efficiency panel filter. Typical face velocity is 0.45 m/s, and initial pressure drop is about 100 to 250 Pa depending on media area and frame depth. Standard frame depths are 50, 75, 100, and 150 mm, and a deeper frame holds more media, so it runs at a lower pressure drop for the same face area. Common modular face sizes are 305×305 mm, 610×610 mm, and 1220×610 mm, which let designers build a filter bank by tiling modules rather than ordering one oversized panel. The media pack is usually 50 to 100 mm of pleated glass fibre at 2 to 3 pleats per millimetre, and the pleat height sets how much dirt the panel can hold before resistance climbs to the change-out limit.
Why Particle Size Defines the Rating
The counter-intuitive fact about HEPA is that 0.3 micrometres is the hardest particle to catch, not the smallest. Particles larger than 0.3 micrometres are removed more easily by impaction and interception, and particles smaller than 0.3 micrometres are removed more easily by Brownian diffusion because they wander into fibres. That is why a filter rated 99.97% at 0.3 micrometres is actually better than 99.99% at both 0.1 and 1 micrometre. Test houses scan the media with a 0.1 to 0.2 micrometre challenge aerosol, such as PAO or DOP, to find the real weak point, then report the MPPS efficiency. A filter that passes only at 0.3 micrometre but leaks at 0.1 micrometre would fail the scan, so the certification is stricter than the headline number suggests. This is also why washing or vacuuming a HEPA destroys it: the fragile media tears, the pleats collapse, and the scan would show dozens of bypass channels across the face.
HEPA Efficiency Classes Under EN 1822 / ISO 29463
European and international standards grade HEPA filters by efficiency at the most penetrating particle size. The table below lists the common classes, their minimum MPPS efficiency, typical applications, and the nominal initial pressure drop you should design around.
| Class | Min. efficiency at MPPS | Typical application | Initial pressure drop (Pa) |
|---|---|---|---|
| H10 | 85% | Pre-filter before H13/H14, rough dust control | 90 |
| H11 | 95% | Commercial HVAC final filter, lobby air | 120 |
| H12 | 99.5% | Pharmacy prep, lab exhaust | 150 |
| H13 | 99.95% | Operating theatres, ISO Class 5 cleanrooms | 220 |
| H14 | 99.995% | Bio-safety cabinets, aseptic filling | 250 |
ULPA classes U15 to U17 capture 99.9995% or better at 0.1 to 0.2 micrometres and are specified only where ISO Class 3 to 4 cleanliness is mandatory, such as lithography bays.
Where HEPA Filtration Is Applied
HEPA filters appear wherever a defined particle count must be held. In healthcare, H13 to H14 units protect operating theatres, isolation rooms, and haematology wards. In pharmaceuticals, terminal HEPA boxes deliver ISO Class 5 air at filling lines. Electronics fabs use H14 and ULPA in laminar-flow hoods where a single 0.5 micrometre defect can ruin a wafer. Commercial buildings add H11 to H12 in AHU final stages to cut asthma-triggering fine dust. Food plants use H13 to keep spores out of dry ingredients. The selection of class follows the required ISO 14644-1 cleanliness and the tolerated pressure-drop penalty on the fan.
How to Choose the Right HEPA Class
- Choose H10 to H11 when the goal is general dust and pollen control in offices or schools and a cleanroom grade is not required.
- Choose H12 when laboratory or pharmacy prep areas need stronger capture but full aseptic Grade A air is unnecessary.
- Choose H13 when the space must meet ISO Class 5 to 7, such as operating theatres and most electronics assembly.
- Choose H14 or ULPA when bio-containment, aseptic filling, or ISO Class 3 to 4 is mandated and the fan can absorb 250 Pa or more.
- Choose a lower class when energy budget is tight and the duty is recirculation rather than primary protection, to keep pressure drop near 120 Pa.
Installation and Maintenance
Correct installation matters more than the label. Seal the frame with gel or knife-edge gasket, scan the perimeter with a PAO or DOP aerosol probe, and record a leak rate below 0.01% before the unit is accepted. In service, watch the manometer: replace the cartridge at the final pressure drop, normally 450 Pa, not when it merely looks dirty. Most commercial H13 units run 6 to 18 months between changes; hospital supply filters are swapped on a tighter scheduled interval tied to pressure trend. Store spares flat in sealed polyethylene, never stacked, to protect the pleats. Annual certified leak testing confirms the seal still holds.
Worked Example: Sizing a Cleanroom Filter Bank
Suppose an ISO Class 7 room is 50 m² with a 2.5 m ceiling, so the volume is 125 m³. At 30 air changes per hour the required supply is 3,750 m³/h, and with 20% recirculation the filter bank must pass about 4,500 m³/h. An H13 module of 610×610 mm at 0.45 m/s face velocity passes roughly 1,500 m³/h, so three modules meet the flow with margin. The initial bank drop is about 220 Pa, and the fan is selected for 450 Pa final, giving a useful life of roughly 9 to 12 months before the manometer triggers a change. If the room were ISO Class 5 the supply would rise to 60 air changes per hour, doubling the bank to six modules and pushing the design flow to 7,500 m³/h, which shows how steeply filter count grows with cleanliness grade.
Common Mistakes to Avoid
- Specifying H14 where H13 already meets the ISO class, because the extra 0.045% efficiency adds 30 Pa of pressure drop and 20% more fan energy for no measurable gain.
- Forgetting the pre-filter, which lets coarse dust blind the HEPA in weeks instead of the normal 6 to 18 months and voids the efficiency warranty.
- Sealing with the wrong gasket so the perimeter leaks above the 0.01% limit and the room never reaches its particle count.
- Storing spares stacked or unsealed, which crushes pleats and creates channel leaks before the filter is even installed.
- Replacing on a fixed calendar rather than on pressure trend, wasting half the media life or risking a bypass at high drop.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
What does a HEPA filter actually remove?
A HEPA filter removes at least 99.97% of particles 0.3 micrometres in diameter, including dust, pollen, bacteria, and most aerosols. Larger and smaller particles are captured even more efficiently by interception, impaction, and diffusion, so the 0.3 micron rating is the hardest test point for every class.
How often should a HEPA filter be replaced?
Replace HEPA filters when the pressure drop reaches the manufacturer final limit, typically 450 Pa, or every 6 to 18 months in normal commercial duty. In hospitals and cleanrooms, scheduled replacement follows the pressure gauge and periodic DOP leak testing rather than a fixed calendar date alone.
Is HEPA the same as ULPA?
No. HEPA filters capture 99.95% to 99.995% at the most penetrating particle size, while ULPA filters capture 99.9995% or higher and target particles near 0.1 to 0.2 micrometres. ULPA is used where ISO Class 3 to 5 cleanliness is required, whereas HEPA suits ISO Class 5 to 8 environments.
Can a HEPA filter be washed and reused?
No. HEPA media is fragile pleated glass fibre or melt-blown synthetics that collapse when wetted, so washing destroys the media and the seal. Discard and replace the cartridge when it reaches final pressure drop; only the metal frame housing is reused after a certified cleaning and leak test.