HEPA filter
A high-efficiency particulate air filter: pleated fibrous media in a sealed frame, rated in United States practice at 99.97 percent efficiency for particles 0.3 micrometers in diameter. HEPA filters supply the air in cleanrooms and protective environment rooms and clean the exhaust from containment systems.
Also called high-efficiency particulate air filter, HEPA, terminal HEPA filter
What is a HEPA filter?
A HEPA filter is a high-efficiency particulate air filter rated to capture 99.97 percent of 0.3 micrometer particles, the grade the CDC cites for protective environment rooms. Cleanrooms and containment exhausts depend on HEPA filters that are also leak tested where they are installed.
HEPA filter: how it works
HEPA media is a dense mat of fine glass or synthetic fibers pleated into a deep frame. Large particles strike fibers because they cannot follow the air around them, small particles wander into fibers through random motion, and particles in between are caught as they pass close to a fiber. The size that slips through most easily sits between those mechanisms, near 0.3 micrometers, which is why efficiency is rated there.
Where you meet the term
- In a cleanroom ceiling, either in ducted terminal housings or inside fan filter units.
- In the exhaust of a hazardous drug room, a glovebox, or a hot cell, where filters sit in isolatable housings.
- In an air handler static pressure budget, entered once at its clean resistance and once at its replacement point.
How to check it
- Read the filter label and certification record. The label and the certifier's report state the filter grade, the rated airflow, and the date of the last installed leak test. Owner check
- Scan or test the installed filter. A certifier introduces a test aerosol upstream and scans the filter face and seal downstream, or samples downstream of a bank, to confirm no leak exceeds the acceptance limit. Technician
- Record pressure drop against the replacement point. A gauge across the filter shows its loaded resistance, which is compared with the replacement pressure drop recorded in the design basis. Technician
Figures you will see
| Condition | Figure | What it tells you |
|---|---|---|
| CDC protective environment room filter | 99.97 percent at 0.3 micrometers | Read against the model documentation |
| DOE in-place system test acceptance for nuclear air cleaning | 0.05 percent maximum leakage | Recommended in DOE-HDBK-1169-2022 for systems designed to its guidance |
HEPA filter: what it is not
- A HEPA filter is not a MERV 16 filter with a better name. The EPA describes MERV as a 1 to 16 scale for particles from 0.3 to 10 micrometers, while HEPA filters are rated by a separate efficiency test at 0.3 micrometers.
- A factory efficiency rating does not prove the installed filter works. Leaks at the gasket, the frame, or a damaged pleat bypass the media, which is what the installed leak test finds.
HEPA filter questions
Why is HEPA efficiency rated at 0.3 micrometers?
Filter fibers catch large particles by impaction and small ones by diffusion, and particles near 0.3 micrometers are too small for one mechanism and too large for the other. Rating efficiency at that size describes the filter at its weakest point, so larger and smaller particles are caught at a higher rate.
How much static pressure does a HEPA filter add?
A HEPA filter adds more resistance than a MERV filter of the same face area, and its resistance grows as it loads. Nuclear-grade HEPA filters qualified under ASME AG-1 are limited to 1 inch of water column clean at rated airflow, and a fan serving HEPA filters is selected at their replacement pressure drop.
How often are cleanroom HEPA filters leak tested?
The governing document sets it. USP 797 requires compounding rooms to be certified every six months, and the FDA aseptic processing guidance gives twice a year as an example interval for an aseptic processing room, with added tests after work that could disturb the filters.
Where this definition comes from
Written from government guidance material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B: AirCDC table of ventilation parameters: airborne infection isolation rooms with at least 12 total and 2 outdoor air changes per hour, air movement into the room, a pressure differential of at least 0.01 inches of water gauge (2.5 pascals), and all air exhausted directly outdoors unless recirculated within the room through HEPA filters; protective environment rooms with at least 12 air changes per hour, positive air movement, and HEPA filters of 99.97 percent efficiency at 0.3 micrometers.
- DOE-HDBK-1169-2022, Handbook for use with DOE-STD-1269-2022, Air Cleaning Systems in DOE Nuclear FacilitiesDOE handbook replacing the 2003 Nuclear Air Cleaning Handbook: primary, secondary, and tertiary confinement zones with air flowing toward the more contaminated zone and pressure differentials that prevent backflow under upset conditions; a 100 feet per minute rule of thumb for capture velocity across a primary confinement breach; positive seals at secondary barrier penetrations; bag-in bag-out side-access housings with upstream and downstream shutoff dampers and provisions for in-place testing; parallel housings with inlet and outlet isolation dampers; a recommended 0.05 percent maximum leakage acceptance for in-place HEPA system tests under ASME AG-1 Appendix TA; backup power for fans, dampers, and controls where safety documentation requires it; and stack guidance of exit velocity at least 1.5 times wind velocity with 3,000 feet per minute recommended, noting that high velocity is no substitute for proper stack location.
- Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing, Current Good Manufacturing PracticeFDA guidance recommending a positive pressure differential of at least 10 to 15 pascals between adjacent rooms of differing classification with doors closed, outward airflow sufficient to minimize ingress when doors open with door-open time strictly controlled, at least 12.5 pascals from an aseptic processing room to adjacent unclassified space, airflow for at least 20 air changes per hour as acceptable for ISO 8 supporting rooms, a critical area velocity of 0.45 meters per second (90 feet per minute) plus or minus 20 percent, and HEPA filter leak testing at installation and at intervals such as twice a year for the aseptic processing room.
- What is a MERV ratingEPA explanation that MERV, minimum efficiency reporting value, runs from 1 to 16 and reports a filter's ability to capture particles between 0.3 and 10 microns, that performance is assessed in the three size ranges 0.3 to 1, 1 to 3, and 3 to 10 microns, that a higher MERV captures more of the targeted sizes, and that a MERV 13 filter removes at least 85 percent of particles in the 1 to 3 micron range.