Air
HEPA
High-Efficiency Particulate Air filtration. Under the U.S. definition, a HEPA filter captures at least 99.97 percent of particles at 0.3 micrometers, a size near the range these filters find hardest to capture, which is why the test uses it.
In more detail
The 0.3 micrometer figure is widely read as a lower limit. It is closer to the opposite. A filter is least effective near its most penetrating particle size, where the two dominant capture mechanisms are both at their weakest: larger particles are intercepted and impacted onto fibers, while much smaller particles move erratically enough by Brownian motion that they collide with fibers and stick. In between, neither works well. The exact most penetrating size varies with media and airflow and often falls somewhere in the 0.1 to 0.3 micrometer range, so testing at 0.3 µm is a practical worst case rather than a universal constant. A filter that meets the requirement there performs better on particles both larger and smaller, which is why HEPA media handle ultrafine particles well.
The definition is not international, and the two frameworks classify on different bases. The U.S. specification is a single-point efficiency requirement at 0.3 µm. EN 1822 instead determines each filter's own most penetrating particle size and classifies against efficiency at that size, with H13 at 99.95 percent and H14 at 99.995 percent. Because the test basis differs, those classes are not directly interchangeable with the U.S. rating, and marketing frequently treats them as if they were.
HEPA-type and HEPA-like are not HEPA. They are unregulated marketing terms for media that did not meet the standard. In an assembled machine the seal around the filter matters as much as the medium: air that bypasses a poorly gasketed frame is unfiltered regardless of what the media is rated for.
Why it matters
HEPA is the filtration level that makes a room defensible during a wildfire event, and the number that actually matters there is delivery rather than efficiency. A filter rated at 99.97 percent moving very little air cleans a room slowly, so the useful figure is clean-air delivery rate measured against the room's volume. In a house, a portable unit sized to a bedroom and a well-filtered central system solve different problems, and the bedroom is usually the one worth solving first.
The common misunderstanding
That HEPA filters air in general — that it removes gases, odors, or carbon dioxide. It is a particle filter and nothing else. Gas-phase removal requires an adsorbent such as activated carbon, and the token carbon layer in most consumer purifiers holds too little media, with too little contact time, to do much for long. The second error is trusting HEPA-type on a box. The qualifier is there because it isn't.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.