What Is a HEPA Filter? Meaning, Materials, and Ratings Explained
HEPA is one word that brings up three common questions, and most confusion about it comes from answering the wrong one. People ask what the letters mean, what the filter is physically made of, and whether a product's "HEPA" claim is actually true.
What HEPA means
A U.S. Department of Energy standard: capture at least 99.97% of particles at 0.3 microns. It's a performance spec, not a brand.
What it's made of
Ultra-fine glass fiber or charged polypropylene, pleated into a dense mat that's nothing like a furnace filter.
How you verify it
"HEPA," "True HEPA," and "HEPA-type" aren't interchangeable. The rating system tells you which one you're actually buying.
The rest of this guide answers each question in detail, then shows how filter efficiency connects to CADR, the rating system that tells you the square footage an air purifier can clean.
What Does HEPA Stand For?
HEPA stands for High Efficiency Particulate Air, and it refers to a filter that captures at least 99.97% of particles measuring 0.3 microns. The U.S. Department of Energy defined this official HEPA standard. It's not a brand name or a marketing term; it's a specific, testable performance threshold.
The technology dates back to the 1940s, developed for the Manhattan Project to capture radioactive particles from the air in weapons facilities. It was declassified after World War II and commercialized in the 1950s. The core filter media hasn't fundamentally changed since because it remains the most effective way to remove airborne particulates without using chemicals or ionization.
What Is a HEPA Filter Made Of?
HEPA filter media is made from one of two materials: borosilicate microfiberglass or non-woven polypropylene. Both are engineered specifically to trap particles far smaller than what a standard HVAC or furnace filter can catch.
- Fiberglass (glass fiber): Extremely fine glass strands packed into a dense mat. Fiberglass typically delivers the highest efficiency but also the highest pressure drop, which means more resistance to airflow.
- Non-woven polypropylene: A synthetic, meltblown fabric that's electrostatically charged during manufacturing. This is the media used in Oransi's filters. The static charge boosts particle capture without adding as much airflow resistance so that the purifier can move more air for the same fan power.
Neither material is "better" because there are differences in the tradeoff between efficiency and airflow. A filter that's harder to push air through needs a stronger fan to hit the same clean-air output, which is why the material choice matters more for engineering the whole system than for bragging rights on a spec sheet.
How Do HEPA Filters Work?
HEPA filters work by forcing air through a dense mat of fibers that trap particles through three separate physical mechanisms, not one. Large particles get caught by direct impaction and interception because they simply can't navigate around the fibers. Very small particles (under about 0.1 microns) move erratically and are caught by diffusion, essentially wandering into a fiber before they can pass through.
The gap between those two mechanisms is where filtration gets hardest. Particles in the 0.1–0.3 micron range are too small to be reliably intercepted and too large to diffuse predictably, called the most penetrating particle size (MPPS), and it's why HEPA efficiency is tested at 0.3 microns: it's close to the worst case a filter will encounter, not the easiest.
HEPA Filter Ratings and Standards, Compared
HEPA filter ratings are measured differently in the U.S. and Europe, and the two systems aren't directly interchangeable. In the U.S., the reference standard is 99.97% efficiency at 0.3 microns. In Europe, EN 1822 rates filters at the MPPS instead, using classes from E10 through H14.
| Rating | Standard | Minimum efficiency | Test point |
|---|---|---|---|
| MERV 16/17 | ASHRAE 52.2 (US) | 95%+ at 0.3 microns | 0.3 microns |
| E11 | EN 1822 (EU) | 95% at MPPS | 0.1–0.3 microns |
| E12 | EN 1822 (EU) | 99.5% at MPPS | 0.1–0.3 microns |
| H13 | EN 1822 (EU) | 99.95% at MPPS | 0.1–0.3 microns |
| H14 | EN 1822 (EU) | 99.995% at MPPS | 0.1–0.3 microns |
The practical difference between HEPA 12 vs. HEPA 13 is roughly one order of magnitude in leakage: an E12 filter lets through about 1 in 200 particles at its test point, while an H13 lets through about 1 in 2,000. In consumer air purifiers, this distinction matters less than it sounds. Once media is pleated and sealed into a filter housing, small losses from folding and edge-sealing can eat into the theoretical rating on the media itself.
HEPA vs. "True HEPA" vs. HEPA-Type Filters
"True HEPA" and "HEPA" are marketing terms without a legal definition, while "HEPA-type" is an admission the filter doesn't meet the actual standard. When HEPA-grade media is pleated into a filter, efficiency typically drops slightly. A 99.97%-rated media might perform closer to 99.7% once pleated, and lower still if the seal around the frame isn't airtight.
In the consumer market, "HEPA" is generally used to mean a high-efficiency pleated filter, not the literal industrial-grade standard used in cleanrooms. That's a reasonable shorthand as long as the manufacturer discloses real test data. It becomes a problem when a company markets an unrated filter as "HEPA-type" to imply HEPA-level performance without the testing to back it up.
Oransi's AirMend and Mod filters are tested to verified HEPA performance levels; we don't use "HEPA-type" language because it signals a filter that hasn't cleared the actual bar.
HEPA vs. ULPA Filters
ULPA (Ultra Low Particulate Air) filters remove 99.999% or more of particles at 0.1 microns or larger — a higher bar than HEPA, at a smaller particle size. ULPA filters show up in cleanrooms and biosafety cabinets where the air needs to be essentially free of any particulate. That level of filtration comes with a steep airflow penalty, which is why it's overkill and often counterproductive in a home. A well-sized HEPA purifier moving more air will typically clean a room faster than an ULPA filter throttled down to compensate for its resistance.
Why CADR Matters More Than the Efficiency Percentage
CADR (Clean Air Delivery Rate) combines filter efficiency with actual airflow, and it's the number that predicts how fast a purifier cleans a room, not the efficiency percentage alone. A filter can be extremely efficient and still perform poorly if it chokes off airflow.
The math is simple: CADR = efficiency × airflow (cfm). A filter that's 100% efficient at 300 cfm has a CADR of 300. Drop efficiency to 99.8% and CADR barely moves, to 299.4. But drop efficiency to 50%, which can happen with poorly sealed or degraded media, and that same 300 cfm purifier effectively delivers a CADR of just 150.
This cuts both ways. A purifier marketed at H14-level filtration but only pushing 250 cfm delivers a CADR of 250. A purifier at a more typical 99.8% efficiency but 300 cfm airflow delivers a CADR of roughly 299, which is a better real-world result, despite the "lower" filter rating. Chasing an efficiency number instead of CADR is one of the most common mistakes in this category.
What HEPA Filters Don't Remove
HEPA filters capture airborne particles like pollen, mold spores, fine dust, pet dander, and smoke particulates, but they do not remove odors, gases, or VOCs. HEPA media works like a mechanical sieve: it's built to physically intercept solid and liquid particles, not to adsorb gas molecules, which are orders of magnitude smaller than even the finest particulate.
This is why a HEPA-only purifier can trap the particulate from cigarette smoke while leaving the smell behind. Odors and gases require activated carbon, which works on an entirely different principle: adsorption into a porous carbon structure rather than fiber interception. If odor, chemical off-gassing, or VOCs are the main concern, see our guide on activated carbon filtration and the best air purifiers for odors.
Are HEPA Air Purifiers Safe?
Yes. Air purifiers that rely only on HEPA filtration are safe because they capture particles mechanically and don't emit anything into the air. There's no chemical reaction, no byproduct, and no pollutant generated as part of the filtration process.
The caveat is for purifiers that combine HEPA with other technologies, particularly electronic or ionizing methods, which can produce ozone as a byproduct. Ozone is a lung irritant, and California's Air Resources Board (CARB) requires that any air cleaning device sold in the state emit no more than 0.050 ppm of ozone. A HEPA-plus-carbon purifier with no ionizer or electronic stage, like Oransi's Mod lines, doesn't carry this risk.
HEPA Filter Sizing: Matching CADR to Your Room
A HEPA purifier's rated coverage only means something if it matches your actual room size — sized too small, even a 99.97% filter won't turn over the air fast enough to matter. The rule of thumb is an air exchange rate of at least 4–5 times per hour for allergy or asthma relief; more for wildfire smoke or heavy pet dander.
| Room size | Recommended model | Filtration type |
|---|---|---|
| Up to 709 sq ft | AirMend 150HB | HEPA |
| Up to 780 sq ft | AirMend 200HB | HEPA |
| Up to 888 sq ft | AirMend 270HB | HEPA |
| Up to 878 sq ft (with odor/VOC concerns) | Mod Jr. | HEPA + activated carbon |
| Up to 1,361 sq ft | Mod+ | HEPA + activated carbon |
If odors, smoke smell, or off-gassing are part of what you're trying to solve, choose from the Mod series or TrueCarbon line rather than HEPA alone. HEPA handles the particulates, while carbon handles the odors and gas.
Frequently Asked Questions
What is a HEPA rating?
A HEPA rating is a certified efficiency level showing what percentage of airborne particles a filter removes at a specific particle size. In the U.S., a true HEPA rating means 99.97% or more removal at 0.3 microns; in Europe, EN 1822 ratings (E11 through H14) measure the same idea against the most penetrating particle size.
What is a HEPA filter made of?
A HEPA filter is made of either borosilicate microfiberglass or non-woven polypropylene, pleated into a dense mat and sealed into a frame. Both materials are engineered to trap particles far smaller than a standard furnace filter can catch.
What does HEPA stand for?
HEPA stands for High Efficiency Particulate Air. It refers to a filter certified to remove at least 99.97% of particles at 0.3 microns under the U.S. Department of Energy's HEPA standard.
What does "HEPA" mean on a mask or respirator?
On a mask, "HEPA" usually refers to the filter media inside an elastomeric respirator, but personal respirators like N95s are actually certified under a separate NIOSH rating system, not the HEPA standard used for air purifiers. An N95 must block at least 95% of particles, which is a different (and lower) bar than the 99.97% HEPA standard used in home air purifiers.
What material is HEPA filter media made from?
HEPA filter media is made from ultra-fine glass fiber or electrostatically charged polypropylene fabric, not paper, cotton, or standard synthetic fibers used in basic air filters.
What is a HEPA filter rating chart?
A HEPA filter rating chart lines up equivalent efficiency classes across standards. For example, MERV 16/17 in the U.S. against E11–H14 in Europe's EN 1822 system, so you can compare filters tested under different regional standards. See the comparison table above.
What are HEPA filter standards?
The main HEPA filter standards are the U.S. DOE standard (99.97% at 0.3 microns) and Europe's EN 1822 standard (E10–H14 classes, tested at the most penetrating particle size). Both are legitimate, but they aren't directly interchangeable numbers.
HEPA 12 vs. HEPA 13 — what's the difference?
HEPA 13 (H13) is roughly ten times more efficient than E12 at its test point, which is about 1 particle in 2,000 gets through an H13 filter versus about 1 in 200 for an E12. In practice, once either grade is pleated and sealed into a consumer air purifier, the real-world difference narrows.
How do HEPA filters work?
HEPA filters work by trapping large particles through direct interception and impaction, and very small particles through diffusion, where erratic particle motion drives them into a fiber. The hardest particles to catch fall in between, around 0.1–0.3 microns, which is why HEPA filters are tested at that range.
What's the difference between HEPA and True HEPA?
"True HEPA" is a marketing term, not a separate legal standard. It's typically used to distinguish a filter that meets the actual 99.97%-at-0.3-microns bar from a "HEPA-type" filter that doesn't. Ask for the manufacturer's actual test data rather than relying on the label alone.
Are HEPA filters safe?
Yes. HEPA filters are safe because they trap particles mechanically and don't emit any byproducts. The only safety consideration is with purifiers that pair HEPA with an ionizer or electronic stage, which can generate ozone.
What size room does a HEPA filter cover?
Coverage depends on both filter efficiency and airflow (CADR), not filtration type alone. As a practical guide: there are compact units for small rooms, a mid-size HEPA purifier for bigger rooms, and large-room units to cover more open spaces. See the sizing table above for specific models.
