I · Air · Explainer · 7 min
What is a HEPA filter?
What the standard actually means, what HEPA captures and what it cannot, and how to read the spec sheet.
By Colpo d’Aria Editorial · Published September 24, 2026 · Last reviewed October 1, 2026
The short answer
HEPA is a performance standard, not a material or a brand. A true HEPA filter captures at least 99.97% of particles at 0.3 microns, the size hardest to trap. It removes particles from the air, including dust, pollen, mold spores, pet dander and many virus-carrying droplets. It does not remove gases, odors or CO₂.
What the standard says
HEPA stands for high-efficiency particulate air. The widely used US definition requires a filter to capture at least 99.97% of particles at 0.3 microns in diameter. That size is chosen because it is the most penetrating particle size: larger particles are caught by impact and interception, smaller ones by diffusion, and 0.3 microns sits in the awkward middle.
A filter that passes the test is a HEPA filter. Everything else is marketing language.
True HEPA versus HEPA-type
Many products are labeled HEPA-type, HEPA-like or 99% HEPA. These terms have no standard meaning. They often describe filters that capture far less than 99.97%, and sometimes much less at the hardest particle sizes.
When comparing products, look for the words true HEPA or an explicit 99.97% at 0.3 microns claim. If the package avoids stating a number, assume it is not HEPA.
What HEPA captures
- Dust and fine particles (PM2.5), including smoke particles
- Pollen, the main trigger of seasonal allergies
- Mold spores, which are typically 3 to 40 microns and easily captured
- Pet dander and dust mite debris while airborne
- Bacteria and virus-carrying respiratory droplets and aerosols
What HEPA does not do
- It does not remove gases: VOCs, odors, formaldehyde and ozone pass through. Those need activated carbon or another sorbent.
- It does not remove CO₂. Only ventilation does that.
- It does not remove allergens that have settled on surfaces. Cleaning still matters.
- It does not fix a moisture problem. A filter captures mold spores but the damp source keeps producing them.
MERV, HEPA and your HVAC system
Furnace and air-conditioning filters use a different scale, MERV, which runs from 1 to 16. MERV 13 approaches HEPA-level capture of fine particles and is the usual recommendation for homes whose systems can handle the added resistance. True HEPA is too restrictive for most residential HVAC systems, which is why HEPA usually appears in portable room purifiers and vacuum cleaners rather than central systems.
How to read a purifier spec sheet
- Filter class: true HEPA or an explicit 99.97% at 0.3 microns claim
- Smoke CADR: the clean air delivery rate. The AHAM guideline is a CADR of at least two-thirds of the room area in square feet
- Room size rating: treat manufacturer claims skeptically and size up so you can run the unit on a lower, quieter setting
- Carbon by weight: a thin carbon sheet does little for odors; meaningful gas filtration is measured in pounds
- Replacement filter cost: check the annual cost before buying the unit
Do HEPA filters help with mold?
They capture airborne mold spores effectively, which reduces what you breathe. They do not solve the problem: mold grows from moisture, so the damp source must be found and fixed, and visible mold growth should be cleaned or remediated. Think of the filter as support while the source is addressed.
Can you clean and reuse a HEPA filter?
Generally no. Washing or vacuuming a true HEPA filter damages the fine fiber structure and reduces performance, and captured particles can be released back into the room. Replace filters on the manufacturer's schedule, sooner in smoky or dusty conditions. Some pre-filters are washable; the HEPA element is not.
Do HEPA filters help with COVID and other viruses?
Yes, as one layer of protection. Virus particles travel on respiratory droplets and aerosols that HEPA filters capture well. The EPA and CDC include portable HEPA air cleaners among the tools that reduce airborne virus particles indoors, alongside ventilation and other measures. A purifier reduces exposure; it does not make a room safe on its own.
Is a more expensive HEPA filter better?
Not necessarily. The standard is the standard. Price differences usually buy quieter fans, better sensors, more carbon, or design. A modest unit with true HEPA and an adequate CADR, run consistently, outperforms an expensive one left off.
How often should I replace a HEPA filter?
Typically every 6 to 12 months, depending on use and air quality. Wildfire smoke, renovation dust and heavy shedding shorten the interval. Many units have filter indicators; a visible gray-brown filter and reduced airflow are reliable signs.