HEPA is the gold standard for particle filtration, but the term gets misused so often that it's lost some meaning. Here's what you actually need to know.
What HEPA Means
HEPA stands for High-Efficiency Particulate Air. A true HEPA filter must capture at least 99.97% of particles that are 0.3 microns in diameter. This standard was developed for industrial and medical applications and has been adopted by consumer products.
Why 0.3 Microns?
Particles of 0.3 microns are actually the hardest to capture—they're in a "sweet spot" that's difficult to filter. Larger particles are caught by direct interception, smaller particles move erratically and get caught by diffusion, but 0.3-micron particles slip through both mechanisms more easily. If a filter captures 99.97% at this size, it captures even more at other sizes.
How HEPA Filters Work
HEPA filters are made of densely packed, randomly arranged fibers. Particles get caught through several mechanisms:
- Interception: Particles following an airstream come close enough to a fiber to be caught
- Impaction: Larger particles can't follow air curves and collide with fibers
- Diffusion: Smallest particles move randomly (Brownian motion) and contact fibers
The combination means HEPA filters work well across a wide range of particle sizes.
True HEPA vs Marketing Terms
Be careful with terminology:
| Term | Meaning | Recommendation |
|---|---|---|
| True HEPA / HEPA | Meets 99.97% at 0.3μm standard | This is what you want |
| H13 / H14 | European grades, equal or better than HEPA | Good choices |
| HEPA-type | No defined standard, lower efficiency | Avoid |
| HEPA-like | No defined standard, lower efficiency | Avoid |
| HEPA-style | No defined standard, lower efficiency | Avoid |
| 99% HEPA | Sounds good, but misses the 99.97% mark | Substandard |
What HEPA Filters Remove
- Dust and dust mites
- Pollen
- Pet dander
- Mold spores
- Some bacteria (those attached to particles)
- Smoke particles
- Fine particulate matter (PM2.5)
What HEPA Filters Don't Remove
- Gases and odors (need activated carbon)
- VOCs (volatile organic compounds)
- Carbon monoxide or other gases
- Viruses that aren't attached to particles (very small)
HEPA Filter Maintenance
Replacement Schedule
Most HEPA filters need replacement every 6-12 months depending on air quality and usage. Check your manufacturer's recommendations. A clogged filter reduces airflow and effectiveness.
Can You Wash HEPA Filters?
Generally no. Water damages the delicate fiber structure. Some filters are marketed as "washable" but these aren't true HEPA—the fibers can't withstand washing. Replace, don't wash.
Signs Your Filter Needs Replacement:
- Reduced airflow (purifier seems weaker)
- Visible discoloration or debris on filter
- Odors from the purifier
- Manufacturer's recommended timeframe passed
The Simple Truth
HEPA filtration isn't magic, but it's very effective at what it does: removing particles from air. For most apartment air quality concerns involving dust, allergens, and smoke, a true HEPA filter is the right choice. Just make sure you're getting actual HEPA, not a marketing term.