HEPA Filters Explained
Everything you need to know about HEPA filters: standards, types, what they filter, and how to maintain them.
What Is a HEPA Filter?
HEPA stands for High-Efficiency Particulate Air. A True HEPA filter captures at least 99.97% of particles with a diameter of 0.3 microns. This includes PM2.5, dust mite allergens, pollen, mould spores, bacteria, and many viruses.
The 0.3-micron size is the “most penetrating particle size” (MPPS) — particles both smaller and larger are actually captured more efficiently.
How HEPA Filters Work
HEPA filters use three mechanisms:
- Interception: Particles following the airstream come within one particle radius of a fibre and stick to it
- Impaction: Larger particles (above 0.5 microns) cannot follow the airstream and collide with fibres
- Diffusion: Very small particles (below 0.1 microns) move randomly due to Brownian motion and get trapped
This combination means that particles of all sizes are captured, with the lowest efficiency at exactly 0.3 microns — still at 99.97%.
True HEPA vs HEPA-Type
This is the most important distinction when buying an air purifier:
| Feature | True HEPA | HEPA-Type / HEPA-Like |
|---|---|---|
| Efficiency | 99.97% at 0.3 µm | Varies (typically 80–95%) |
| Standard | EN 1822, IEST-RP-CC001 | No standard |
| Certification | Independently tested | Self-declared |
| Cost | Higher | Lower |
| Effectiveness | Proven for PM2.5 | Marginal for fine particles |
Always choose True HEPA. In India, many budget purifiers advertise “HEPA” but deliver HEPA-type filters that are ineffective for PM2.5.
HEPA Grades (EN 1822 Standard)
| Grade | Efficiency at MPPS | Typical Use |
|---|---|---|
| H10 | 85% | Industrial pre-filtration |
| H11 | 95% | Lower-end air purifiers |
| H12 | 99.5% | Mid-range air purifiers |
| H13 | 99.95% | High-end air purifiers |
| H14 | 99.995% | Hospitals, clean rooms |
| U15–U17 | 99.9995–99.999995% | Pharmaceutical, nuclear |
For home use in Indian cities, H13 or H12 is recommended. H13 captures 99.95% of particles including PM2.5 and most airborne bacteria.
What HEPA Filters Capture
- Particulate matter: PM10, PM2.5, PM1, ultrafine particles
- Allergens: Pollen, dust mite debris, pet dander, cockroach allergens
- Microorganisms: Bacteria (0.5–5 µm), mould spores (1–30 µm), some viruses (0.02–0.3 µm)
- Not captured: Gases, VOCs, NO2, SO2, CO — these require activated carbon
Maintenance and Lifespan
- Pre-filter: Clean every 2–4 weeks with a vacuum or wash (if washable)
- HEPA filter: Replace every 6–12 months depending on usage and pollution levels
- Carbon filter: Replace every 3–6 months (in combined filter units, when HEPA is replaced)
- In Delhi-NCR: Expect to replace HEPA filters every 6–8 months due to high particulate load
- In cleaner cities: HEPA filters can last 12–18 months
Never wash a HEPA filter — it destroys the fine fibre structure. Only replace it.
Key Takeaways
- True HEPA filters capture 99.97% of particles at 0.3 microns, including PM2.5
- HEPA-type filters lack certification and are significantly less effective
- H13 grade is the recommended minimum for Indian homes
- HEPA filters do not capture gases — you need activated carbon for that
- Replace HEPA filters every 6–12 months; clean pre-filters monthly