An H13 HEPA filter captures 99.97% of particles at 0.3 microns—the most penetrating particle size—and is the standard for home air purifiers. H14 captures 99.995% and is used in medical and clean-room settings. For most homes, H13 is the right choice: it provides near-perfect particle removal without the airflow restriction and cost premium of H14.
HEPA stands for High Efficiency Particulate Air. The term was coined during the Manhattan Project in the 1940s for filters used to capture radioactive particles. Today, HEPA is a performance standard—not a material or brand. The classification system (H10 through H14 and beyond) defines exact minimum efficiency at the hardest-to-capture particle size.
HEPA Filter Classification: The Complete Breakdown
HEPA filters are classified under the European standard EN 1822-1 (adopted globally). Each class has a minimum efficiency at the Most Penetrating Particle Size (MPPS), which is approximately 0.3 microns for most HEPA media.
The critical threshold is H13. Below H13, filters are technically classified as EPA (Efficient Particulate Air), not HEPA. When a product says "HEPA-type," "HEPA-style," or "99% HEPA," it's almost always E11 or E12—which sounds close to H13 but lets through 5–167× more particles at MPPS.
How HEPA Filters Actually Work
HEPA filters don't work like a simple sieve. They use four distinct physical mechanisms to capture particles across the entire size spectrum:
Mechanism 1: Interception
Particles following the airstream pass close enough to a filter fiber that the particle's edge touches the fiber and sticks. This is the primary mechanism for particles between 0.3–1.0 microns. The particle doesn't need to hit the fiber dead-on—it just needs to pass within one particle radius of the fiber surface.
Mechanism 2: Impaction
Larger particles (above 0.5 microns) have enough inertia that they can't follow the airstream as it curves around filter fibers. They break away from the streamline and collide directly with the fiber. The heavier the particle and the faster the airflow, the more effective impaction becomes.
Mechanism 3: Diffusion
The smallest particles (below 0.1 microns) exhibit Brownian motion—they zigzag randomly as they're bumped by air molecules. This random movement increases the probability that the particle contacts a filter fiber and sticks. Diffusion is more effective at lower airspeeds, which is why HEPA filters actually become more efficient for ultrafine particles at lower flow rates.
Mechanism 4: Electrostatic Attraction
Many HEPA filters (especially synthetic ones) carry an electrostatic charge that attracts particles. This charge enhances capture efficiency beyond what the mechanical structure alone would achieve. However, the electrostatic charge degrades over time and when the filter gets wet—which is why washing a HEPA filter destroys its performance.
The 0.3 Micron "Sweet Spot" of Difficulty
At approximately 0.3 microns (300 nanometers), particles are too large for diffusion to be highly effective and too small for impaction and interception to dominate. This creates a trough in capture efficiency—the Most Penetrating Particle Size (MPPS).
This is why HEPA testing uses 0.3 microns. It's not the smallest particle the filter catches; it's the hardest. Particles both larger AND smaller than 0.3 μm are captured at even higher rates.
H11 vs. H13 vs. H14: The Practical Differences
H11 (E11): 95% Efficiency
H11 filters capture 95% of particles at MPPS. That means 5 out of every 100 particles at the hardest-to-catch size pass through. For larger particles like pollen (10–40 μm) and most dust (1–10 μm), H11 still catches 99%+.
Where H11 is used: Budget air purifiers, automotive cabin filters, basic commercial HVAC, and low-cost residential units. Many purifiers marketed as "HEPA-type" or "99% efficient" use H11 or similar-grade media.
The gap from H13: H11 lets through approximately 167× more particles at MPPS than H13 (5% vs 0.03% penetration). For large particles, the difference is negligible. For fine particles like smoke, bacteria, and virus-carrying aerosols, the gap is significant.
H13: 99.97% Efficiency — The Gold Standard
H13 is what "True HEPA" means. It captures 99.97% of particles at MPPS, letting only 0.03% through. This is the standard used in hospital ventilation, pharmaceutical manufacturing, and the overwhelming majority of quality consumer air purifiers.
Where H13 is used: Virtually all reputable consumer air purifiers (Coway, Blueair, Winix, Austin Air, IQAir, Levoit), hospital patient rooms, pharmaceutical facilities, and laboratory settings.
Why H13 is the sweet spot: H13 provides near-perfect particle removal while maintaining reasonable airflow. The denser filter media of H14 creates more airflow resistance, requiring a stronger (louder, more power-hungry) fan to achieve the same CADR.
H14: 99.995% Efficiency — Medical Grade
H14 captures 99.995% of particles at MPPS—letting only 0.005% through. That's 6× better penetration performance than H13.
Where H14 is used: Surgical suites, isolation rooms, cleanrooms, pharmaceutical compounding, BSL-3 laboratories, and some premium consumer purifiers (like the IQAir HealthPro Plus).
The trade-off: H14 filters are denser, creating 30–60% more airflow resistance than H13. A purifier using H14 needs a significantly more powerful fan to achieve the same CADR, which means more noise and energy consumption. For most home applications, the marginal improvement from H13 to H14 (0.03% → 0.005% penetration) isn't worth the trade-offs.
The "HEPA-Type" vs. "True HEPA" Problem
The air purifier market has a labeling problem. Only H13 and H14 filters are technically HEPA under the EN 1822 standard. But many manufacturers use misleading terms:
"HEPA-type" — Usually E11 (95%) or E12 (99.5%). Legally distinct from HEPA but designed to confuse consumers.
"HEPA-style" — Same issue. No defined standard; could be anything from 85% to 99% efficiency.
"99% HEPA" — Sounds impressive but is actually worse than H13's 99.97%. That 0.97% gap means an H13 filter captures 32× more particles at MPPS than a "99% HEPA" filter.
"HEPASilent" — Blueair's proprietary term combining electrostatic charge with a mechanical filter. Their combination achieves H13-equivalent performance (independently verified) with lower airflow resistance, allowing higher CADR at lower noise. This is one of the few proprietary terms that delivers genuine HEPA-class results.
If a product doesn't explicitly state "H13" or "True HEPA" with a specific efficiency claim of 99.97% at 0.3 microns, assume it's not H13. Check for third-party verification from AHAM (Verifide program) or independent lab reports. Legitimate H13 filters are tested and certified; vague marketing language almost always indicates a lower-grade filter.
Real-World Examples: When Filter Grade Matters
Example 1: Allergy Sufferer in a Suburban Home
Rachel has seasonal allergies (tree pollen, grass pollen, ragweed). Pollen particles range from 10–70 microns—far above the 0.3 μm MPPS. Even an H11 filter captures 99.9%+ of pollen. An H13 filter makes no meaningful difference over H11 for pollen alone.
However, Rachel also reacts to dust mite allergens (fecal particles, 1–5 μm) and mold spores (2–20 μm). At these sizes, H11 still performs well. Verdict: H13 is recommended for the all-around protection, but H11 would handle her specific triggers adequately.
Example 2: Wildfire Smoke in California
Mike faces PM2.5 levels of 50–150 μg/m³ during fire season. Wildfire smoke particles concentrate heavily at 0.1–0.5 μm—right around the MPPS. At 0.3 μm, the difference between H11 (95%) and H13 (99.97%) means H11 lets through 167× more smoke particles per pass.
Over multiple air changes, this compounds. After 5 air changes with H13, only 0.7% of original particles remain. With H11, approximately 23% remain after 5 passes. Verdict: H13 is significantly better than H11 for smoke. H14 provides marginal additional benefit.
Example 3: Immunocompromised Patient at Home
Sarah is recovering from chemotherapy and has a severely suppressed immune system. Her oncologist recommends medical-grade air filtration. Airborne bacteria (0.3–5 μm) and virus-carrying aerosols (0.1–1 μm) are critical concerns.
At these sizes, every fraction of a percent matters. H14 (99.995%) reduces pathogen exposure by 6× compared to H13. Her IQAir HealthPro Plus with H14 filter in a small bedroom achieves 12+ ACH with medical-grade filtration. Verdict: H14 is justified for immunocompromised patients where infection risk is life-threatening.
Example 4: Budget-Conscious General Use
Tom wants cleaner air in his apartment but has a $150 budget. A $50 unit with an H11 filter (95% efficiency) removes most dust, pollen, and pet dander effectively. A $150 unit with an H13 filter costs 3× more but provides dramatically better fine-particle filtration.
For general indoor air quality improvement without specific health concerns, an H11 unit provides noticeable improvement at a lower price point. Verdict: H13 is always preferred if affordable, but H11 is a reasonable budget choice for general use without specific smoke or pathogen concerns.
HEPA Filters and Specific Contaminants
HEPA filters do NOT remove gases, odors, or VOCs. No amount of HEPA efficiency will capture formaldehyde, cooking odors, or chemical fumes. These are molecules, not particles. You need activated carbon filtration for gas-phase contaminants. The best purifiers combine H13 HEPA with activated carbon for comprehensive air cleaning.
HEPA Filter Lifespan and Maintenance
How Long Do HEPA Filters Last?
HEPA filter lifespan depends on particle loading (how much contamination is in your air), airflow rate, and filter surface area. General guidelines:
| Condition | H13 Filter Lifespan | H14 Filter Lifespan |
|---|---|---|
| Low pollution, light use | 12–18 months | 18–24 months |
| Average residential use | 6–12 months | 12–18 months |
| High pollution / pets / smoke | 3–6 months | 6–12 months |
| Wildfire season (active smoke) | 1–3 months | 3–6 months |
| Commercial / high-traffic | 3–6 months | 6–12 months |
Why You Can't Wash Most HEPA Filters
Most HEPA filters use either glass fiber or synthetic polypropylene media. The fibers are arranged in a random mat with specific spacing optimized for particle capture. Washing disrupts this structure and—critically—destroys the electrostatic charge that enhances capture efficiency.
Some manufacturers (like Blueair) sell washable pre-filter covers that protect the inner HEPA media. These pre-filters can be washed; the HEPA filter inside them cannot.
A handful of units use washable HEPA-type filters (typically H11 grade). These sacrifice some efficiency for reusability. If the manufacturer says "washable HEPA," verify the filter class—it's almost certainly not H13.
When to Replace
Replace your HEPA filter when:
- The manufacturer's recommended interval has passed
- Airflow noticeably decreases (the fan sounds louder but moves less air)
- Your air quality monitor shows rising PM2.5 that the purifier isn't controlling
- A filter life indicator (if equipped) signals replacement
- You notice odors passing through (carbon component exhausted)
MERV vs. HEPA: Understanding the Different Rating Systems
MERV (Minimum Efficiency Reporting Value) is the rating system for HVAC filters. HEPA (H10–H14) is the rating system for high-efficiency filters in air purifiers and specialized applications. They measure different things at different particle sizes.
The best residential HVAC filter (MERV 16) is roughly equivalent to an H11 HEPA. True H13 HEPA filtration is available only in dedicated air purifiers and specialized HVAC systems.
Key Takeaways
- H13 (True HEPA) captures 99.97% at 0.3 μm—the optimal choice for home use
- H11 captures 95%—adequate for pollen and dust but lets 167× more fine particles through than H13
- H14 captures 99.995%—warranted only for medical use or immunocompromised individuals
- "HEPA-type" and "HEPA-style" are NOT true HEPA—always look for explicit H13 or "99.97% at 0.3 μm"
- HEPA filters capture particles smaller than 0.3 μm more efficiently, not less—0.3 μm is the hardest size
- HEPA does not remove gases or odors—you need activated carbon for VOCs and smells
- Never wash a HEPA filter unless the manufacturer explicitly certifies it as washable
- MERV 16 (best HVAC filter) roughly equals H11—HEPA purifiers far exceed HVAC filtration