True HEPA vs HEPA-type: H13 and H14 standards, and what "99.9 percent" claims miss

What true HEPA means and why 0.3 microns matters

HEPA stands for High-Efficiency Particulate Air, and the standard is defined by ASME B29.100 in the United States and EN 1822 in Europe. Both standards define HEPA as 99.97 percent efficiency at 0.3 microns.

The 0.3 micron size is chosen because it is the Most Penetrating Particle Size (MPPS) for fibrous filters. At this size, filters are at their worst performance: the particles are too small to be blocked by direct interception, but too large to be moved by diffusion. Particles larger or smaller than 0.3 microns are actually easier to capture.

A filter rated at 99.97 percent efficiency at 0.3 microns will perform better at other sizes. The standard guarantees worst-case performance, not average performance. This is why HEPA is the reference standard for all air purifier claims.

How HEPA filters trap particles

• Direct interception: particles follow airflow but are close enough to fibers to stick
• Impaction: large particles cannot follow air curves and hit fibers head-on
• Diffusion: very small particles bounce around randomly and hit fibers
• Electrostatic attraction: some fibers carry a charge that attracts particles

HEPA under US standard (ASME B29.100)

The US standard tests HEPA filters under ASME B29.100 using monodisperse aerosol particles at six sizes: 0.3, 0.5, 1, 2, 3, and 5 microns. The filter is rated at the size showing the lowest efficiency.

A true HEPA filter must achieve 99.97 percent or higher at all six sizes, with the worst-case at 0.3 microns. This means a HEPA filter performs at 99.97 percent or better across all common particle sizes.

Certification requires testing by an independent lab and documentation of the test results. If a purifier manufacturer claims HEPA, they should be able to provide a test certificate with a lab certification number.

HEPA under European standard (EN 1822: H13 and H14)

Europe uses EN 1822, which defines HEPA filters and also introduces intermediate classes: H10, H11, H12, H13, H14, and U15.

H13 is equivalent to HEPA: 99.95 percent efficiency at 0.3 microns. H14 is 99.995 percent (also called ULPA, Ultra-Low Penetration Air). H10 is lower and used for light commercial applications.

In the US market, a European H13 filter meets the HEPA standard. H14 is higher and is used in hospitals and laboratories. Both require certification and testing.

HEPA-type: a marketing term with no meaning

HEPA-type is not a standard. A manufacturer can use this term without testing the filter or meeting any efficiency requirement. A HEPA-type filter might be 80 percent efficient, or 95 percent, or 99 percent. The term makes no promise.

HEPA-type filters are often found in budget air purifiers marketed to buyers who do not know the difference between true HEPA and marketing language. The cost saving is small (a true HEPA filter might cost 20 to 30 dollars more), while the performance gap is large.

The "99.9 percent" claim and what it hides

Marketers sometimes claim "99.9 percent efficiency" without naming the particle size or standard. This is legally safer than claiming "HEPA" because they cannot be charged with false advertising if they do not define what they are testing.

A filter might be 99.9 percent efficient at 5 microns but only 95 percent at 0.3 microns. Or it might be 99.9 percent efficient at one airflow rate but only 90 percent at the actual operating rate. Without the standard and particle size, the number is meaningless.

Always ask the manufacturer: at what particle size and under what standard is the efficiency measured? If they cannot or will not answer, the claim is not trustworthy.

How to verify HEPA certification

Step one: look for the words "HEPA" or "H13" or "H14" on the filter or in the product description. Not HEPA-type, not HEPA-like, just "HEPA".

Step two: ask the manufacturer for a test certificate or certification number. A legitimate HEPA filter has been tested by an independent lab (like NAFA or TÜV) and has a certification number you can verify.

Step three: check the filter replacement model number. The replacement filter should also carry the HEPA certification. If the unit claims HEPA but the replacement filter does not, the purifier loses performance after the first filter.