What Is MPPS? A Direct Answer
MPPS stands for Most Penetrating Particle Size, the particle diameter at which a filter has its lowest capture efficiency. For glass-fibre HEPA media it usually falls between 0.1 and 0.25 µm. Because a filter performs worst at the MPPS, EN 1822 measures efficiency at exactly this size, so the class rating (H13, H14, U15–U17) is a guaranteed minimum, not an average.
Why the Smallest Particles Are Not the Hardest to Catch
Filtration efficiency follows a U-shaped curve driven by three capture mechanisms. Large particles are caught by inertial impaction and interception; the smallest are caught efficiently by diffusion (Brownian motion). Between them lies a weak point, the MPPS, where no single mechanism is dominant.
| Particle size | Dominant mechanism | Capture |
|---|---|---|
| Above 1 µm | Impaction and interception | High |
| 0.1–0.25 µm (MPPS) | Weakest overlap | Lowest |
| Below 0.1 µm | Diffusion | High |
How MPPS Defines the Filter Class
Under EN 1822, the manufacturer first determines the MPPS for the specific media, then measures integral and local efficiency at that size. The result sets the class: H13 ≥ 99.95%, H14 ≥ 99.995%, and the ULPA classes above. See how these classes compare in our guide to HEPA vs ULPA filters and the full EN 1822 standard.
MPPS in Real Operating Conditions
The exact MPPS shifts with media, fibre diameter and face velocity. Higher airflow reduces diffusion capture and can move the MPPS, lowering efficiency, which is one reason a HEPA filter should run near its design airflow. A single “99.97% at 0.3 micron” figure is not the same as an EN 1822 MPPS classification; always check the test size and method.
Why It Matters When You Specify
Because MPPS is the worst case, a filter classified at MPPS gives a conservative, reliable performance guarantee, and installed integrity testing then confirms it in place. Browse Safe Air HEPA filters and request an EN 1822 classified filter, or send your specification for a quotation.
Frequently Asked Questions
MPPS stands for Most Penetrating Particle Size, the particle diameter at which a filter has its lowest capture efficiency. It represents the worst-case performance and is the basis for EN 1822 classification.
No. For many HEPA media the MPPS falls roughly between 0.1 and 0.3 microns, but the exact value depends on the media, fibre diameter and face velocity. The 0.3 micron figure comes from older test methods and should not be assumed universally.
Testing at MPPS measures the filter where it performs worst, so the stated efficiency is a guaranteed minimum. For every other particle size the filter performs at least as well.
Very small particles move by diffusion (Brownian motion) and collide with fibres readily, while larger particles are caught by impaction and interception. Particles near the MPPS fall between these mechanisms, so they are the hardest to capture.
Yes. A higher face velocity reduces diffusion capture and can shift the MPPS, lowering efficiency. This is one reason a HEPA filter should be operated close to its design airflow.
Not necessarily. That figure refers to a fixed 0.3 micron size, while EN 1822 classes such as H13 and H14 are defined at the actual MPPS with integral and local efficiency requirements. Always check the standard and test size behind any efficiency claim.
