Air filters easily catch both large and small particles. It’s the intermediate regime where things are hard. The size where the filter performs worst is called the Most Penetrating Particle Size (MPPS). Typically this is around 0.15 microns.
The EU HEPA filter spec—yours to download today for a bargain $1148.24—says that filters should meet their guarantees at their MPPS. An E12 filter must block 99.5% of particles at its worst-case particle size, while an H13 filter must block 99.95%.
DYNOMIGHT via Hacker News