The Most Penetrating Particle Size (MPPS) is the particle size that is most difficult for a filter to capture. For HEPA filters, this is typically around 0.3 microns. Particles at the MPPS are small enough to follow airflow streams through the filter without being intercepted but large enough to avoid the random motion (diffusion) that aids in capturing even smaller particles.
This concept is crucial because filters are tested and rated based on their efficiency at the MPPS. A filter that performs well at the MPPS will generally perform even better at capturing both larger and smaller particles due to the mechanisms of impaction and diffusion.