MPPS stands for the ‘Most Penetrating Particle Size’. For air filters, this is the size of particle that is easiest to get through the filter. It is a number, typically given in microns or nanometers.
The EN1822 standard defines the MPPS as:
MPPS Most penetrating particle size (= particle size, for which the filtration efficiency is a minimum)
What is the most penetrating particle size (MPPS) for air filters?
Typically, the most penetrating particle size for an air filter is around 0.3 microns (0.3µm). That’s why you’ll see many HEPA filters being marketed as capturing 99.97% or 99.95% of 0.3 microns, just like this one:
However, 0.3 microns is not always the MPPS for an air filter or HEPA filter. The MPPS can vary depending on several factors.
Wha affects the most penetrating particle size (MPPS) for air filters?
The speed of air travelling through a HEPA filter can affect the MPPS. Think of the case where air is travelling faster through a filter. Bigger particles will have more momentum, and be able to push more easily through the filter. As the air speed through a HEPA filter increases, the MPPS decreases.
This graphic from a study on the MPPS of HEPA filters shows the effect the air speed has on a filter’s MPPS. The MPPS (marked by the lowest point on the line) increases in size from 0.3 microns to 0.5 microns as the air speed decreases.