In general, we think that the finest particles are the most difficult to be captured by the mask. But this is not a correct concept. In filtration theory, there are five main filtration mechanisms :

1. Brownian Diffusion - the smaller the particle is, the stronger the effect is.
2. Interception - the larger the particle is, the better the effect is.
3. Inertial Impaction -- the larger the particle is, the better the effect is.
4. Gravitational Settling -- the larger the particle is, the better the effect is.
5. Static Attraction -- the smaller the particle is, the stronger the effect is. Based on these, we can know that (1) and (5) are effective for smaller particles while (2), (3), and (4) are effective for larger particles.

Therefore, the Most Penetrating Particle Size is called MPPS. The size of MPPS is around 0.6 micron. The International Standard such as EN149:2001, NIOSH 42CFR84, and AS1716 all use MPPS as the testing particle size. If only if you purchase the approved masks, you are well covered.