Modern air filtration devices such as face masks, HEPA filters, HME filters, etc., use synthetic or non-woven fabrics as a filtration layer. The filtration layer offers a barrier to particles - either mechanically or electro-statically - and thus particles entering the filter are removed and clean air is inhaled at the other side of the device.
Tests show that filter materials are very efficient at removing small particles (< 0.1 of µm diameter) such as bacteriophage from air streams because electrostatic charge and Brownian motion trap these fine particles within the matrix of the non-woven fabric. Larger particles (>1.0µm in diameter) such as bacteria or fungi are also efficiently removed by mechanical filtration. However, filtration media are generally poor at removing particles in the range from 0.1µm to 0.5µm in diameter. Somewhat counter intuitive, these mid size particles are small enough to escape the forces of mechanical filtration yet are large enough to avoid being entrapped by electrostatic or Brownian motion. These particles are often referred to as the Most Penetrating Particle Size (MPPS).
Unfortunately, many enveloped viruses are in this range of dimension and thus can pass through filtration media. In fact, Influenza virus (A, B, H1N1, H5N1) the SARS virus, pediatric Respiratory Syncitial Virus have a particle diameter between 0.08 µ and 0.3 µ and thus are poorly removed by air filters.