Filtration involves the passage of a fluid through some material having small openings called pores that are small enough to inhibit the passage of most microbes.  Filter types include ground porcelain, sand, diatomaeceous earth and man-made nitrocellulose filters (above).  Though pore sizes can be as small as 0.01 mm and trap even larger viruses, most biological filters with pore sizes between 25 mm and 0.2 mm are not small enough to trap mycoplasmas and viruses.


High Efficiency Particulate Air (HEPA) filters have pore sizes as small as 3 mm and are used to filter air in devices such as the laminar flow hood (used in laboratories to reduce contamination from outside air), as well as closed buildings using HVAC systems, operating rooms and burns wards.