HEPA filter

HEPA is an acronym standing for High-Efficiency Particulate Air (or Absorbance) (or Arrestance) and pronounced  [hep-pa].  A filter designated as HEPA  has increased efficiency in removing particles produced by dust, pollen, moisture, bacteria and viruses from the air you breathe.  It was first produced in the 1950s but it has escaped from its very specialised context of use to become something that the average traveller on an aeroplane really wants to have. 

 HEPA filters are composed of a mat of randomly arranged fibres, usually polypropylene or fibreglass.  The big particles are caught in the net. The small ones move more slowly than the big ones and are knocked around by the air molecules as they move through the twisted pathways. They end up crashing into the fibres and being caught.