An Air shower is an extensive (many kilometres wide) cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic ray (i.e. one of extraterrestrial origin) enters our atmosphere. The term cascade means that the incident particle, which could be a proton, a nucleus, an electron, or (rarely) a positron strikes an atom in the air so as to produce many high energy ions (secondaries), which in turn create more, and so on. The original particle having arrived with high energy and hence velocity near the speed of light, the products of the collisions tend also to move generally downward, while to some extent spreading sidewise.
The term air shower can refer to the specialized antechamber which is passed through before entering a cleanroom in order to blow off excess dust particles from cleanroom personnel before they enter, to minimize contamination. HEPA filters are generally used to decontaminate a room by filtering out particle less than .3 microns in size.