Penetration of a HEPA filter
as a function of particle size

18TH DOE NUCLEAR AIRBORNE WASTE MANAGEMENT AND
AIR CLEANING CONFERENCE, Baltimore 1984.

Also see:
Prototype Firing Range Air Cleaning System 
GLISSMEYER et al

Experimental penetration of particles through a HEPA filter – determination that approximately 0.1% in the 0.1 micron particle range will pass through the filter. If there are 100,000 particles 0.1 micron in diameter per cubic centimeter of air, then 120 per cubic centimeter of air will pass through a HEPA filter. In one day an average man will inhale 28 million particles in the 0.1 micron range through a HEPA filter.