HEPA is defined as a filter that captures 99.97% of particles down to 0.3 microns and is tested using standard IEST-RP-CC001.6. Standard EN1822-5 further tests filters for capture of 99.95% of particles of the most penetrating size, usually around 0.1 microns. Dyson HEPA filters pass both of these standards.
For reference, IEST-RP-CC001.6 is a test standard that covers basic provisions for HEPA (high efficiency particulate air) and ULPA (ultra-low penetration air) filter units as a basis for agreement between customers and suppliers.
EN 1822 is a specification used in the European Union which defines several classes of HEPA filters by their retention at the given most penetrating particle size (MPPS).