A HEPA or High Efficiency Particulate Air Filter is a filter that will capture particles down to .3 microns with 99.97% efficiency. Most household vacuums and a few commercial and ‘ShopVac’ type vacs built today come with a HEPA filter of some kind. The problem with most of these is that they are cheaply made and not all exhaust air is channeled through the HEPA exhaust filter.
EPA’s definition of a HEPA vacuum from 40 CFR 745.83:
“HEPA vacuum means a vacuum cleaner which has been designed with a high-efficiency particulate air (HEPA) filter as the last filtration stage. A HEPA filter is a filter that is capable of capturing particles of 0.3 microns with 99.97% efficiency. The vacuum cleaner must be designed so that all the air drawn into the machine is expelled through the HEPA filter with none of the air leaking past it.”
Before investing 100′s of dollars, be sure the HEPA vacuum you’re purchasing is certified to the correct standard. Simply retrofitting a HEPA filter to an existing vac may not ensure you are 100% compliant.