Product Audits are one of the tools used to verify consistent quality of respirator units.  Filtering facepiece units are purchased from the open market (where available) and tested according to the certification test criteria.  Other types of units are examined at NPPTL under the default to test program.  This program ensures that a unit that has not been tested during a given period of time (1-2 years depending on the type of unit), is tested prior to issuing an extension of approval for a change by the approval holder that would not typically require testing.  And finally, under the product audit program NPPTL has started looking at testing units that have been stored for a period of time.  A formal study may be warranted to collect and establish if stored units are less effective over time.