From the 2005 Deparment of Energy standard on HEPA Filters:
5.2 Performance Requirements: Mandatory performance requirements for HEPA
filters are set out below. These performance requirements shall be demonstrated by
test and inspection by the manufacturer. These performance requirements shall also be demonstrated by test and inspection by the FTF when applicable (see Section 4.1).
5.2.1 Penetration: Aerosol penetration for any HEPA filter shall not exceed
0.03% (0.0003) at 0.3 micrometer particle size.
And then from the 2015 Standard:
High Efficiency Particulate Air (HEPA) Filter: A throwaway, extended-medium, dry type
filter with a rigid casing enclosing the full depth of the pleats. The filter shall exhibit a
minimum efficiency of 99.97% when tested with an aerosol of 0.3 micrometer diameter.
It seems like the requirement for particulate of 0.3 micrometers(PM0.3) at 99.97% must be met in order for a filter to qualify as a HEPA filter. This would exceed the the original claim for only 99% thus validating it.
So if you have an approved HEPA filter, then the standard guarantees the 99.97% for 0.3 micrometer diameter particles.
I do remember seeing that this standard is for DOE contractors. Consumer product qualifications may be held to a lower standard. I don't know.