Basically a HEPA/electrostatic filter works in two ways. As a “physical” filter (think fishing net), works well on big particles. As an electrostatic filter,

Basically a HEPA/electrostatic filter works in two ways. As a “physical” filter (think fishing net), works well on big particles. As an electrostatic filter, works best with really small, works by electrically attracting particles and having them stick. 2.5 micron is in the middle crossover range where total net effectiveness is worst. Respirators are rated at the same size for the same reason.