To be a HEPA filter, the filter has to trap at least 99.97% of particles such as dust, pollen, mould and bacteria at 0.3 micron. As a comparison to 0.3 micron, a human hair is approximately 70 microns.
HEPA filters are classified as either H13 or H14. If you see something classified as ‘True HEPA’ it isn’t a real definition. The filter specifications are an important balance of filter efficiency – which is the percentage of dirt a filter moves, life – how long the filter lasts before clogging up and flow – how easily the air flows through the filter.
At H13 or H14 the filter needs to still be efficient but not immediately block or restrict the air flow. HEPA filters are used in a variety of industries from cabin air filters in Tesla vehicles to air purifiers.