HEPA filters are technically specified to capture airborne particles measuring what specific diameter?

0.3 microns

The performance standard for HEPA filtration is calibrated against a particle size that represents a significant challenge for filtration technology. Specifically, HEPA filters must demonstrate the capability to capture 99.97% of airborne particles that are exactly 0.3 microns in diameter. This particle size is often cited because it represents the Most Penetrating Particle Size (MPPS) for standard fibrous filter media; particles smaller or larger than this size are theoretically captured with even higher efficiency by the filter's mechanisms (diffusion and interception). Targeting the 0.3-micron size ensures the filter is robust enough to handle the vast majority of sub-micron threats that earlier technologies could not reliably address.