HEPA filters don’t stop working below 0.3 µm. Around 0.3 µm is usually the most penetrating particle size (MPPS)—the “weak spot” that’s hardest to filter. Below that, diffusion (Brownian motion) becomes very effective, so HEPA can capture nanoparticles extremely well.
ALT Closeup of Smart Air HEPA filter
ALT Microscopic view of HEPA filter