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As I understand HEPA filters usually remove particles down to about 20nm, which will capture a good chunk, PM2.5 is 2500nm so far smaller than that. Although it may well be that the very small particles are most able to penetrate our biological barriers, and are therefore the most dangerous. This is an active area of research and I think still not fully understood.

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The study linked in this comment tree mentions

> The study was also the first to investigate the filtering of very small nanocluster particles down to 1.3 nanometres in size. These are close to being gas molecules, but their filtration properties are different. All of the air filters that were tested effectively removed nanocluster particles smaller than 3 nanometres, which means that particles of this size are unlikely to enter indoor areas.

With HEPA filters being the most effective ones removing "100% of root nanoparticles"

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