According to a NASA study, true medical grade HEPA filters can filter out ultrafine (<0.01 μm) sized particulate matter.
"The phenomena associated with particulate matter removal by HEPA media filters and packed beds of granular material have been reviewed relative to their efficacy for removing fine (<2.5 μm) and ultrafine (<0.01 μm) sized particulate matter. Both HEPA media filters and packed beds of granular material, such as activated carbon, which are both commonly employed for cabin atmosphere purification purposes, are found to have efficacy for removing nanoparticulate contaminants from the cabin atmosphere. When used alone, HEPA-rated media provides superior performance for removing virtually 100% of particulates."