Abstract: A device for filtering nanoparticulate matter below 300 nm in size from ambient air within a defined enclosed space comprises at least one filter medium containing at least one filter material, and a fan or blower, which blows or draws the air through the filter medium does so with sufficiently low air velocity in the filter medium that the air has a long residence time in the filter medium, during which time adhesive encounters between nanoparticulate matter and the solid phase of the filter material are highly probable. This air velocity is also being sufficiently low to ensure that, once adhesion has taken place, kinetic de-adhesion by the air flow is highly improbable. Such a device allows to keep the nanoparticle count inside the enclosed space kept below 5000/cc, equivalent to ‘natural woodland’, even though external levels may exceeded 200,000/cc: the removal of nanoparticles by cleaning making this <2.5% the external count.