EN 149 is a European standard of testing and marking requirements for filtering half masks. Such masks cover the nose, mouth and chin and may have inhalation and/or exhalation valves. EN 149 defines three classes of such particle half masks, called FFP1, FFP2 and FFP3, according to their filtering efficiency. It also classifies masks into “single shift use only” or “re-usable ”, and an additional marking letter D indicates that a mask has passed an optional clogging test using dolomite dust. Such mechanical filter respirators protect against the inhalation of particulates such as dust particles, droplets, and aerosols.