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With or without face masks on, the quality of air you breathe matters.  HEPA stands for "high efficiency particulate air (filter)" and it is the industry standard in medical settings, hospitals and surgery centers as well as in homes and offices.  A HEPA "...air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns (µm). The diameter specification of 0.3 microns responds to the worst case; the most penetrating particle size (MPPS). Particles that are larger or smaller are trapped with even higher efficiency. Using the worst case particle size results in the worst case efficiency rating (i.e. 99.97% or better for all particle sizes).  All air cleaners require periodic cleaning and filter replacement to function properly. Follow manufacturer's recommendations on maintenance and replacement.."  (source EPA website)

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