What is a HEPA filter?

 A HEPA filter is a type of mechanical air filter that works by forcing air through a fine mesh that traps harmful particles such as pollen, pet dander, dust mites, and tobacco smoke ¹². The acronym HEPA stands for High-Efficiency Particulate Air ¹. Filters meeting the HEPA standard must satisfy certain levels of efficiency. Common standards require that a HEPA air filter must remove from the air that passes through at least 99.95% (ISO, European Standard) or 99.97% (ASME, U.S. DOE) of particles whose diameter is equal to 0.3 μm, with the filtration efficiency increasing for particle diameters both less than and greater than 0.3 μm ¹² . HEPA filters capture pollen, dirt, dust, moisture, bacteria (0.2–2.0 μm), virus (0.02–0.3 μm), and submicron liquid aerosol (0.02–0.5 μm) ¹² . Some microorganisms, for example, Aspergillus niger, Penicillium citrinum, Staphylococcus epidermidis, and Bacillus subtilis are captured by HEPA filters with photocatalytic oxidation (PCO) . A HEPA filter is also able to capture some viruses and bacteria which are ≤0.3 μm . A HEPA filter is also able to capture floor dust which contains bacteroidia, clostridia, and bacilli .