HEPA filters were originally designed in the 1940s and used in the Manhattan Project to prevent the spread of airborne radioactive contaminants. Today, to be considered “True HEPA,� a filter must be capable of removing at least 99.97% of airborne particulates 0.3 microns or larger. The 40850-NQH replacement HEPA filter is compatible with the NQ Clarifier Standard, NQ Clarifier Medical, and NQ Clarifier Medical Plus air purifiers, and is made of fine glass-impregnated cellulose fibers. NQ Industries subjects its HEPA filters to rigorous DOP testing, which has proven that the 40850-NQH replacement HEPA filter actually has over 99.99% efficiency at removing harmful pollutants from the air you breathe.Ensure optimal performance of your NQ air purifier by replacing the HEPA filter with the 40850-NQH every 24 months (18 months recommended).