Surround Air HEPA filter is 99.97% efficient at 0.3 microns for capturing particles including dust and mold spores. Replace XJ-3000 HEPA filter every 1-2 years or when unit has been shut off for more than 3 days. Continuous air flow is required through HEPA filters or moisture can accumulate in which bacteria can grow. Use for XJ-3000C, XJ-3000D, XJ-3000E and Million-Aire XJ-3000F units (not included). Note: Filters are not returnable unless deemed defective after inspection by the manufacturer. Please be careful to order the correct size filter for your air purifier.