The HEPA (High Efficiency Particulate Air) filter can be installed in addition to the standard filter to improve the filtration. By adding this filter you will get a triple filtration: 1. Fleece dust bag 2. Standard main filter 3. HEPA filter. Please note that adding this filter will not make a dust extractor DE 1230 an H-class extractor, even though the filter itself fulfills HEPA class H13 requirements. When installing a HEPA-filter for the first time, the HEPA Filter Cover for DE 1230 is necessary.