H Class Vacuums otherwise known as Dust Class "H" (High) dust extractors, tackle high hazardous dust, filtering out 99.997% of all dust that is extracted. The air that passes through this system is cleared of the finest dust particles. If you are dealing with MDF or asbestos dust, which is particularly harmful due to its fine nature, then this is the extractor class you will need.
The HEPA (High-Efficiency Particulate Air) filters you find on H Class vacuum extractors are considerably more effective than the filters on L and M Class extractors and collect 99.997% of the dust passing through them.