This filters are for use with the Misonix Downflow Workstation part #1-4820. Hydrocarbon filters should be used if chemicals or solvents are to be used in the unit. If powders and particules are being used, a HEPA filter part #1-4822 should be used instead. The Particulate Pre-Filters should be replaced any time a main filter HEPA or Hydrocarbon is being replaced, or in the event of low air flow through the unit. Pre-Filters come in a pack of 10.