This is a replacement cartridge for the Elegoo USB Air Filter for Elegoo Resin Printers. It is refillable with activated carbon and there is a slot for a HEPA filter pad. The filter pads are for the AirSense 10 CPAP and are cut to length using the filter template included on the second build plate. Available from Amazon here https://a.co/d/g2fHX6J. To assemble, fill the bottom chambers with activated carbon, anything with a minimum diameter greater than 2mm should work, the carbon I have has a diameter of 4mm. Insert one HEPA filter pad into the filter template and cut it using a utility knife using the outer edge of the upper frame as a guide. The shortened filter pad is inserted into the channels on the sides of the top filter chamber, soft side up, and inserted until it slots into the rear channel. You then slide the cover from the top side of the filter cartridge until it nests into the bottom of the cartridge sealing off each filter chamber. Make sure the thumbnail recess is on the outside of the cover with the point facing towards the bottom for ease of opening later.