Introduction: 3D Printed Activated Carbon P100 3M Respirator Filter

This is a reusable P100 filter with allowances for activated carbon inserts. The activated carbon is intended to reduce exposure to volatile organic compounds, bacteria, and viruses. It is intended to reduce the risk of exposure to the Novel Coronavirus (COVID-19). The filter fits 3M 6000 series and 7000 series respirators with bayonet connections. If you want a full respirator design that uses this filter, visit:

I call it the NORA filter, and would like remixes of it to carry the word NORA in some way. Novel-coronavirus, ORganic-compound and Asbestos filter.

Please feel free to adapt this design to fit other respirators, but keep me informed.


A single filter is comprised of these parts:

  1. 3D printed parts from
    • 1X Base part
    • 3X Ring part
    • 1X Top part
  2. HEPA filter material, one 40mm by 40mm square
  3. Activated carbon filter material, three 40mm by 40mm squares
  4. 6 pieces M3-0.5 x 30mm bolts (Hex socket head HIGHLY recommended)
  5. 6 pieces M3 nuts

The P100 material (HEPA, 0.3 micron) and activated carbon sheets are easily obtained from Amazon, often in all-in-one packages for air purifier filter replacements.

My Amazon purchases:

"GHM True HEPA Filter Plus 4 Carbon Replacement Pre-Filters A Winix 115115 Size 21 Compatible with Winix Air Purifier WAC5300 WAC5500 WAC6300 5300 5500".

"M3-0.5x40mm Socket Head Cap Bolts Screws, 304 Stainless Steel 18-8, Allen Socket Drive, Fully Machine Thread, Bright Finish, 50 pcs by Eastlo Fastener"

"uxcell a16033100ux0563 M3 Carbon Steel Grade 8 Hexagon Hex Nut Black Carbon Steel (Pack of 100)"

Now, on to the next part.

Step 1: Harvest HEPA Filter Material

My HEPA filter and activated carbon material were made to fit a Winix air purifier (PIC1). Most HEPA filters are box-like structures consisting of filter material folded accordion-style, held together by wax, and supported on four sides by cardboard.

  1. With a utility knife, cut away the cardboard sides (PIC2).
  2. Cut closely along the wax strips, to obtain the widest possible strips of HEPA material (PIC3). Discard the wax strips (PIC4).
  3. There is a LOT of HEPA material in one air purifier filter. Bag it up for ease of handling (PIC5).
  4. The activated carbon sheets come ready to use (PIC6).

Now, on to the next part.

Step 2: Prepare Filter Material and Base Part

The stack-up allows 4 stages of filter membranes. I recommend 1X P100/HEPA and 3X activated carbon if you are focusing on protection against COVID-19.

  1. Cut out a 40mm by 40mm square of HEPA material (PIC1).
  2. Cut out three 40mm by 40mm squares of activated carbon material. Cut off the corners just enough such that they become octagonal (PIC2). You can discard the corners, or use the to stuff the voids between filter stages for additional adsorption.
  3. Pull the nuts into their seats on the BASE part using the bolts (PIC3). This will seat them and make it easier for assembly.
  4. Examine the base part (PIC4). Nuts should all fit. The mating surface of the connector should be smooth.

The bolt holes in the base may exhibit "stringing" from the print process; do not worry. When you seat the nuts, the stringy plastic should be pushed out of the way for the most part.

Now, on to the next part.

Step 3: Assembly

The filter stages are sealed together by the clamping force exerted by the fasteners (nuts & bolts). The plastic is flexible enough to distort slightly, such that a good seal can be achieved without breakage. It is important to tighten the bolts the same way one tightens lug nuts/bolts: get them all lightly screwed down, before tightening them one-by-one in opposite pairs.

The fuzzy side of the HEPA material should face the outside (you will understand upon handling it). The HEPA filter(s) should be closest to the bayonet connector, or you may be exposed to particles from the activated carbon material. I recommend 1X P100 and 3X activated carbon if you are focusing on protection against COVID-19.

  1. Thread two bolts through the TOP part (thickest ring-like part) and center the activated carbon sheet over the hole (PIC1).
  2. Add a RING part to the stack-up, and place the next activated carbon sheet over that hole (PIC2).
  3. Do the same with the next two RING parts (PIC3). The filter at the very top (closest to the BASE) should be HEPA/P100.
  4. Put the BASE on the stack-up and carefully drive the bolts through the nuts with a hex wrench. Having seated the nuts earlier, this should not be too difficult. Try not to shake the filter elements out of place.
  5. Tighten everything down the way one tightens lug nuts/bolts. There should be little to no gap between stages (PIC4). Check that all the nuts are seated properly (PIC5).
  6. Duct tape can be used to seal around the cylinder (PIC6). Cut off any excess

Now, on to the next part.

Step 4: Final Test

There is a decorative shroud you can use to cover the assembly (PIC1 & PIC2). This should only be done when the filter is screwed onto the respirator.

If you made only one NORA filter, the other filter port on your respirator should be CAPPED.

  1. Wear the respirator with NORA filter attached. At this stage, don't use the decorative shroud.
  2. Check for a good seal by covering the inlets with your hands/putty/something. This should cut off airflow and cause the onset of suffocation - a sure sign that the assembly is airtight.
  3. Remove your hands/putty/something from the inlets.
  4. You can attach the decorative shroud now.

The filter should attenuate smells significantly. This is a sign that volatile organic compounds are being bound to the activated carbon through the adsorption process. Bacteria, mold spores, and viruses are reduced through the same mechanism. Like any other filter, it will not completely eliminate the risk of contracting COVID-19, but it'll offer far greater protection than a paper/cloth N95 mask. Remember, your eyes are also a virus ingress point - so wear sealed goggles.

Feedback is appreciated. Please adapt this to fit other respirators if possible, and keep me posted.