My 3M respirator is comfortable, durable, tight-fitting, and easy to clean. Unfortunately it was designed only to protect the wearer, and so it does not filter exhaled air, only incoming air. This Instructable will show you how to print a filter that fits over the exhaust valve to filter outgoing air with minimal resistance to breathing. I am an essential worker, and this filter as well as a previous version have allowed me to wear my respirator to work for months.

Supplies

3M Half-Mask 7502 Respirator
3M Particulate Filter
12 mm M3 Screw and Nut (If you use another set, the nut must be able to fit in a 5.8 mm slot)
Small Zip Ties (Optional substitute for the above screws and nuts)
Hot Glue Gun
Surgical Mask
Hex Wrench Set

Step 1: Printing

Print the Main Piece.stl with support in the orientation shown in photo 1. If you're having trouble with the supports in that orientation, print it vertically instead. Printing vertically will make the piece weaker, and take longer however, so only do it if you can't get it to work with the pictured orientation.
Print the Cover.stl with support in the orientation shown in photo 2.
Remove the support material from both pieces.

Step 2: Assembly

Press the main piece into the exhaust valve on the respirator and then slide on the cover to check that they both fit. See photos 1 and 2.
Remove both pieces.
Quickly squeeze hot glue around the base of the circled part of the main piece in photo 3, then press it into the respirator before the glue can cool.
Squeeze more glue along the sides of the main piece to hold it securely to the respirator. See photo 4.
Place the surgical mask over the main piece and then put the cover over both. See photo 5.
Push a screw through one of the three holes in the front of the cover. Put a nut on the other side and thread the screw through it. Don't tighten it fully yet. Repeat for the other two holes on the front. Do the last two screws on the bottom of the cover last. See photos 6-8. Use zip ties instead of screws and nuts if you prefer.
Ensure that the surgical mask is held down tightly by the cover, then tighten all of the screws fully. If you're using zip ties, cut off the excess with scissors after fully tightening them.
Use scissors to trim away the excess from the surgical mask.
Attach the pink 3M particulate filters to the sides of the mask.