Do you hate every dust mask on the market? Me too. Most of them are more trouble than they're worth. The cheap throwaways never fit your face well enough, and if you just need one for a trip up to the attic or something it might not make sense to go buy an expensive respirator.
Unless you're really into steamed up glasses and breathing in the dust coming in around the damn thing anyway, try making one of your own. If it's going to be for a one time use, you can throw it away or recycle it when you're done. Or you can keep it, as it's also useful in some other ways. I use mine to wrap up my chisel when I carry it in my toolbag. It's handy for a rag, too.
You can just tie a bandanna around your face, but it only ties in one place and can slip off. Cutting a tee shirt in the way I'm going to show allows you to tie it in two places. It's very comfortable.
Step 1: Cut the Bottom
Lay the shirt out flat. Cut about a handspan's width out of the bottom. When you lay your hand down, you don't need to spread your fingers out - just place it there. Your handspan will cover your face holes. Put your hand up to your face right now and see! That's how wide the fabric should be.
After you cut off the bottom, make one cut to free the ends. Put the two ends together and fold the long piece in half to make the rest of the cuts.
Step 2: Cut Out the Ties
Measure out three handspans. The first represents the part that covers your face and the other two represent the length of the ties. If your piece is more than three handspans long, you can cut that last bit off or leave it for longer ties.
Cut out the center, leaving a big fat thumb's width on the upper and lower edges for the ties. I cut these two sections one handspan at a time because I use these scraps, as you'll see.
Step 3: Tie It On
Grab two of the ties, pull the mask across the bridge of your nose and tie them around the back of your neck. Grab the other two ties (pulling the lower part of the mask under your chin) and tie them up on top of your head.
Step 4: Bonus Protection!
Don't throw those scraps away - you can use them as extra layers to maximize filtration! Stuff them in the gaps beside your nose, or sew them in.
Step 5: Go Fearlessly Forward to Some Probably Horrible Task