I cannot stress enough, the importance of a good respirator.
Go to your local Lowe's and pick up the 3M Professional Series Organic Vapor/Pesticide OV/P95 Respirator. It will cost you $40. It will save your life. (Comes with 2 carbon cartridges and 4 HEPA filters)
What you risk, when you buy a cheap respirator, is your life. Working with a sub-par respirator that lets particulate into your airways degrades your lungs, and shortens your life. The effect isn't only on your lungs. Your whole body gets poisoned when Polyurethane particulate absorbs into your blood. You're better off not letting any in at all, by using a more-expensive, but proven respirator.
At the very least, the medical bills you'll face if you get too much in your lungs far exceeds the $30 extra you'll pay on this respirator. Ask Nintendude. He seems to be going through troubles, related to working without a respirator. He may never be able to armor again. Is that something you want?
(Sorry if I've mistaken your issue, Nintendude, and sorry if I offended you by using your name. If you see this and want me to remove it, tell me so, via email or my build thread, or whatever.)