Here's some more good info regarding respirator ratings:
http://www.cdc.gov/niosh/npptl/topics/respirators/factsheets/respfact.html
http://www.lni.wa.gov/wisha/rules/respirators/PDFs/HT3-Resources.pdf
I use a magenta color coded, P-100 respirator for car paint, a substance that I believe is far nastier than polyurethane and I can detect no odor whatsoever in an area full of the vapors.
As for finishing, I did 2 areas of cork flooring. The first one using a lambs wool pad and the second using a 3/16 knap mohair roller. Several years later, the area finished with the lambs wool is still showing streaks and "brush marks" while the area finished with the roller is perfectly smooth.