I actually do have a high quality respirator, and it's useful for strong but short lasting fumes.
In this cases the urethane doesn't have much fumes, but it's like a 4-5 day process of putting 2 coats on per day, so it adds up. I can't very well wear a respirator for 4 days.
Normally I would open some windows and put some fans and get some good air flow going.. but this summer it's been like 80% humidity constantly, so I'm really reluctant to do that as I fear it will interfere with the drying.