[stevenoble]

Unless you are using the 2K clear with a full size spray gun on an industrial level everyday you don't need an air fed mask. Those full size guns really do emit some fumes, it's like a fog. I used to work in the car spraying industry and we did use an air fed mask there when using the 2K and it really was essential. Using it through an airbrush produces much less spray mist. If you have a proper, purpose made extractor and the relevant 3M spray mask, you can use it quite safely at home. I stress though that you do need the extractor and the proper mask. I would never spray it without those. It's not a case of spray it and you suffer irreparable damage to your lungs at the first usage. The symptoms build over time and end up much like asthma sufferers feel.
To answer the original question Zero 2K seems to be the urethane clear of choice for many, myself included...