Here is some info re Isocyanates:
Respiratory Protection
Where the potential exists for overexposure to isocyanates, a NIOSH-approved supplied-air respirator with a full facepiece operated in a pressure-demand or other positive-pressure mode should be used.
For increased protection use in combination with an auxiliary self-contained breathing apparatus operated in a pressure-demand or other positive-pressure mode.
Can I wear an air-purifying, cartridge-style respirator for protection against isocyanates?
Air-purifying respirators can be used with contaminants that have poor warning properties, such as isocyanates. However, you must check with the manufacturer of your respirator for specifics.
For example: 3M and MSA have a respirator change out formula for some isocyanates. If you know the airborne concentration in parts per million (ppm), you can calculate approximately how long a cartridge will last.
The best respirator for spraying Isocyanates is probably MSA supplied air. But spaying outside with fans I'd do an MSA full face with a GMC-H cartridge in it and call it good. Double check that cartridge with MSA as they may have something a little better for Isocyanates since it's been quite awhile since I checked.
