For many health reasons, painters should wear a supplied-air respirator when spraying paints containing highly toxic chemicals such as isocyanates. (Isocyanates are a family of highly reactive, low molecular weight chemicals.  Isocyanates are powerful irritants to the mucous membranes of the eyes and gastrointestinal and respiratory tracts. Direct skin contact can also cause marked inflammation. Isocyanates can also sensitize workers, making them subject to severe asthma attacks if they are exposed again. Death from severe asthma in some sensitized subjects has been reported.)  In fact, some paint manufacturers say painters must wear supplied-air respirators when spraying isocyanate-containing paints. See more info.