It's not that tough to figure out, if you understand how Isocyanides work. Isocyanides react to moisture. Any polyurethane will be catalyzed with an isocyanide, and the percentage will depend on the particular paint formula....some require a high percentage of isocyanides and some (especially waterbornes) require a low percentage in the catalyst, because of reasons I'll explain next. In a sense, the Isocyanides behave somewhat like electricity, in that they follow the path of least resistance. This means that in a non-waterborne paint, the isocyanides are seeking moisture, and our bodies are made up primarily of water, thus they are mostly attracted to us, and tend to enter through the soft tissue (eyes, nose, ears,etc.). If they are already happily reacting in a waterborne solution, they tend to stay there (remember, path of least resistance). Any catalyzed paint is most dangerous when the catalyst is being mixed in, as the catalyst is a "free radical" at that time, and is seeking moisture to react with. Thus, a waterborne is the safest of all catalyzed polyurethanes.