Thanks AandC,
The industrial chemist I chatted to has spent a working lifetime formulating paint and ink, my gut feeling is that he was speaking from a technically sound viewpoint. He did explain a very technical argument as to why 2 pack acrylic was better than 2 pack polyurathane - unless your spraying a very large lorry!! The toughness and flexibility of 2 pack acrylic was just one of the reasons he mentioned.
My understanding is that single pack polyurathane is a lot softer than 2 pack when cured.
If you want to be a purist there are still some places where you can get cellulose paints.
The point being made was both are a danger to health.