On Wednesday, February 17, 2016 at 6:22:51 PM UTC-5, Andrzej Kobus wrote:

Jim, the release of isocyanates is caused by heat from sanding. The higher the heat the more isocyanates will be released. The amounts we are talking about are not large but almost all of us have been exposed to isocyanates over our lifetime and the effects of this chemical are cumulative so it is wise to avoid it as much as possible. If you use water you are generating very little heat plus isocyanates bind to water which is a plus in this case.

Thanks for the reference. The study in question considered "workers'
exposure to isocyanates during cutting, grinding and orbital sanding
operations." So it sounds like we're talking about a lot less heat
when we're just color sanding by hand. Still and all, there's no
reason to take chances, and I can't see any real advantage to dry
sanding. We've color sanded one set of (Grob 103) wings painted
with polyurethane, and it was wet sanding all the way.

(I'll admit that I have, on rare occasions, sanded out a run
or two in polyurethane that I caused.)

Jim Beckman