Stuart,

I am replying on the list because of the seriousness of the isocyanate
issue.

You said: "an OSHA approved organic vapor mask will absorb the isocyanates.
 Only a pro working with this stuff every day in a production environment
needs the forced air system."

You are right about the organic vapor mask, but the problem is that you
cannot tell if the canister is working since iso's are odorless.  The
canisters have a short life after they are opened and a finite shelf life
if unopened.  The pressure in such a mask is negative when you inhale, so a
poor seal to the face lets the iso's in and you can't smell them.  (I have
a beard.)  The positive pressure fresh air system keeps you healthy and is
foolproof.

Mike