Sintra doesn't off gas alltogther that much. I've now been involved with heating and bending four sets of armor with a heat gun, and until you see discoloration of the surface there is no apparent offgassing. The good thing, if you can call it that, is that the fumes put off by either left over vinyl chloride monomer or by the breakdown of the polymer into chlorine gas are very pungent - a little will be noticeable.
The necessary surface temp to bend the stuff is anything above about 150F. If you were patient enough, you could set your oven to 'warm' which is usually 180F, or even just to 225F, and in all likelihood you wouldn't be at any more risk than the boiling water method.
I do agree though that the heat gun is the best option, because you can take it to a well ventilated place. Boiling water scares me, because it can spill and will burn very large areas of your body very easily.
Care must be taken no matter what method is used.
-Brian
