Any product, not just SFX materials, if used incorrectly is dangerous.
Some products such as silicones can be used without a respirator (we always advise wearing one no matter what you are doing in the shop) but are messy!
While others such as urethanes, polyesters and epoxies emit dangerous fumes during the curing process or while in liquid form (or while sanding!)
Just because you can't smell the urethane does not mean it is not there so we advise to only use ANY sfx material in a well ventilated area along with the proper PPE equipment such as
- Nitrile or latex gloves
- Dust mask (if sanding)
- Respirator (with organic filter) for casting, molding or sanding
- Eye protection
- Old clothes (don't want to destory your new clothes, do you?)
In addition, it is good practice to keep your work area free from dust by having a workshop vac on hand to clear up any sanding debris from becoming airbone and having a fan blowing any fumes from casting away from your (respirator wearing) face