100% silicone is safe after fully cured so long as there are no mildew-resistant chemicals in it. Styrofoam is okay, but it does have formaldehyde in it. Most pre-made styrofoam backgrounds are coated to keep them safe. I coat all styrofoam in 100% silicone and allow to dry for at least a week. There should be no smell to it once cured and it will feel like rubber.

I use GE silicone II ...