I haven't used 100% silica powder myself but I know a lot of people have and that a lof of people have been afraid to try after seeing a lot of users on YouTube and Specktra talking about how it can be dangerous and can cause silicosis.
At first I thought that maybe it really was dangerous but I found it weird that a huge company like Makeup Forever would sell it if there really was a health risk.
Silica has been used in all sorts of makeup for a long time and of course there's a difference between it being an ingredient in a liquid foundation and it beingn a pure powder that you can actually inhale but it still seemed weird to me that it could be that dangerous.
Now, I haven't studied the difference of silica crystalline and microspheres myself but I found this blog entry and I think it's super interesting and I'd really like you all to read it if you haven't already! :)