Apparently Crystal deodorant contains Potassium Alum or Ammonium Alum, Alzheimer anyone?

There is no credible scientific studies that establish a causal link between aluminium and Alzheimer's.

But, if you subscribe to this theory you better stop using toothpaste too. It is a source of aluminium.

As far as I know, crystal deo doesn't quite "contain" alum salt as much as "consist of pure alum salt".

Alum salt has a long history in men's cosmetics, most of our grandfathers used alum sticks to treat cuts after shaving... While metallic aluminum is toxic, I do not know of any scientific study about the harmful effects of alum salt.

That said, I personally do not use alum, instead go for a combination of talc and perfume for deodoration and perspiration control (something I adopted due to a skin rash, and stuck with it because it beats most off-the-shelf deodorants).