Ask A Scientist Chemistry Archive

Question:
Why do contacts have to be soaked in a saline solution when not in use?
 bridgette m wolf

Answer:  Soft contact lenses are 70% water.  If you don't soak them, they'll 
dry out and shrivel up.  We use saline solution because (1) it's sterile and 
hostile to bacteria, thereby avoiding infection, and (2) it prevents the 
chemicals in the lens from degrading and dissolving into the solution (there 
are ionic salts, silica salts, in the lense itself; if you soak lenses in pure 
water for a very long time they will start to diffuse into the water).
 
That said, it's okay to use distilled water to soak your lenses
once in awhile but best to use sterile saline solution.
 
-topper