Safety glasses are not regular glasses and your everyday glasses should not be a replacement for safety glasses. Proper eye protection is important.
One major difference is the lens material. Safety glasses are made with high impact resistant material, the same material is also used for aircraft windshields. Your eyes are protected as the chances of the lenses shattering on impact is slim to none; these lenses are tested to withstand high impact projectiles that regular glass or plastic lenses would not.
Safety glasses are a complete unit, lenses fitted to frame which have passed rigorous testing. Safety frames give wraparound coverage. Modern safety glasses are also very comfortable, fine to be worn all day.
Wearing your regular prescription glasses under safety glasses do not allow safety glasses to sit where they are supposed to or, in some cases, to seal appropriately to your face.
If you need prescription lenses, and you wear safety glasses on a regular basis, the best option for you is prescription safety glasses, ask us about protecting your eyesight.