These lenses are made out of polycarbonate, a type of light weight polymer used in certain military helments. It is several times stronger than regular plastic and will stop a BB (but not a .22). I had a patient (10 y.o. boy) who got accidently (?) shot with a BB gun and these type of lenses "saved" his eye. You can have regular prescription glasses made out of this plastic; it is not just for safety glasses.
Wear them whenever you are working in the shop or engaged in potentially hazardous activities. Despite advances in modern medicine and surgery, it is still easier to prevent an accident than to try to surgically repair an eye...