In terms of safety glass construction, polycarbonate has become the gold standard due to its ultra thin and light construction. Just because it is light, however, does not mean that it is not strong and in fact it is the strongest of the three materials used (glass, plastic CR-39, polycarbonate). Polycarbonate lenses offer higher impact resistance, lighter weight, inherent ultraviolet filtering and increased durability as compared to conventional plastic or glass lenses.This material is used in a number of areas including bullet proof glass and astronaut helmet shielding.
While glass stands highest in optical clarity, as a safety glass, it is extremely inefficient due to its propensity toward breakage which, more often than not, results in shattering. Shattered glass poses a number of threats to the eye including blindness. Also, glass offers no inherent UV protection whereas polycarbonate gives complete protection from the sun's harmful rays. Also, glass is less scratch resistant than polycarbonate which, in turn, is less scratch resistant that plastic CR-39.
Plastic CR-39 was created in the mid twentieth century and is still a highly used lenses material due to its scratch resistance and greater optical clarity then polycarbonate. If you were purchasing personal prescription eyewear, this would be the best choice. However, for safety conditions, CR-39 is being phased out much like glass has been since plastic CR-39 also shatters at a certain impact load.
Polycarbonate is the strongest material on the market with an ability to block all UV rays as well as having a high threshold for scratch resistance. All of these attributes will protect your eyes better than glass and plastic CR-39 ever could while alleviating any fears you may possess that your safety glasses could one day shatter.