The only material that can guarantee fully correct optical precision is glass. All frames apply pressure to keep a lens in place. This pressure causes weaker materials like CR-39 to bend and distort, which for a person who requires corrective eyeglasses can create huge vision problems and headaches. As a result the user could be more likely to experience a workplace injury. Glass provides the sharpest vision of any kind of lens on the market, better than polycarbonate and CR-39 by leaps and bounds.
Most people who choose polycarbonate do so because of the fear of glass prescription safety goggles shattering if dropped. However, workers clean their eye wear far more frequently than they drop it. Polycarbonate lenses are significantly more easy to scratch. So much so that you could take them off for a quick cleaning on the job, only to put them on a few seconds later and not be able to see anything through the scratches. Jobs that require safety glasses are typically dangerous ones, and the risk of losing visibility due to an improper cleaning, could be considered a major safety hazard.
While not recommended glass lenses are less likely to be ruined when an employee ops to use his shirt for a quick clean. With polycarbonate and CR-39 lenses, you should not only be using a microfiber cloth but also a liquid cleaner. It is inevitable that dirt, debris and especially fingerprints will accumulate on eye wear, and with the plastics this can mean a trip to a locker or bathroom for rinsing and drying. A microfiber cloth alone may not scratch, but it will smear and streak, making for cloudy vision. With glass lenses, cleaning a few specs off your glasses is a quick fix, then you are back to working safely. No cloudy smears, no scratches, no long trip to the bathroom and an angry boss.