David Walser mentioned the ANSI Z87.1 standard which governs the various levels of impact resistance. Since the standard is periodically revised, there will also be a year shown following the ANSI Z87 number and applicable suffixes such as the plus sign. Up until recently, the latest revision year shown was 2003, but the 2010 revision is now the latest. Some of the changes to the most recent version have to do with harmonizing the various standards around the world, but mainly it now focuses on specific hazards so that industry is better able to relate specific hazards against the ratings. Previous versions were somewhat abstract in interpreting how various tests related to real-world situations. The EU standards have been somewhat better than our standards in that regard for a number of years. Here is a link to a brief FAQ sheet about changes in the Z87.1 -2010:
MSA FAQ Sheet
I have a 3M Airstream PAPR that I have used for around 6 years as well as several different simple face shields. I have just recently started to do a bit of research on the subject and I am becoming increasingly aware of what levels of protection these various pieces of safety gear are able to provide. The bottom line based on my gut feeling is that woodturners may be placing too much reliance on face shields to provide impact protection as a substitute for insuring that what they are doing is safe.
Primarily, face shields provide eye and face protection against various things like small projectiles, splashes, and dust. A faceshield or helmet is not capable of protecting a woodturner from anything that is large and heavy. The reason is fairly simple -- they are not energy absorbing devices -- unlike safety features in an automobile, there is no crumple zone to dissipate the energy in the structure nor things like air bags to distribute the energy over a large area. Instead, most of the kinetic energy from an impact can be transferred to the wearer's head if the impact is straight-on. At best, a glancing blow may result in a less serious injury.
This doesn't mean that face shields are not worthwhile. Protecting our face and eyes from smaller objects, chips, bark pieces, and dust are extremely important. It just means that we need something else as our primary protection for things where face shields can't help. The "something else" is, of course, safe turning practices that avoid dangerous situations.