We live in a world of labels. We trust some labels to be precise: We expect a 250 mg capsule to deliver 250 milligrams of medication, and our 2-liter soft drink bottle undoubtedly holds two liters.

Many users in a group may obtain values close to the labeled NRR, but a substantial portion typically do not. The only way to determine a person's attenuation level is with a fit test system. Editor's ...