The NIOSH respirator protection approval regulation (42 CFR 84) defines the term "N95" to refer to a filter class, not a respirator. However, many filtering facepiece respirators have an N95 class filter and many people refer to them, and have come to know them, as N95 respirators. A filtering facepiece respirator that filters out at least 95% of airborne particles during "worst case" testing using a "most-penetrating" sized particle is given a 95 rating.