What is a respirator?

NIOSH - NPPTL - Respirator Trusted-Source Information Page: "A respirator is a personal protective device that is worn on the face, covers at least the nose and mouth, and is used to reduce the wearer’s risk of inhaling hazardous airborne particles (including dust particles and infectious agents), gases, or vapors. Respirators should only be used as a 'last line of defense' in the Hierarchy of Controls when engineering and administrative controls are not feasible or are being put in place.
Respirators protect the user in two basic ways. The first is by the removal of contaminants from the air, called air-purifying respirators (APR), APRs include particulate respirators, which filter out airborne particles, and “gas masks,” which filter out chemicals and gasses. Other respirators protect by supplying clean respirable air from another source. Air-supplying Respirators (ASR) comprising this category of respirators includes airline respirators, which use compressed air from a remote source; and self-contained breathing apparatus (SCBA), which include their own air supply."