A device designed to protect the wearer from noxious gases, vapors, and aerosols or to supply oxygen or doses of medication to the wearer. Respirators are used widely in industry to protect workers against harmful atmospheres, and in the military to protect personnel against chemical, biological, or radioactive warfare agents. Respirators are classified according to whether they are atmosphere-supplying or air-purifying.

Atmosphere-supplying respirators are used in atmospheres deficient in oxygen or extremely hazardous to the health of the wearer. Such atmospheres can occur in unventilated cellars, wells, mines, burning buildings, and enclosures containing inert gas. The self-contained breathing apparatus (SCBA) is a completely self-contained unit with the air supply or the oxygen-generating material being carried by the wearer. Air-supplied respirators are equipped with the same variety of facepieces as the SCBA, however these respirators can have the air supplied to the facepiece by means of a hose and a blower—the hose mask—or from a compressed-air source equipped with proper airflow and pressure-regulating equipment—the air-line mask.

In an air-purifying respirator, ambient air is passed through a purifying medium to remove the contaminants. However, these devices do not provide oxygen or protect against oxygen-deficient atmospheres. A widely used air-purifying respirator is the nonpowered, or negative-pressure, respirator . Ambient air is inhaled through the purifying medium in the replaceable cartridges and exhaled through an exhaust valve. In the case of the powered air-purifying respirator, an external blower, usually powered by a belt or helmet-mounted battery pack, forces air through the purifying medium and supplies it to the wearer under positive pressure, thus minimizing the problem of face-seal leakage.