In the most general sense, a respirator mask is a device that prevents the user inhaling dangerous gas, fumes and vapors. It also provides protection from inhalation of dust and airborne biological pathogens. By inhalation, we mean any means of entrance into the body via either the nose or mouth. Some respirators also protect against invasion of foreign matter via the eyes.
However, within this broad definition, there are many subcategories. We set out below a high level description of the taxonomy of respirator masks:
The masks all require a mechanism to ensure a hermetic seal against the face. This avoids leakage of noxious fumes or particles through the seal between the masks and face. Some respirators achieve this via earloops while others use headstraps;
Respirators may filter the hazardous atmosphere to provide a breathable air supply to the user. Alternatively, they may provide a separate air supply;
Some respirator masks only cover the lower half of the face. These are not intended to protect the eyes or the upper half of the face. Others provide full face coverage;
Some respirators have highly efficient filtering mechanisms that protect the wearer from both gases/vapors and particulate matter. Others will provide protection only from particulate matter.
The respirator masks in this section are all half face, air filtering, particulate respirators. They protect against non oil particulate matter of size roughly 0.3 microns and larger. For reference, 0.3 microns is roughly the size of a grain of salt. Specifically, these respirator masks do not protect against noxious gases, vapors or fumes.