A respirator is a device designed to protect the wearer from inhaling harmful dusts, fumes, vapors, or gases. Respirators come in a wide range of types and sizes used by the military, private industry, and the public. Respirators range from cheaper, single-use, disposable masks to reusable models with replaceable cartridges.
There are two main categories: the air-purifying respirator, which forces contaminated air through a filtering element, and the air-supplied respirator, in which an alternate supply of fresh air is delivered. Within each category, different techniques are employed to reduce or eliminate noxious airborne contents.
All respirators function by forming a seal on the users face with the respirator itself. This is essential, as respirators are designed to come in contact with all air flowing through them, which is then delivered to the user. Fit testing uses simple equipment that places the users face and head into a hood, into which flavored mist is sprayed. Usually either bitter or sweet in flavor, with bitter flavors being favored for the almost involuntary reaction of users. Following a standard procedure, the user then breaths through the chosen respirator, and indicates whether they can detect the mist. If they cannot, the filter has passed the basic requirement of a face seal and the user also understands how to fit the mask.
Mechanical filters remove contaminants from air in the following ways:
- by particles which are following a line of flow in the airstream coming within one radius of a fiber and adhering to it, called interception;
- by larger particles unable to follow the curving contours of the airstream being forced to embed in one of the fibers directly, called impaction; this increases with diminishing fiber separation and higher air flow velocity
- by an enhancing mechanism called diffusion, which is a result of the collision with gas molecules by the smallest particles, especially those below 100 nm in diameter, which are thereby impeded and delayed in their path through the filter; this effect is similar to Brownian motion and increases the probability that particles will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities
- by using certain resins, waxes, and plastics as coatings on the filter material to attract particles with an electrostatic charge that holds them on the surface of the filter material;
- by using gravity and allowing particles to settle into the filter material (this effect is typically negligible); and
by using the particles themselves, after the filter has been used, to act as a filter medium for other particles