they may have variable filter composition, and they may have differing modes of operation (e.g., powered vs. nonpowered). The performance of a respirator or medical mask depends on the efficiency of the filter (how well it is able to collect airborne particles) and fit (how well it prevents leakage around the facepiece).
Media used for the filtration of airborne particles do not work by the same principles as those used for the filtration of liquids. Filters used in respirators and medical masks must allow the user to breathe and thus cannot clog when particles adhere to their fibers. Respirator and medical mask filters are typically composed of mats of nonwoven fibrous materials, such as wool felt, fiberglass paper, or polypropylene (see Box 2-1). The material creates a tortuous path, and various mechanisms result in the adhesion of particles to the fibers without necessarily blocking the open spaces, still allowing air to flow easily across the filter (Revoir and Bien, 1997).
This chapter will discuss three mechanisms of removing particles from the airstream: inertial impaction, diffusion, and electrostatic attraction (see Figure 2-1). Mechanisms for removing large particles differ from those for small particles.
The model postulates that inertial impaction is effective for aerosol particles that are approximately 1 µm and larger. Such particles have enough inertia that they cannot easily flow around the respirator fibers. Instead of flowing through the filter material, the large particles deviate from the air streamlines and collide with the fibers and may stick to or be caught in them.
For much smaller particles—those that are 0.1 µm and smaller—diffusion is regarded as an effective filtration mechanism. Brownian motion—the process by which the constant motion of oxygen/nitrogen molecules causes collisions between particles—results in a “wandering” pathway. The complex path that is followed by the small particles increases the chance that they will collide with the filter fiber and remain there.
Another efficient method of capturing both large and small particles from the airstream is said to be electrostatic attraction, in which electrically charged fibers or granules are embedded in the filter to attract oppositely charged particles from the airstream. The attraction between the oppositely charged fibers and particles is strong enough to effectively remove the particles from the air. The first electrostatic filters used resins