Mechanical filters are a class of filter for air-purifying respirators that mechanically stops particulates from reaching the wearer's nose and mouth. They come in multiple physical forms.
Mechanism of operation
Mechanical filter respirators retain particulate matter such as dust created during woodworking or metal processing, when contaminated air is passed through the filter material. Wool is still used today as a filter, along with plastic, glass, cellulose, and combinations of two or more of these materials. Since the filters cannot be cleaned and reused and have a limited lifespan, cost and disposability are key factors. Single-use, disposable and replaceable-cartridge models exist.
Mechanical filters remove contaminants from air in the following ways:
by interception when particles following a line of flow in the airstream come within one radius of a fiber and adhere to it; by impaction, when larger particles unable to follow the curving contours of the airstream are forced to embed in one of the fibers directly; this increases with diminishing fiber separation and higher air flow velocity by an enhancing mechanism called diffusion, where gas molecules collide with 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 electret filter material (usually, electrospun plastic fibers) to attract or repel particles with an electrostatic charge, so that they are more likely to collide with the filter surface by using certain coatings on the fibers that kill or deactivate infectious particles colliding with them (such as salt) 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.
Considering only particulates carried on an air stream and a fiber mesh filter, diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the most penetrating particle size of 0.3 μm, diffusion and interception predominate.
For maximum efficiency of particle removal and to decrease resistance to airflow through the filter, particulate filters are designed to keep the velocity of air flow through the filter as low as possible. This is achieved by manipulating the slope and shape of the filter to provide larger surface area.
High-efficiency particulate air (HEPA)filters are all filters meeting certain efficiency standards. A HEPA filter must remove at least 99.97% (US) or 99.95% (EU) of all airborne particulates with aerodynamic diameter of 0.3 μm. Particles both smaller and larger are easier to catch, and thus removed with a higher efficiency. People often assume that particles smaller than 0.3 microns would be more difficult to filter efficiently; however, the physics of Brownian motion at such smaller sizes boosts filter efficiency (see figure).
Materials
Mechanical filters can be made of a fine mesh of synthetic polymer fibers. The fibers are produced by melt blowing. The fibers are charged as they are blown to produce an electret, and then layered to form a nonwoven polypropylene fabric.
Exhalation valves
Some masks have check valves, that let the exhaled air go out unfiltered. The certification grade of the mask (as N95 or FFP2) is about the mask itself and it does not warrant any safety about the air that is expelled by the wearer through the valve. A mask with valve will reduce inwards leakages, thus improving the wearer protection.
Unfiltered-exhalation valves are sometimes found in both filtering facepiece and elastomeric respirators; PAPRs cannot by nature ever filter exhaled air. As a result, these masks are believed to be incapable of source control, which is protecting others against an infection in the wearer's breath. They are not generally designed for healthcare use, . Despite the aforementioned belief, a 2020 research by the NIOSH and CDC shows that an uncovered exhalation valve already provides source control on a level similar to, or even better than, surgical masks.
During the COVID-19 pandemic, masks with unfiltered-exhalation valves did not meet the requirements of some mandatory mask orders. It is possible to seal some unfiltered exhalation valves or to cover it with an additional surgical mask; this might be done where mask shortages make it necessary.