Mechanical filter (respirator). Mechanical filters, a part of particulate respirators, 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

More obscure mechanisms include:
 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)

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.

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.

Uses

Filtering facepiece respirators 
Filtering facepiece respirators consist mainly of the mechanical filtration medium itself, and are discarded when they become unusable due to damage, dirt, or excessive breathing resistance. Filtering facepieces are typically simple, light, single-piece, half-face masks and employ the first three mechanical filter mechanisms in the list above to remove particulates from the air stream.  The most common of these is the white, disposable standard N95 variety; another type is the Surgical N95 mask. It is discarded after single use or some extended period depending on the contaminant. NIOSH recommends not reusing filtering facepieces in biosafety level 2 or 3 laboratories.

Elastomeric respirators

Powered air-purifying respirators (PAPRs)

Filtration standards

U.S. standards (N95 and others)

European standards (FFP2 and others)

Other standards (KN95 and others) 

Respirator standards around the world loosely fall into the two camps of US- and EU-like grades. According to 3M, respirators made according to the following standards are equivalent to US N95 or European FFP2 respirators "for filtering non-oil-based particles such as those resulting from wildfires, PM 2.5 air pollution, volcanic eruptions, or bioaerosols (e.g. viruses)":
 Chinese KN95 (GB2626-2006): similar to US. Has category KN (non-oily particles) and KP (oily particles), 90/95/100 versions. EU-style leakage requirements. In China, KN95 respirators are made by companies such as Guangzhou Harley, Guangzhou Powecom, Shanghai Dasheng and FLTR.
 Korean 1st Class (KMOEL - 2017–64), also referred to as "KF94": EU grades, KF 80/94/99 for second/first/special. In Korea, KF94 respirators are made by companies such as LG, Soomlab, Airqueen, Kleannara, Dr. Puri, Bluna and BOTN.

The NPPTL has also published a guideline for using non-NIOSH masks instead of the N95 in the COVID-19 response. The OSHA has a similar document. The following respirator standards are considered similar to N95 in the US:
 Japanese DS2/RS2 (JMHLW-Notification 214, 2018): EU-like grades with two-letter prefix – first letter D/R stands for disposable or replaceable; second letter S/L stands for dry (NaCl) or oily (DOP oil) particles. Japanese DS2 respirators are made by companies such as Hogy Medical, Koken, Shigematsu, Toyo Safety, Trusco, Vilene and Yamamoto Safety.
 Mexican N95 (and others) (NOM-116-2009): same grades as in NIOSH.
 Brazilian PFF2 (ABNT/NBR 13698:2011): EU-like grades.

Disinfection and reuse
Hard filtering facepiece respirator masks are generally designed to be disposable, for 8 hours of continuous or intermittent use. One laboratory found that there was a decrease in fit quality after five consecutive donnings. Once they are physically too clogged to breathe through, they must be replaced.

Hard filtering facepiece respirator masks are sometimes reused, especially during pandemics, when there are shortages. Infectious particles could survive on the masks for up to 24 hours after the end of use, according to studies using models of SARS-CoV-2; In the COVID-19 pandemic, the US CDC recommended that if masks run short, each health care worker should be issued with five masks, one to be used per day, such that each mask spends at least five days stored in a paper bag between each use. If there are not enough masks to do this, they recommend sterilizing the masks between uses. Some hospitals have been stockpiling used masks as a precaution. The US CDC issued guidelines on stretching N95 supplies, recommending extended use over re-use. They highlighted the risk of infection from touching the contaminated outer surface of the mask, which even professionals frequently unintentionally do, and recommended washing hands every time before touching the mask. To reduce mask surface contamination, they recommended face shields, and asking patients to wear masks too ("source masking").

Apart from time, other methods of disinfection have been tested. Physical damage to the masks has been observed when microwaving them, microwaving them in a steam bag, letting them sit in moist heat, and hitting them with excessively high doses of ultraviolet germicidal irradiation (UVGI). Chlorine-based methods, such as chlorine bleach, may cause residual smell, offgassing of chlorine when the mask becomes moist, and in one study, physical breakdown of the nosepads, causing increased leakage. Fit and comfort do not seem to be harmed by UVGI, moist heat incubation, and microwave-generated steam.

Some methods may not visibly damage the mask, but they ruin the mask's ability to filter. This has been seen in attempts to sterilize by soaking in soap and water, heating dry to , and treating with 70% isopropyl alcohol, and hydrogen peroxide gas plasma (made under a vacuum with radio waves). The static electrical charge on the microfibers is destroyed by some cleaning methods. UVGI (ultraviolet light), boiling water vapour, and dry oven heating do not seem to reduce the filter efficiency, and these methods successfully decontaminate masks.

UVGI (an ultraviolet method), ethylene oxide, dry oven heating and vaporized hydrogen peroxide are currently the most-favoured methods in use in hospitals, but none have been properly tested. Where enough masks are available, cycling them and reusing a mask only after letting it sit unused for five days is preferred.

It has been shown that masks can also be sterilized by ionizing radiation. Gamma radiation and high energy electrons penetrate deeply into the material and can be used to sterilize large batches of masks within a short time period. The masks can be sterilized up to two times but have to be recharged after every sterilization as the surface charge is lost upon radiation.

A recent development is a composite fabric that can deactivate both biological and chemical threats.

References 

Respirators
1956 introductions