Mechanical filter respirator


Mechanical filter respirators are a class of respirator that removes particulates from the air using a filter that mechanically stops them from reaching the wearer's nose and mouth.

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:
  1. by interception when particles following a line of flow in the airstream come within one radius of a fiber and adhere to it;
  2. 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
  3. 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
  4. 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 filter surface;
  5. by using gravity and allowing particles to settle into the filter material ; and
  6. 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 0.3 μm most penetrating particle size, 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.
A substantial advance in mechanical filter technology was the HEPA filter. A HEPA filter can remove as much as 99.97% of all airborne particulates with aerodynamic diameter of 0.3 μm, particles both smaller and larger are removed with an efficiency >99.97%.
operations. Even "clean" industrial processes often generate large amounts of harmful particulate matter and require breathing protection.

Physical form

Filtering facepiece respirators

Disposable filtering facepiece respirators are discarded when they become unsuitable for further use due to considerations of hygiene, excessive resistance, or physical damage.

Elastomeric respirators

Elastomeric respirators are reusable devices with exchangeable cartridge filters that offer comparable protection to N95 masks. The filters must be replaced when soiled, contaminated, or clogged.
They may have exhalation valves. Full-face versions of elastomeric respirators seal better and protect the eyes. Fitting and inspection is essential to effectiveness.

Powered air-purifying respirators (PAPRs)

are masks with a electricity-powered blower that blows air through a filter to the wearer. Because they create positive pressure, they need not be tightly-fitted. PAPRs typically do not filter exhaust from the wearer.

Filtration standards

U.S. standards (N95 and others)

In the United States, the National Institute for Occupational Safety and Health defines the following categories of particulate filters according to their NIOSH air filtration rating as of 2011:
Oil resistanceRatingDescription
Not oil resistantN95Filters at least 95% of airborne particles
Not oil resistantN99Filters at least 99% of airborne particles
Not oil resistantN100Filters at least 99.97% of airborne particles
Oil resistantR95Filters at least 95% of airborne particles
Oil resistantR99Filters at least 99% of airborne particles
Oil resistantR100Filters at least 99.97% of airborne particles
Oil proofP95Filters at least 95% of airborne particles
Oil proofP99Filters at least 99% of airborne particles
Oil proofP100Filters at least 99.97% of airborne particles

European standards (FFP2 and others)

EN 143 defines the 'P' classes of particle filters that can be attached to a face mask, and European standard EN 149 defines the following classes of "filtering half masks" or "filtering face pieces", that is respirators that are entirely or substantially constructed of filtering material:
ClassFilter TypeFilter penetration limit Inward leakageTypical Elastic Band
FFP1MaskFilters at least 80% of airborne particles<22%Yellow
FFP2MaskFilters at least 94% of airborne particles<8%Blue or White
FFP3MaskFilters at least 99% of airborne particles<2%Red
P1AttachmentFilters at least 80% of airborne particlesN/AN/A
P2AttachmentFilters at least 94% of airborne particlesN/AN/A
P3AttachmentFilters at least 99.95% of airborne particlesN/AN/A

Both European standard EN 143 and EN 149 test filter penetration with dry sodium chloride and paraffin oil aerosols after storing the filters at 70 °C and −30 °C for 24 h each. The standards include testing mechanical strength, breathing resistance and clogging. EN 149 tests the inward leakage between the mask and face, where 10 human subjects perform 5 exercises each. The truncated mean of average leakage from 8 individuals must not exceed the aforementioned values.

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 ":
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: