Respirators

See also: N95 masks can be effectively sterilized and reused and Using a hepa filter to make a DIY protective mask (better than N95!)
A **respirator** is a device designed to protect the wearer from inhaling hazardous atmospheres, including particulate matter such as dusts and airborne microorganisms, as well as hazardous fumes, vapours and gases.

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 *impaction*, when particles larger than the maximum distance between fibers are forced to embed in the fibers directly; this increases with diminishing fiber separation and higher air flow velocity
2. by *interception* when particles following a line of flow in the airstream come within one radius of a fiber and adhere to it;
3. by *diffusion* when the Brownian motion of the smallest particles, especially those below 0.1 μm in diameter, raises the probability that a particle will be stopped by interception; this mechanism becomes dominant at lower air flow;
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 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 micrometres or greater.

### NIOSH

In the United States, the National Institute for Occupational Safety and Health defines the following categories of particulate filters as of 2011:

The first part of the filter's classification uses the letters N, R, or P to indicate the filter's ability to function when exposed to petroleum.

* "N" = not resistant to petroleum
* "R" = somewhat resistant to petroleum
* "P" = strongly resistant to petroleum

The second part lists the percentage of particles that the mask is certified to block (such as 95, 97 or 99 percent).

The most common is N95, which is recommended by the US Centers for Disease Control and Prevention (CDC) for most cases of air contamination. These filters are designed to seal tightly around mouth and nose and are made of material certified to block 95% of particles 0.3 μm or larger in diameter, roughly the size of a single virus and include PM2.5.

They are however "... relatively difficult to breath through ...", and their full effectiveness also depends on a good fit – NIOSH recommends that each respirator wearer receive "... an initial fit test and annual fit tests thereafter".

### EN 143

European standard EN 143 defines the following classes of particle filters that can be attached to a face mask:

 P2
 P3

### EN 149

European standard EN 149 defines the following classes of “filtering half masks” (also called “filtering face pieces”), that is respirators that are entirely or substantially constructed of filtering material:

 FFP2
 FFP3

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 ten human subjects perform 5 exercises each and for 8 individuals the average measured inward leakage must not exceed 22%, 8% and 2% respectively, as listed above.

**NIOSH air filtration rating** refers to the publications of National Institute for Occupational Safety and Health (NIOSH) of the US government pertaining to respirators and masks worn to filter contaminated air, regardless of cause.

### Contrast with plain surgical mask

 **Surgical masks**, also known as a **procedure masks**, are intended to be worn by health professionals during surgery and during nursing to catch the bacteria shed in liquid droplets and aerosols from the wearer's mouth and nose. They  are not designed to protect the wearer from inhaling airborne bacteria or virus particles and are less effective than respirators, such as N95 or NIOSH masks, which provide better protection due to their material, shape and tight seal.

Plain surgical masks are standard for staff in hospital operating rooms, and often recommended to the public as part of avoiding seasonal flu.
They do not carry a NIOSH rating. They are designed to filter out relatively large particles, such as sputum droplets and hair.

The World Health Organization, Society for Healthcare Epidemiology of America, the Infectious Diseases Society of America, the Association for Professionals in Infection Control and Epidemiology, and the American College of Occupational and Environmental Medicine all recommend these except in cases of "high risk".

## HEPA

**High-efficiency particulate air** (**HEPA**), also known as **high-efficiency particulate absorbing** and **high-efficiency particulate arrestance**, is an efficiency standard of air filter.

Filters meeting the HEPA standard must satisfy certain levels of efficiency. Common standards require that a HEPA air filter must remove—from the air that passes through—at least 99.95% (European Standard) or 99.97% (ASME, U.S. DOE) of particles whose diameter is greater than or equal to 0.3 μm. See the Specifications section for more information.

HEPA was commercialized in the 1950s, and the original term became a registered trademark and later a generic term for highly efficient filters. HEPA filters are used in applications that require contamination control, such as the manufacturing of disk drives, medical devices, semiconductors, nuclear, food and pharmaceutical products, as well as in hospitals, homes and vehicles.

### Mechanism

HEPA filters are composed of a mat of randomly arranged fibres. The fibers are typically composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers. Key factors affecting its functions are fiber diameter, filter thickness, and face velocity. The air space between HEPA filter fibers is typically much greater than 0.3 μm.

The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. HEPA filters are designed to target much smaller pollutants and particles. These particles are trapped because they stick to a fiber through a combination of the following mechanisms:

* Interception: Particles following a line of flow in the air stream come within one radius of a fiber and adhere to it.

* Diffusion: An enhancing mechanism that is a result of the Brownian motion of the smallest particles, especially those below 0.1 μm in diameter, which raises the probability that a particle will be stopped by interception; this mechanism becomes dominant at lower air flow

Of course particles larger than the maximum distance between fibers will become embedded in them. This is called *impaction*. This effect increases with diminishing fiber separation and higher air flow velocity.

Diffusion predominates below the 0.1 μm diameter particle size, whilst impaction predominates above 0.4 μm. In between, near the most penetrating particle size (MPPS) 0.21 μm, both diffusion and interception are comparatively inefficient. Because this is the weakest point in the filter's performance, the HEPA specifications use the retention of particles near this size (0.3 μm) to classify the filter.

### Gas filtration

HEPA filters are designed to arrest very fine particles effectively, but they do not filter out gasses and odor molecules. Circumstances requiring filtration of volatile organic compounds, chemical vapors, cigarette, pet, and/or flatulence odors call for the use of an activated carbon (charcoal) or other type of filter instead of or in addition to a HEPA filter. Carbon cloth filters, claimed to be many times more efficient than the granular activated carbon form at adsorption of gaseous pollutants, are known as **HEGA** filters ("**H**igh **E**fficiency **G**as **A**dsorption") and were originally developed by the British military as a defense against chemical warfare.

### Pre-filter and HEPA Filter

A HEPA filter can be used in conjunction with a pre-filter (usually carbon-activated) to extend the usage life of the more expensive HEPA filter. In such setup, the first stage in the filtration process is made up of a pre-filter which removes most of the larger dust, hair, PM10 and pollen particles from the air. The second stage high-quality HEPA filter, which filters out the finer particles that escapes from the pre-filter.

### Specifications

HEPA filters, as defined by the United States Department of Energy (DOE) standard adopted by most American industries, remove at least 99.97% of airborne particles 0.3 micrometers (μm) in diameter. The filter's minimal resistance to airflow, or pressure drop, is usually specified around 300 pascals (0.044 psi) at its nominal volumetric flow rate.

The specification used in the European Union: European Standard EN 1822-1:2009, defines several classes of HEPA filters by their retention at the given most penetrating particle size (MPPS):

See also the different classes for air filters for comparison.

Today, a HEPA filter rating is applicable to any highly efficient air filter that can attain the same filter efficiency performance standards as a minimum and is equivalent to the more recent National Institute for Occupational Safety and Health N100 rating for respirator filters. The United States Department of Energy (DOE) has specific requirements for HEPA filters in DOE-regulated applications.

### Marketing

Some companies use a marketing term known as "True HEPA" to give consumers assurance that their air filters meet the HEPA standard, although this term has no legal or scientific meaning. Products that are marketed to be "HEPA-type," "HEPA-like," "HEPA-style" or "99% HEPA" do not satisfy the HEPA standard and may not have been tested in independent laboratories. Although such filters may come reasonably close to HEPA standards, others fall significantly short.