Respiratory Protection for Airborne Exposures to Biohazards from 3M

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Technical Data Bulletin from 3M

Respiratory Protection for Airborne Exposures to Biohazards

Abstract

Diseases can be transmitted via many routes, including inhalation of aerosols. When properly fitted, selected, used, and maintained, particulate-removing respirators have been demonstrated to reduce the amount of aerosols – both bioaerosols and non-biological aerosols – that are inhaled by the wearer. In contrast, most surgical masks are not designed to seal tightly to the face, and research has shown that they do not achieve the level of contaminant reduction provided by a certified respirator that is used correctly. Although questions have been raised as to whether or not biological particles can be filtered as well as non-biological particles, research shows that a respirator filter will remove bioaerosols in a similar manner as it removes non-biological aerosols from air that passes through it.

When respiratory protection is needed to help reduce exposure to bioaerosols, the user should select a certified particulate respirator according to recommendations from CDC, WHO or applicable local agencies. Once a respirator has been selected, a continuing, effective respiratory protection program as specified by applicable local regulations must be implemented. This includes training on the respiratory hazards, fit testing, and proper maintenance and disposal of the respirators.

Introduction

Recently there has been growing interest in the use of respirators to help protect against certain airborne biohazards. Diseases that may be caused by inhalation of airborne biological organisms include tuberculosis (TB), Hantavirus, anthrax, coronaviruses (including SARS, MERS and others), and influenza. Biohazards may become airborne, perhaps as the agent itself, such as an anthrax spore, or as the agent riding on some other material that becomes airborne, such as dusts, mists or droplet nuclei. Hantavirus infection has been caused by people inhaling soil dust that became airborne after rodents shed virus via urine, feces or other materials into the soil. In fact, it is generally thought that airborne viruses are normally attached to other particles and

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Inhalation of these bioaerosols may be reduced by wearing respirators. The US Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), and many national health authorities have made numerous recommendations for respirator use where they believed the potential for the spread of disease through the airborne route exists. Considerations for selection and use of respirators to help reduce exposure to bioaerosols include:

• Filtration
• Microorganism survival on the filter
• Potential re aerosolization of the bio aerosol
• Reuse of the respirator
• Fit and the assigned protection factor of the respirator

These topics are addressed in this bulletin.

Routes of Exposure

Inhalation is not the only route of exposure for biohazards. Infection can occur from other routes of exposure, such as ingestion, skin and mucous membrane penetration (including the eyes) and animal and insect bites. Skin and mucous

rarely exist as naked organisms.

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membrane penetration can occur by direct contact with aerosols or secondarily – e.g., a hand touching a contaminated surface and then touching a mucous membrane.

How a disease is spread informs what types of controls are useful in preventing its spread. If the disease can be spread by contact, preventing surfaces from becoming contaminated and hand hygiene will be very important. Surgical masks may be worn by infected people in order to help reduce the spread via exhaled aerosols. Surgical masks, safety glasses or goggles, and faceshields may be used to help shield the healthcare worker’s mucous membranes (eyes, nose, and mouth) from large sprays of blood and other body fluids. Use of respirators may also be appropriate.

2 Particles ranging in size from sub micron to100 μm can remain airborne for extended periods of time. Particles smaller than

100 μm in size can enter the nose, mouth and throat and are considered “inhalable.” Particles smaller than 10 μm can reach the large bronchioles and are considered the “thoracic” fraction, and particles smaller than approximately 5 μm can enter the

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Certain diseases can be spread through the airborne route. This means that if the organism that causes the disease is aerosolized the potential exists for illness. Tuberculosis is one disease that is spread through the airborne route. Healthcare workers have been known to acquire tuberculosis infections during care for tuberculosis patients. In one particular case, it is believed by investigators that the transmission occurred during an aerosol-generating procedure that a healthcare worker

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deep lung and are considered the “respirable” fraction.

performed without wearing a respirator, according to guidance at the time.
airborne route is one of the ways that severe acute respiratory syndrome (SARS) and seasonal influenza can be spread. Analysis of various typical ventilation systems in surgical suites suggests that airborne particles are quickly and evenly

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risk of exposure, infection, and illness. With so many respirator use recommendations being made on websites and other sources, it is important to understand respirators and the role they have in helping to reduce exposures to bioaerosols.

Terminology

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  1. A filtering half facepiece (sometimes called a disposable respirator), where the filter is virtually the entire respirator

  2. An elastomeric (reusable) half facepiece with a particulate filter

  3. An elastomeric (reusable) full facepiece with a particulate filter

  4. A powered air purifying respirator (PAPR) that includes a particulate filter

Particulate respirators are classified by their performance against local certification standards. In the US, testing is done by the National Institute for Occupational Safety and Health (NIOSH). In Europe respirators are tested against the relevant European Standard and are approved to the PPE Directive 89/686/EEC or the replacement PPE Regulation (EU) 2016/425.

Filtration efficiency is one of the performance parameters evaluated for certification. The filtration efficiency tests are designed to be very stringent. Table A contains some of the minimum filtration requirements according to US and European standards. There are many test variables that affect performance such as type of aerosol, particle size, flow rate, whether the aerosol has been charge-neutralized to the Boltzmann equilibrium state, etc. For a more detailed comparison of global filtering facepiece respirator regulations, see the 3M Technical Bulletin - Comparison of FFP2, KN95, and N95 and Other Filtering Facepiece Respirator Classes.

When airborne, viruses and bacteria can be filtered by respirators with particulate filters. Because no respirator will prevent the inhalation of all particles, such as viruses and bacteria, respirators cannot eliminate the

distributed throughout the room.

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A respirator is a device designed to help provide the wearer with respiratory protection against inhalation of a hazardous

Bioaerosols are those airborne particles that are living or originate from living organisms. fragments, toxins, and particulate waste from all varieties of living things.

They include microorganisms and

For bioaerosols, particulate-removing respirators are often recommended to help reduce exposure. Particulate respirators are available as:

atmosphere.

Evidence has been presented that indicates the

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Table A. Filtration Requirements Per U.S., European, and China Standards

Standard

Classification

Filter Efficiency

NIOSH 42 CFR 84

N95

≥ 95%

NIOSH 42 CFR 84

N99

≥ 99%

NIOSH 42 CFR 84

N100

≥ 99.97%

EN 149:2001

FFP1 (filtering facepiece)

≥ 80%

EN 149:2001

FFP2 (filtering facepiece)

≥ 94%

EN 149:2001

FFP3 (filtering facepiece)

≥ 99%

EN 143:2000, EN 140:1999, EN136:1998

P1 (elastomeric facepiece)

≥ 80%

EN 143:2000, EN 140:1999, EN136:1998

P2 (elastomeric facepiece)

≥ 94%

EN 143:2000, EN 140:1999, EN136:1998

P3 (elastomeric facepiece)

≥ 99.95%

GB2626-2006

KN/KP90

≥ 90%

GB2626-2006

KN/KP95

≥ 95%

GB2626-2006

KN/KP100

≥ 99.97%

Please note that penetration of particles through the filter is only one of the possible sources of exposure to contaminants. Other potential sources such as faceseal leakage, leakage as a result of improper maintenance, or not wearing the respirator when necessary may contribute more to exposure than filter penetration. Each of these factors must be addressed and controlled. For example, all particulate respirators that are designed to seal to the face (including filtering facepiece respirators) can be fit tested using the saccharin or BitrexTM qualitative fit test methods (with the exception of full-face masks in some countries in Europe), or using appropriate quantitative fit test methods such as the ambient particle counting method using the TSI® PortaCount®. Wearers must be trained how to properly use and maintain their respirators and the importance of wearing them all of the time during potential exposure.

Please also note that respirators help reduce exposure to airborne contaminants but do not prevent the inhalation of all particles. As a result, when properly selected, used and maintained, respirators can lower exposures to concentrations considered safe for most non-biological particles. However, they do not eliminate the risk of exposure, infection, or illness since safe exposure levels have not been established for biological particles. In many countries, types or classes of respirators are given an “assigned protection factor,” or APF. APF is the expected ability of the respirator to reduce exposure when used according to an effective respiratory protection program. For example, an APF of 10 means that a respirator may reduce exposure by a factor of 10 (or 90%) when properly selected, used, and maintained. Therefore, even if a filter could be hypothetically 100% efficient, the expected amount of exposure reduction would be limited by the APF. Because no respirator will prevent the inhalation of all particles, none can entirely eliminate the risk of exposure, infection, and illness.

For more information on the proper selection, use, and maintenance of respiratory protection, please see the United States (US) OSHA standard for respiratory protection (29 CFR 1910.134), EN 529 Respiratory protective devices: Recommendations

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A surgical mask is an infection control device designed to help prevent the spread of infection from the wearer’s exhaled

for selection, use, care and maintenance — Guidance document,

or any applicable local standards and guidance.

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a barrier for large droplets expelled by the wearer. However, since surgical masks are not tested in the same way as respirators,

breath to potentially susceptible persons.

A surgical mask may help reduce contamination of the environment by providing

any “filtration efficiency” claims cannot be directly compared to those for a respirator.
for its ability to reduce exposure of the wearer against fluid splashes or high velocity streams. Most surgical masks are not designed to seal tightly to the face, and research has shown that they do not achieve the level of contaminant reduction

provided by a NIOSH-certified respirator.

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This has been found to be true both in laboratory studies and in healthcare

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masks, and workers who wore neither, N95 respirators were found to be significantly protective against bacterial and viral

workplaces.

When infection rates were tracked among workers who wore N95 respirators, workers who wore medical

Surgical masks have not been assigned protection factors by OSHA and should not be relied upon to help reduce exposure to inhalable airborne particles. For a more detailed comparison, see the 3M

infections, while surgical masks were not.
Technical Bulletin - Respirators and Surgical Masks: A Comparison.

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A surgical mask may also be tested

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In a few cases, a certified respirator may also have the attributes of a surgical mask. These are sometimes referred to as “Surgical N95 Respirators.” These products can help block large droplets expelled by the wearer, but also have been shown to have efficacy at filtering smaller particles and are designed to fit tightly to the face. Because of the additional use as a respirator, this type of surgical mask (i.e. surgical respirator) must also be fit tested. For an exploration of when surgical N95 respirators are needed, see 3M Technical Bulletin- Surgical N95 vs. Standard N95 – Which to Consider?

Filtration

A number of questions have been raised regarding the use of respirators against biological agents. The primary question is whether or not particulate respirators can filter small particles such as fungal spores (2 to 5 μm), bacteria (0.3 to 10 μm), or

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organisms may be carried on other particles including dust, blood, saliva, etc.

viruses (0.02 to 0.3 μm).

The physical size of various organisms is shown in Table B. As noted previously, biological

Droplets generated from coughing, or sneezing and talking will quickly dry in the air to form droplet nuclei. Droplet nuclei

16, 17 generated from coughs, sneezes, and speaking have been found to range from submicron to over 20 microns. Influenza

viruses, and other viruses, have been collected from exhaled breath.
Mycobacterium tuberculosis may range from less than 1 μm to greater than 5 microns.
influenza viruses have been sampled from the air of hospital rooms containing influenza patients and found to be in the size

4 particles can be filtered by particulate respirators.

range from less than 1 μm to greater than 4 μm.

Understanding filtration mechanisms can help answer whether or not these

Many particulate respirators use a non-woven fibrous filter media to capture particles. Fibers from less than 1 μm to 100 μm in size crisscross to form a web of many layers which is mostly air due to the spaces between the fibers. It is these spaces between fibers that allow for breathability. Particles are trapped, or captured, when flowing through the layers of filter media, and a particle becomes attached to a fiber due to a number of different mechanisms. The most common of these are

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gravitational settling, inertial impaction, interception, diffusion, and electrostatic attraction. Table B. Size of Various Microorganisms

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It is thought that droplet nuclei that contain

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Airborne particles containing

Microorganism (common name or disease)

Physical Size (μm)

Hepatitis virus (Hepatitis B)

0.042 - 0.047

Adenovirus (respiratory infections)

0.07 - 0.09

Filoviruses (Ebola)

0.08 diameter 0.79 - 0.97 length

Bunyaviridae (Hantavirus)

0.08 - 0.012

Orthomyxoviridae (Influenza A, B, & C)

0.08 - 0.012

Coronaviridae (SARS - CoV & MERS - CoV)

0.125

Variola Virus (Smallpox)

0.14 - 0.26 diameter 0.22 - 0.45 length

Mycobacterium tuberculosis (TB)

< 1 to > 5 diameter

Bacillus anthracis spore (Anthrax infection)

1.0 - 1.5 diameter

To understand how a particle is captured, one must first consider the movement of air through the filter media. The path of the air around a fiber may be described in terms of imaginary streamlines. Any particle carried by the air may or may not stay within the streamlines depending largely upon the particle's size (aerodynamic diameter).

Very large particles (< 100 μm) in slow moving airstreams may settle out due to gravity. However, most respirable particles are too small for this mechanism. Respirable particles above 0.6 μm in diameter are typically captured efficiently by

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Inertial impaction occurs when a particle cannot follow an air streamline around a fiber because of its inertia and instead impacts into the fiber. In the interception mechanism, the particle holds to the streamline,

interception and inertial impaction.
but that streamline will naturally bring the particle close enough to come in contact with the fiber. In contrast, diffusion is

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typically very efficient for particles smaller than 0.1 μm. Random movements of air molecules collide with these very small particles and cause them to wander across streamlines until they come in contact with a fiber.

Because of the various mechanisms by which particulate filtration occurs, the smallest particles are typically not the most

1 difficulttofilter.Mostparticulatefiltershavearegionoflowerfiltrationefficiencysomewherebetween0.05-0.5μm. Particles

in this range are large enough to be less effectively pushed around by diffusion, but small enough to be less effectively captured by interception or impaction. The most penetrating particle size (MPPS) will depend on the filter media, air flow, and electrostatic charge on the particle. Filters that use electrostatic attraction may have a MPPS shifted to a slightly smaller size range.

Filtration efficiencies of six different commercially available US N95 filtering facepiece respirators as tested by 3M are shown in the left side of Figure 1. (Previous research has shown that for 3M products, European FFP2 respirators have equivalent or better filtration efficiency in tests representative of health care environments.) Averaged filtration efficiencies are shown as a function of different sized sodium chloride particles at a flow rate of 85 liters per minute.

Figure 1. Averaged Filtration Efficiency for Six N95 Respirators* (on the left), and Size Distribution of Droplet Nuclei from a Sneeze (on the right).

* Data generated 2006

While there was variability between different samples of the same respirator model, and between different models, the MPPS included particles with a diameter between 0.04 and 0.1 μm. As seen in Figure 1, particles that are smaller or larger than the MPPS are captured with higher filtration efficiency. Filtration via diffusion (most noticeable for particles smaller than 0.1 μm) actually increases as particle size decreases. Other research has confirmed that filter efficiency increases with decreasing

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A size distribution from a sneeze is shown on the right side of Figure 1.
larger than the MPPS. In other words, droplet nuclei that may contain microorganisms will be filtered with high efficiency by these respirators.

Filtration claims, especially regarding bioaerosols, can be confusing. To help maximize the level of protection, make sure to use a respirator that has been tested and approved per all applicable local regulations. And, as mentioned above, filtration efficiency is just one of the required components that needs to be considered when selecting and using a respirator.

An often-expressed question is whether biological aerosols are removed by respirator filters the same as non-biological aerosols. Due to concerns on the efficacy of respirator filters for Mycobacterium tuberculosis (TB), many studies have been conducted using bioaerosols. These filter evaluations were conducted over a range of test conditions (flow, humidity), biological species representing various shapes (spheres, rod, and rod/sphere shape) and sizes, filter performance levels and

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particle size, even for particles as small as 0.003 μm (much smaller than that of virus).

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It should be noted that most of the droplet nuclei are

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varying filter media (mechanical and electret; polypropylene and fiberglass). These experiments24-29 found no significant difference in the filtration of biological aerosols and non-biological aerosols with similar physical properties – in other words,

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Spherical particles were found to be usually more penetrating than rod-shaped particles with equivalent aerodynamic diameter over a range of particle sizes.

filtration efficiency is based on particle size rather than the nature of the particle’s origin.

Studies in which researchers challenged NIOSH-certified N95 and P100 filtering facepiece respirators and filter cartridges with viable virus aerosols – including H1N1 influenza aerosols – indicated that respirators capture viable H1N1 influenza and

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filtration efficiency of respirator filters challenged with nanometer-sized particles. These studies have found that NIOSH-

other virus aerosols as well as or better than their respective N95 or P100 rating.
certified respirators show filtration efficiencies similar to what would be expected based on their approval category.

Where penetrations have slightly exceeded 5%, the results were not statistically significantly higher than 5%.

Microorganism Survival on Filters

Another area of interest is regarding the survival of microorganisms on respirator filters. This could impact storage and handling procedures. Several studies have been conducted regarding survival on filters. Over 18 types of respirator filters and

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The polypropylene filters used in these studies were then checked for survival of microorganisms ranging from immediately after loading to as many as 28 days later, depending on the experiment. These studies found there were surviving organisms immediately after loading and that they survived for varying lengths of time depending on the storage conditions of the study. Usually storage under high-humidity conditions was the most favorable for long term survival. However, these storage conditions are not typical of respirator storage practices in most respirator programs. Storing filtering facepieces used against bioaerosols in resealable plastic bags may be inappropriate, since the filters may be moist from use, and storage in plastic will keep the humidity level high.

One of these studies looked for migration of the organism to the inside of the filtering facepiece respirator and concluded that respirators may be reused over time with little risk of internal contamination, even after a week’s time, provided the respirator

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five surgical masks have been studied using several types of microorganisms followed by storage at various humidities.
The filters were typically loaded with the microorganisms at experimental concentrations that were higher than those typically expected in most work environments.

40 contamination from environmental bacteria was due to handling (removal from bag to sample).

is carefully handled and stored (handled by non-filter components, e.g., straps).

The investigators felt any internal

These filters were inoculated with Stachybotrys atra and stored at relative humidity (RH) as high as 100% for 86 days. S. atra grew and produced toxins on these

One study looked at two high efficiency filters with varying percentages of cellulose.

cellulose filters at the high RH conditions. Again, these conditions are not typical during most normal respirator use and storage.

These concerns have prompted some to state that a traditional filter without a nanoparticle coating of a biocide would turn into a breeding ground for a virus or bacterial agent. The studies mentioned above do not support this claim. While it may be relatively easy to load a filter with a biocide, determining its efficacy is more difficult. Close examination of the claim needs to be made. Claims often relate to protection of the product, such as from microbial decay rather than protection of the wearer. Many countries require that a product claim of biocidal effectiveness for protecting the wearer must be in compliance with local regulations. In the US, claims are regulated by the Environmental Protection Agency (EPA). In Europe, claims must be in compliance with the Biocide Product Regulation (EU) (528/2012). If the claims have not been approved or are not in compliance, they may be inappropriate. Very little peer-reviewed research has been conducted on respirators that currently claim antimicrobial properties.

One investigation of respirators incorporating antimicrobial-treated filter media found that there was non-detectable or no

Having the filter treated with a biocide may only be beneficial in extending the useful life of the filter. While most of the virus would be deposited on the filter as a result of breathing through the filter, bioaerosols may also be deposited on the straps,

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organisms captured on untreated filters and those filters treated with iodine and similar environmental conditions.

effect on the viability of penetrating particles.

Another study found an insignificant difference in the fractions of surviving 42

Other studies have investigated the

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3M Personal Safety Division

exhalation valve cover (if present), and nose clips etc. Thus caution in handling the respirator must still be taken and a biocide filter treatment may not prevent the spread of disease by contact with these respirator components.

Overall, these studies suggest careful consideration for filter handling, reuse and respirator disposal, especially where the organism can be spread by contact. Precautionary measures might include the use of gloves and washing hands after handling the respirator. For organisms transmitted only by inhalation, respirator handling may not be critical. One investigator suggested training for respirator users might be necessary to recognize when exposures would require immediate disposal of

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Reaerosolization of Microorganisms Collected on Respirator Filters

Once a particle is collected onto a fiber, it will adhere to the filter fiber due to Van der Waals forces. Therefore, filters are likely

to be good collectors of small particles. In contrast, reaersolization is the process by which any aerially deposited material on

the filter can be re-suspended into the air. It could be hypothesized to happen if there was high air flow back through the filter

such as if the wearer were to cough or sneeze while wearing the respirator. In this regard, one experiment used three

microorganisms and two surrogate particles [NaCl and Polystyrene latex (PSL) particles] of various size ranges from 0.6 μm

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respirators.

They were loaded onto three models of filtering facepiece particulate respirators. The reentrainment velocity was 300 cm/sec. Reaerosolization was significant only for larger test particles (3 and 5 μm) into dry air. There was no reaerosolization when the RH levels were greater than 35%. These authors concluded that reaerosolization of collected TB bacteria and other particles less than a few microns in size is insignificant at conditions encountered in respirator wear. They also speculated that the conclusions were valid for other fibrous filters as well.

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to 5.10 μm.

The two models of filtering facepiece particulate respirators were loaded with ~ 20 million particles. The respirators were then dropped three feet onto a hard

In a second study, investigators used 1μm PSL particles to simulate anthrax spores.

surface. The amount released ranged from 0 to 0.5% and the average release measured 0.16% and 0.29% for the 2 models. While this loading represents a much higher degree of loading than would be expected in typical work environments, this study indicates a small, but consistent fraction of 1 μm particles captured by a respirator filter may be released into the air. These results suggest caution in handling and disposing of respirators contaminated with anthrax spores.

Selection and Use

When respiratory protection is needed for exposures to bioaerosols, the user should select a certified/approved particulate respirator according to recommendations from CDC, WHO or applicable local agencies. Remember that the NIOSH particulate filter rating does not include faceseal leakage – only filter penetration. European EN performance standards do include a Total Inward Leakage performance requirement, however, these are not necessarily good indicators of product performance in the workplace. Therefore, the assigned protection factor must be considered to ensure the expected reduction in respiratory exposure is adequate for your intended application.

Once a respirator has been selected, a continuing, effective respiratory protection program as specified by applicable local regulations must be implemented. This includes training on the respiratory hazards, as well as the respirator fit testing, maintenance, disposal, etc. Of course, all respirators must be used in accordance with the applicable user instructions.

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  24. Willeke, K., Qian, Y., Donnelly, J., Grinshpun, S.A. and V. Ulevicius. Penetration of airborne microorganisms through a sur- gical mask and a dust/mist respirator. AIHA Journal. 57:348-355; 1996.

  25. Richardson, A.W., Eshbaugh, J.P, Hofarce, K.C. and P.D. Gardner. Respirator Filter Efficiency Testing against Particulate and Biological Aerosols under Moderate to High Flow Rates. ECBC-CR-085; 2006.

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3M Personal Safety Division

  1. Garner, P.D., Eshbaugh, J.P., Harpest, S.D., Richardson, A.W., and K.C. Hofacre. Viable viral efficiency of N95 and P100 respirator filters at constant and cylclic flow. J Occ. Environ. Hyg.10(10):564-572; 2013.

  2. Harnish,D.A.,Heimbuch,B.K.,Husband,M.,Lumley,A.E.,Kinney,K.,Shaffer,R.E.,andJ.D.Wander.ChallengeofN95 filtering facepiece respirators with viable H1N1 influenza aerosols. Infect. Cont. Hosp. Ep. 34(5): 494-499; 2013.

  3. Balazy, A., M. Taivola, A. Adhikari, S.K. Sivasubramani, T. Reponen and S.A. Grinshpun. Do N95 respirators provide 95% protection level against airborne viruses and how adequate are surgical masks. Am. J. Infect. Control. 34:51-57; 2006.

  4. Eninger, R.M., Honda, T., Adhikari, A., Heinonen-Tanski, H., Reponen, T. and S. A. Grinshpun. Filter Performance of N99 and N95 Facepiece Respirators against Viruses and Ultrafine Particles. Ann. Occup. Hyg. 52(5):385-396; 2008.

  5. Rengasamy, S., Eimer, B.C. and R.S. Shaffer. Comparison of Nanoparticle Filtration Performance of NIOSH-approved and CE-Marked Particulate Filtering Facepiece Respirators. Ann. Occup. Hyg. 53(2): 117-128; 2009.

  6. Rengasamy, S., Verbofsky, R., King, W.P. and R. E. Shaffer. Nanoparticle Penetration through NIOSH-Approved N95 Fil- tering Facpiece Respirators. Journal of the International Society for Respiratory Protection. Vol. 24: 49-59; 2007.

  7. Brosseau, L.M., McCullough, N.V., and D. Vesley Bacterial survival on respirator filters and surgical masks. J. Am. Biol. Saf. Assoc. 2:232-243; 1997.

  8. Reponen, T.A., Wang, Z., Willeke, K. and S.A. Grinshpun. Survival of mycobacteria on N95 personal respirators. Infect. Control Hosp. Epidemiol. 20:237-241; 1999.

  9. Wang, Z., Reponen, T.A. and K. Willeke. Survival of bacteria on respirator filters. Aerosol Sci. Tech. 30 (3), pp. 300-308; 1999.

  10. Pasanen, A., Nikulin, M., Berg, S. and E. Hintikka. Stachybotrys atra corda may produce mycotoxins in respirator filters in humid environments. Am. Ind. Hyg Assoc. J. 55:62-65; 1994.

  11. Johnson, B., Winters, D.R., Shreeve, T.R. and C.C. Coffey. Respirator filter reuse test using the laboratory simulant myco- bacterium tuberculosis (H37RA strain). J. Am. Biol. Saf. Assoc. 3:105-116; 1998.

  12. Eninger,R.M.,Adhikari,A.,Reponen,T.,andS.A.Grinshpun.DifferentiatingBetweenPhysicalandViablePenetrations When Challenging Respirator Filters with Bioaerosols. Clean 36(7), 615-621; 2008.

  13. Wu, C.-Y., Lee, J.-H., Riemenschneider, L, and A.D. Theodore. Evaluation of the performance of iodine-treated biocide filters challenged with bacterial spores and viruses. Air Force Research Laboratory Report, AFRL-RX-TY-TR-2008-4511; 2008.

  14. Qian, Y., Willeke, K. Grinshpun, S.A and J. Donnelly. Performance of N95 respirators: reaerosolization of bacteria and solid particles. Am. Ind. Hyg Assoc. J. 58:876-880; 1997.

  15. Kennedy,N.J.andW.C.Hinds.Releaseofsimulatedanthraxparticlesfromdisposablerespirators.JOcc.Environ.Hyg.1:7- 10; 2004.

  16. Holden, K.L. Unmasking leading to a healthcare worker Mycobacterium tuberculosis transmission. Journal of Hospital Infection. 100(4): e226-e232, 2018.

  17. Memarzadeh F, Manning AP. Comparison of operating room ventilation systems in the protection of the surgical site. ASHRAE Transactions. 2002;108(2):3–15

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