Filtering Facepiece Respirators, and Respirator Filters, Some Considerations

This Post is about Respirators, (not Surgical Masks)

Also See:

Respirators Still a Hot Item, So Should Be Knowledge of Their Use and Styles 04-08-2020

Respiratory Protection, Things You May Need to Know as a User or Employer 03-30-2020

Respirators, Things You Need to Know! 02-01-2020

Hard to Believe Respirator vs Virus Size

Recently, I discovered some information on the relative size of various viral particles vs the size of particle that common Air Purifying Respirators are made to Filter. Air Purifying Respirator Filters, according to the information I found, remove particles 0.3 um (Micro-Meters) and larger at the specified efficiency (95% thru 99.97% depending on the type chosen). The virus in the news constantly now is reportedly actually smaller than the variant of it in the table above. But, it ranges to, or nearly to, the size specified.

The virus in the news is 0.125 um in diameter or less, 2.4 (or more) x smaller than the ability of the filter to remove it according to the stated specifications. So, at minimum, it’s akin to a 6’ man standing in a 14’ 4” Hole in the Wall. That’s a lot of space above, below, and to the sides.

Virus vs Opening in Respirator Filter

The above image is the size of the Smallest Particle Common Respirator Filters are rated to remove (Black) to their stated efficiency, and the virus particle is shown (Blue). So it’s quite smaller than the openings in the respirator filter. But the virus often combines with other viruses, increasing the size, and from what I read, come viruses can combine (by contact) with particles in the air that are larger. But even entering nearly alone, they tend to snag on things, and the filter is designed internally to provide a series of rapid changes in the direction of air currents flowing through, thereby significantly increasing the chance that the particle will become snagged (a slalom if you will to cause particles to impact with the filtering media).

This is only part of the problem with Negative Pressure Air Purifying Respirators. Most of what we picture as respirators are Negative Pressure in Design. In order to filter something, a media of some sort needs to be between you and the substances to filter. Think of a Water Filter, or an Oil Filters. In fluids (including Air Flow), they measure the fluid differential across the filter, a new filter has low differences in pressure between the inlet and outlet sides. As the filter plugs, the differential pressure increases, since the pressure at the input side is increasingly higher than the pressure at the outlet side. In industry, and likely commercial enterprises, they use the differential pressure as an indicator of when to change the filter. With Respirator Cartridges, Cartridge-Filters Combination, or Filters, as the Respirator Filtering Element plugs, the user draws a breath, and with an increasing plugged filter, the pressure inside the Respirator decreases more and more, creating a low pressure area, and encouraging outside air to infiltrate the Respirator Face to Facepiece Seal, thereby letting in contaminants. 

OSHA Mandates a Respirator Cartridge/Filter Change-Out-Schedule for some substances, Benzene requires them to be changed at the beginning of each 8-Hour Shift. Benzene is adsorbed (not absorbed) by specially prepared carbon particles. These particles have tiny pores on the surface. The Benzene impacts this media on it’s way through the cartridge and the Benzene is, loosely speaking, precipitated onto the particles, and therefore is removed from the air. The reason for the Change-Out-Schedule mandated by OSHA is severalfold.

  1. The Carbon Particles will only hold so much, then the Benzene will pass through unfiltered.
  2. Moisture is also Precipitated on the Carbon Particles, and other Organic Vapors, and once the tiny pores are filled, everything passes through.
  3. After the work shift is done, and the cartridges have successfully filtered the Benzene and other organic vapors from the air, when the cartridge sits idle, the substances evaporate from the particles, and then may be breathed when the respirator is again used.

Various substances have different requirements for Changing the Cartridges and Filters. One common to them all requires the Cartridges or Filters to be changed when an increase in Breathing Resistance is noted by the wearer. Other requirements may require them to be changed more frequently, but if it’s harder to breath, even if the Cartridge or Filter Change-Out-Schedule doesn’t call for a change, they must be changed. On Negative Pressure Air Purifying Respirators this is key to reducing leaks at the Respirator to Facepiece Seal. Remember, the harder it is to breathe, the lower the pressure is inside the mask as the user breathes in, and the more likely that leaks will occur between the Face and Facepiece Seal.

But even with new Cartridges and/or Filters, some Facepiece Leakage occurs. We had a yearly Respirator Fit Test, it measured particles outside the respirator and compared them to the particles inside the respirator. Typically dust alone sufficed to provide the particles, but the in-Plant Hospital was new, the dust minimal, so they added an Ultrasonic Humidifier to the room. When used with Tap Water, everything gets ejected, including minerals in the water, so, it’s much more than just water vapor. The Device, a Porta-Count, was connected to a computer that showed the progress of the fit test, as each of the following listed items was done in stages, measuring the particles at each stage.

  • Normal breathing
  • Deep breathing
  • Turning head side to side
  • Moving head up and down
  • Talking
  • Grimace
  • Bending over
  • Normal breathing

We were tested to see how the respirator fit overall. On the Computer Screen, there was an Activity Bar, Green for Good, Yellow for Increasing Particles Detected Leaking into the Respirator, and Red for Too Much Leaking Into the Respirator due to a Bad Face to Facepiece Seal. There were short periods of Red, some yellow, and mostly green. If the Average Seal was sufficient, you were able to wear that manufacturer’s Respirator in that Size for the next year, using Cartridges, Filters, and Cartridge/Filters, and Replacing them, as the situation demanded.

An Air Purifying Respirator cannot be used (this assumes you have the correct Chemical Cartridges, Chemical Cartridge/Filters, or Filters):

  • For Unknown Exposures
  • For a Known Exposure if the level is Immediately Dangerous to Life and Health
  • For a Level of the Contaminant that exceeds the Assigned Protection Factor of the Respirator (Except as permitted for short periods under specific Standards). The Maximum Use Concentration is the Maximum Amount of a Substance that the User can be exposed to wearing a specific type of Respirator and is based on the Assigned Protection Factor and other specifics not discussed here.
  • If the Oxygen in the Area is below 19.5% [Air Purifying Respirators Don’t Supply Oxygen] Air is normally 21% Oxygen, likely at Sea Level, and Decreases with an Increase in Altitude) and if it’s not 21% adjusted for your area, you should understand what is causing the reduction in Oxygen to be certain the Respiratory Protection you’re using is suitable for the conditions.

If you abide by the above, and a little leaks in, it’s not critically important. It’s a known exposure, it’s not Immediately Dangerous to Life and Health, there’s enough Oxygen, and the Levels don’t exceed the respirator design. This is for industry. Hopefully the little that leaks in, or gets through the filters, is so low that it might take 20 years of such exposures to develop an Industrially Related Condition.

But what about Pathogens? I am not qualified to speak of them, but I’m going to touch upon them. Remember, Conditions Need to be significantly controlled to use an Air Purifying Respirator, because they WILL NOT completely eliminate the contaminant, they will reduce it to levels inside the respirator that, in many cases, if those conditions were found outside the respirator, that the respirator wouldn’t even need to be worn.

For Example, Benzene. You’re allowed to be exposed to 1 Part Per Million (PPM) averaged over 8 hours without a Respirator. A Half Face Respirator has an Assigned Protection Factor of 10. With Organic Vapor Cartridges, you can be exposed to 10 ppm while wearing a Half Face Respirator because the respirator will reduce the level inside the Respirator to 1 ppm or less. But it doesn’t reduce it to zero.

But with Pathogens, how many are required to infect? I don’t know. But I looked at the size of the virus in the news, it’s 0.125 um (Micro Meters) and some sites have it smaller, and saw that particulates removed by Particulate (Solid and Liquid Particles) Filters is rated as 95%, 99%, or 99.97% of particles 0.3 um or larger. If I calculated correctly, the virus particle would look like the example shown above.

Some Links, Selected Excerpts, and My Commentary 

3M PDF: Key Differences Between Respirators and Masks

3M PDF: Respirators and Surgical Masks: A Comparison

3M PDF: Respiratory Protection for Airborne Exposures to Biohazards

Here, read 3M’s own words:

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 effective 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. Particles ranging in size from submicron to 100 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 deep lung and are considered the “respirable” fraction.

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 performed without wearing a respirator, according to guidance at the time. Evidence has been presented that indicates the 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 distributed throughout the room. When airborne, viruses and bacteria can be filtered by properly selected and worn respirators with particulate filters. Because no respirator will prevent the inhalation of all particles, including viruses and bacteria, respirators cannot eliminate the 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.


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.

The above paragraphs are what I’ve been laboring to say. To make matters more complicated, OHSA, under the recommendations of the Filtering Facepiece Respirator Manufacturers, gave them an Assigned Protection Factor of 10 . Various sources can be found of Unions and others in Industrial Hygiene stating that the Assigned Protection Factor of Disposable Filtering Facepiece Respirators should be 5 (bear in mind, this was for protection against larger particles – smaller, pathogen sized particles will enter the Filtering Facepiece Respirator with more ease). In the opinion of, perhaps many, the Filtering Facepiece Respirators do not seal as tightly to the face as Elastomeric Respirators do (see the Featured image atop this post, on the right side, this is an Elastomeric without the required Filters attached).

Here’s a Letter describing the Lower APFs of Filtering Facepiece Respirators, and as I’ve Explained, maybe with particulates of minimal threat, OSHA assigning an APF of 10 to Filtering Facepiece Respirators will have little consequence, but are they safe around pathogens?

PDF: Letter Including Section on Assigned Protection Factors (APFs):

3M PDF: 2019 Novel Coronavirus Outbreak Technical Bulletin

3M PDF: Nuevo Coronavirus 2019 y brote de enfermedad COVID -19 (The Links don’t seem to work in the Document)

3M PDF That Must be Downloaded to View: 3M Technical Bulletin – Cleaning and Disinfecting 3M Reusable Elastomeric Half and Full Facepiece Respirators following Potential Exposure to Coronaviruses

3M PDF: Surgical N95 vs. Standard N95 – Which to Consider?

PDF: Comparison of FFP2, KN95, and N95 and Other Filtering Facepiece Respirator Classes

U.S. Govt Agency Link, Thorough: Filter Performance Of N99 And N95 Facepiece Respirators Against Viruses And Ultrafine Particles

Very Technical and Thorough Article: Viable Viral Efficiency of N95 and P100 Respirator Filters at Constant and Cyclic Flow

Infectious viral aerosols are not well characterized and are poorly understood in the real world. A single virus can range in size from 20 to 300 nm. Although a single virus may be smaller than a bacteria particle (300 to 10,000 nm in size), viral aerosols likely exist as agglomerates or attached to inert particles. Infectious aerosols generated by coughing, sneezing, talking, and breathing create a diverse size of particles ranging from less than 1 μm to 100 μm.

PDF: Simple Respiratory Protection—Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20–1000 nm Size Particles

NIOSH-Approved Particulate Filtering Facepiece Respirators

The above page now refers readers to the following:

NIOSH-Approved Air Purifying Respirators for Use in Health Care Settings During Response to the COVID-19 Public Health Emergency

Letter of Authorization

EUA Clarification Letter on Respirators 

Imported, Non-NIOSH-Approved Disposable Filtering Facepiece Respirators

Letter of Authorization

Non-NIOSH-Approved Disposable Filtering Facepiece Respirators Manufactured in China

Letter of Authorization

Face Shields

Letter of Authorization

Emphatically, I am NOT SUGGESTING THAT RESPIRATORS ARE NOT NEEDED, I am saying that many people are wearing them recklessly because they are overconfident in the belief they are protected. I’m Suggesting that Elastomeric Respirators (usually reusable, though some disposable are made, but Elastomeric Respirators are Silicone or Rubber usually Half Face Respirators that commonly take 2 Filters or Cartridges) are more protective from exposure to pathogens as the respirator more than likely fits better, the nature of the Filters are, I think, more conducive to the removal of the pathogen from the air, especially when used with P100 Filters that stop 99.97% of particles of a specific size from passing through the filter. But some of the reading today suggests that Elastomeric Respirators are not permitted in Hospitals, nor, the article seemed to suggest, are Filter Facepiece Respirators with Exhaust Valves Permitted in Hospitals as any pathogens from the wearer is expelled into the room unfiltered.

Author: Dr-Artaud

A Doctor that is not a Doctor, but named after a character in the movie "No Such Thing", as is the Avatar.

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