Airborne transmission likely the dominant route of Covid-19 infection
In an open letter published by the Infectious Diseases Society of America in July, 239 scientists urged the World Health Organization and U.S. medical authorities to "recognize the potential for airborne spread of COVID-19." The scientists claimed to have "demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets (aerosols) small enough to remain aloft in air and pose a risk of exposure."
Now, many scientists think airborne transmission may be the dominant route of Covid-19 transmission.
Tuberculosis is an excellent example of aerosol-based transmission of disease. In 1956, tuberculosis was a major global killer. Then, as now with COVID-19, health authorities held that tuberculosis and other respiratory diseases were spread by large infected droplets through person-to-person contact or contaminated surfaces. They turned out to be wrong. Tuberculosis can only be transmitted through aerosols (airborne). Similar to Covid-19, tuberculosis outbreaks often occurred in crowded, poorly ventilated environments such as ships, buses, schools, houses of worship, and bars. What if, like tuberculosis, the Covid-19 virus only transmits through the airborne route? Scientists have pointed out that many super-spreader events cannot be explained by other transmission routes. In the Skagit County church choir case, a single infector infected 52 of the other 60 people at a choir rehearsal. That is an 86.7% infection rate. Attendees had used hand sanitizer and avoided hugs and handshakes. It is therefore unlikely all 52 attendees caught the virus from surfaces or being hit by droplets from one infectious person.
Our current situation cannot be more dire. We added 1 million new infections in a single week of November. Covid-19 death has surpassed the 250,000 mark with more than 11.6 million infections as of 11/18/2020. Every 2 days, more lives are lost than from the 9/11 attacks. And experts are warning that it will get worse.
Studies have shown that a carrier can release virus-laden aerosols simply by exhaling, without coughing or even talking. CDC’s recent estimate (11/10/2020) states that asymptomatic carriers account for more than 50% of Covid-19 transmissions.
This makes Covid-19 doubly dangerous. We don’t know who is infectious, and we can’t see the virus released.
Isn’t it time that we rethink our approach? As Einstein once said, “Insanity is doing the same thing, over and over again, but expecting different results.”
Nothing can be too prudent at this point, especially when we are losing the battle badly against an enemy that is invisible and can be anywhere and everywhere.
What is aerosol (airborne) transmission?
Aerosols infect through inhalation. Aerosols are particles of saliva or respiratory fluid, but they are smaller than droplets, from less than 1 µm (micron) to about 100 μm. For this reason, they can linger in the air, from tens of seconds to hours, and can travel longer distances. They infect by being inhaled through the nose or mouth, or (less likely) by deposition on the eyes. Depending on their size, they stay longer and travel further in the air, and they also reach different parts of the human respiratory tract.
Infectious aerosols travel far beyond 6 feet and can stay afloat for hours
Figure 1 shows that 5 µm (micron) aerosols can travel to about 300 feet even when the indoor air velocity is low at 2inch/second. At a higher speed, that distance goes up to about 1300 feet.
- Size of respirable aerosols that can reach deep into the lungs - ≤5µm (micron).
- Aerosol sizes that dominates Covid transmissions are around 3 µm.
- 3 µm aerosols take about 1.5 hours (90 minutes) to settle to the ground in still air, from the height of a person. Covid-19 virus remains infective in the air for about 1-2 hours at typical room temperature (70 oF).
A study published in Building & Environment (June 2020 issue) shows that large droplet exposure during talking is very minor when compared to aerosols exposure with the subjects being more than 0.5 m (1.6 feet) apart. (Figure 3)
Aerosol sizes that dominate Covid-19 transmission are thought to be around 3 µm (microns). Aerosols less than 5 µm can reach deep into the lungs.
Tuberculosis is a classic model of aerosol-based infection. According to a slide in CDC’s “Tuberculosis Aerosols 101” presentation, 3 µm aerosols in still air settle to the ground in about 1.5 hours (90 minutes), from the height of a person. Researchers believe Covid-19 virus remains infective in the air for about 1-2 hours at typical room temperature (70o F).
Long story short, the virus-laden aerosols exhaled by a carrier can float in the air for an hour and a half, can travel far beyond the “social distance” standard of 6 feet, and are potently infectious while airborne.
Sources:
- Nazaroff, Linsey C. Marr 2020
- Matthews, T.G. Thompson, C.V., Wilson, D.L., et al., 1989, Air velocities inside domestic environments: An important parameter in the study of indoor air quality and climate, Environ Internat
- Baldwin, P.E., Maynard, A.D., 1998, A survey of wind speeds in indoor workplaces, Ann Occup Hyg
- CDC “Tuberculosis Aerosols 101” presentation.
- “FAQs on Protecting Yourself from COVID-19 Aerosol Transmission” https://tinyurl.com/FAQ-aerosols Version: 1.79, 5-Nov-2020. Lindsey Marr, Shelly Miller, Kimberly Prather, et al
- Chen, W., Zhang, N., Wei, J., et al., June 2020, Short-range airborne route dominates exposure of respiratory infection during close contact, Building & Environment
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Masks are not PPE, respirators are PPE
Masks and respirators are two completely different categories, though they may look alike. We primarily wear masks to protect others, not ourselves. FDA refers to face masks as source control, meaning to contain infectious contaminants at its source, the wearer.
Find out how FDA defines face masks and respirators below.
“Source control refers to the use of a facemask or cloth face covering over the mouth and nose to contain that individual’s respiratory secretions to help prevent transmission from infected individuals who may or may not have symptoms of COVID-19.
Face Mask – A mask, with or without a face shield, that covers the user’s nose and mouth and may or may not meet fluid barrier or filtration efficiency levels. Face masks are for use by the general public and HCP (health care personnel) only as source control in accordance with CDC recommendations.
Filtering Facepiece Respirator – A filtering facepiece respirator (FFR) is a device that is a disposable half-face-piece non-powered air-purifying particulate respirator intended for use to cover the nose and mouth of the wearer to help reduce wearer exposure to pathogenic biological airborne particulates.”
Researchers tested three types of commonly available face masks and an N95 respirator for their protective efficiency. The air flow was set at 8.75 L/min to simulate a near resting state respiratory ventilation rate. Aerosol particle size was 1.0 to 2.5 microns. Aerosol sizes thought to dominate Covid-19 transmission are around 3 microns.
Source: Southern Research Institute, published in Applied Biosafety Vol. 15, No. 2, 2010. www.absa.org
Disclaimer: Filtration results may vary widely across studies due to a number of factors: experiment design, mask material, fabrication, test particle size, air flow velocity, etc.
It is worth noting that face masks for civil use are not regulated in US or China, nor is there any national standard established for them. It is our humble opinion that any KN95 is superior than cotton masks or surgical masks, because they use non-woven and melt-blown (electrostatically charged polypropylene fiber) materials. You can find more comparison between masks and KN95 below.
"The gold standard is a N95 or a KN95 respirator, which, if properly fitted, filters out and prevents the wearer from breathing in at least 95 percent of small aerosols. The efficacy of surgical masks against aerosols varies widely." — Linsey C. Marr, Virginia Tech Professor. Preeminent expert on the airborne transmission of infectious diseases. New York Times, 7/30/20
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Respirators have to meet stringent standards and undergo vigorous testing. Masks do not.
| Main Requirements/Features |
KN95 |
Mask† |
Protection
Protects people around the wearer |
 |
|
Protection
Protects the wearer from inhaling virus laden aerosols |
|
A
little
|
Filtration Efficiency of Non-Oily Particles
Filtration efficiency is 95% or higher at any time during the tests for all 40 respirators.
- A test aerosol (sodium chloride) about 0.3 µm in size is used, which is in the Most Penetrating Particle Size (MPPS) range.
- Airflow rate of 85 L/min, which represents a high work rate. Adults breathe at a rate of approximately 7.5 L/min while resting and 13 to 25 L/min during light exercise (Adams, 1993).
- Aerosol concentration is conducted at a minimum of 200 mg/m3, which represents a very hazardous respiratory exposure. A moderate dust storm is only about 40 mg/m3.
|
|
 |
Filtration Efficiency of Oily Particles
Paraffin oil test at 95 L/min. (EU FFP2 standard)
Filtration efficiency is 94% or higher for all 40 tested respirators. |
|
|
Breathability
This ensures the breathing comfort through the respirator.
An initial breathing resistance (resistance to airflow) not exceeding 35 mm water column height pressure and initial exhalation resistance not exceeding 25 mm water column height pressure.
*Millimeters (mm) of water column is a unit of measurement of small pressure differences. |
|
|
Certification
Certified under various standards: China KN95, EU FFP2, US NIOSH N95 in process Pre-Certification stage. |
|
|
Inward Leakage
The total inward leakage consists of three components: face seal leakage, exhalation value leakage, and filter penetration. At least 46 out of 50 individual exercise results (i.e. 10 subjects x 5 exercises - walk, head tilt side to side, then up and down, talk, walk) for total inward leakage does not exceed 11% and, in addition, at least 8 out of the 10 individual wearer arithmetic means for the total inward leakage is not greater than 8%. |
|
|
Clean Room
GMP Clean Room Class 100,000 standard workshop. |
|
|
Fluid Resistance
Provides a physical barrier against body fluid, keeping it from reaching wearer's mouth and nose. Body fluid, such as saliva, respiratory droplets and aerosols produced during a person’s breathing, speaking, singing, sneezing, and coughing may contain viruses and bacteria. |
|
|
Electrostatic Charges
Filter fibers are electrostatically charged.
Electrostatic attraction adds an additional filtration mechanism to the respirator's mechanical filtration. This is very important for respirators to meet the stringent filter efficiency AND breathing resistance requirements because it enhances particle collection without increasing breathing resistance. It works with small and large size particle, in other words, it is size blind. |
|
|
CO2 Content of Inhaled Air
The carbon dioxide content of the inhaled air (dead space) does not exceed an average of 1.0 % (by volume). |
|
|
QC Testing
Vigorous in-house testing capacity ensures all quality control standards are met consistently by every respirator/mask. |
|
|
Flammability
Not of highly flammable nature.
When tested, the respirator does not burn or does not continue to burn for more than 5s after removal from the flame. |
|
|
† Face Covering (Mask, Bandana, etc.)
Surgical and cotton masks are not designed to seal to the face. Though they can block large droplets, inhalation creates a negative pressure surrounding the wearer’s nose/mouth area. Some air will leak in from the sides without going through the masks’ filter materials, thereby letting in virus-laden small aerosols.
KN95 respirators are tested on 10 human subjects with each performing exercises in a test chamber. The amount of a test aerosol (0.5 µm) that enters the tested respirator via both filter penetration and face seal leakage is measured. A total inward leakage cannot exceed 8%.
With masks, we protect others.
With respirators, we can protect ourselves WHILE we protect others. They filter both inhaled and exhaled contaminants including pathogenic biological airborne particulates by at least 95%.
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How does a KN95 respirator work?
The following information is taken directly from the CDC website. For more, please visit www.cdc.gov.
With respiratory protection, whether the goal is to prevent the outward escape of user-generated aerosols or the inward transport of hazardous airborne particles, there are two important aspects of performance.
- First, the filter must be able to capture the full range of hazardous particles, typically within a wide range of sizes (<1 to >100 µm) over a range of airflow (approximately 10 to 100 L/min).
- Second, leakage must be prevented at the boundary of the facepiece and the face.
Filter Performance
The filters used in modern surgical masks and respirators are considered “fibrous” in nature—constructed from flat, nonwoven mats of fine fibers. Fiber diameter, porosity (the ratio of open space to fibers) and filter thickness all play a role in how well a filter collects particles. In all fibrous filters, three “mechanical” collection mechanisms operate to capture particles: inertial impaction, interception, and diffusion. Inertial impaction and interception are the mechanisms responsible for collecting larger particles, while diffusion is the mechanism responsible for collecting smaller particles.
In some fibrous filters constructed from charged fibers, an additional mechanism of electrostatic attraction also operates. This mechanism aids in the collection of both larger and smaller particle sizes. This latter mechanism (electrostatic attraction) is very important to filtering facepiece respirator filters that meet the stringent NIOSH filter efficiency and breathing resistance requirements because it enhances particle collection without increasing breathing resistance.
- Inertial impaction: With this mechanism, particles having too much inertia due to size or mass cannot follow the airstream as it is diverted around a filter fiber. This mechanism is responsible for collecting larger particles.
- Interception: As particles pass close to a filter fiber, they may be intercepted by the fiber. Again, this mechanism is responsible for collecting larger particles.
- Diffusion: Small particles are constantly bombarded by air molecules, which causes them to deviate from the airstream and come into contact with a filter fiber. This mechanism is responsible for collecting smaller particles.
- Electrostatic attraction: Oppositely charged particles are attracted to a charged fiber. This collection mechanism does not favor a certain particle size.
As seen on the graphic, there is a particle size at which none of the “mechanical” collection mechanisms (interception, impaction, or diffusion) is particularly effective. This “most penetrating particle size” (MPPS) marks the best point at which to measure filter performance. If the filter demonstrates a high level of performance at the MPPS, then particles both smaller AND larger will be collected with even higher performance.
Particles in the size range of 0.1-0.5 µm are the most difficult to filter. The aerosol used in N95 and KN95 respirator testing is 0.3 μm, which is in the MPPS range of most filters. Pollution aerosols are mostly ~0.3 μm.
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How is it made?
Our KN95 respirator consists of multiple layers of randomly arranged non-woven fabric made from polypropylene.
The most misunderstood aspect of filter performance: Filters do NOT act as sieves. Unlike sieves or membrane filters where particles smaller than openings or pores can pass through, respirators are designed to target a range of particle sizes. Particles get trapped in a respirator as they are forced to make twists and turns through the dense fiber forest of the materials, some of which are as thin as a single micron. KN95 also has electrostatically charged material to further attract particles. Once removed from the airstream by, the particles cannot no longer break loose from the filter fibers as they are strongly held by molecular attractive forces.
- Exterior spun bound: Made of 15-35 micron wide non-woven polypropylene threads. Fluid resistant. Blocks larger particles.
- Melt-blown filter: Made of much smaller non-woven polymers fibers less than 1 micron wide. They are electrostatically charged to capture oppositely charged particles of all sizes, including those in the 0.1-0.5 µm range, also referred to as the Most Penetrating Particle Size (MPPS), that the mechanical filtration mechanism is not too effective, (both layers of meltblown fabric)
- Hot air cotton: fluffy, elastic, and moisture proof.
- Contact layer: skin friendly bio-compatible non-woven fabric.
New York Time has an animation that take you inside a mask and an N95 Respirator and let you follow the particles as they make twists and turns through the dense fiber forests of the filter materials and get trapped along the way. https://www.nytimes.com/interactive/2020/10/30/science/wear-mask-covid-particles-ul.html?referringSource=articleShare
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Why Clean Room 100,000 workshops are recommended to produce KN95?
Respirators are literally air filters you wear on your face. They are engineered to capture particles (dirt, germs, contaminants) from the air. The electrostatically charged fibers in the KN95 can attract oppositely charged particles in the air without too much air flow. That can occur as long as the fibers are exposed during the production, storage, usage, etc.
This is why it is preferrable that production workshops can meet Clean Room Class 100,000 standards and why it is important to keep your respirators in a sealed packaging.
Professor Jiaxing Huang, a materials scientist at Northwestern University working to develop a new type of medical face mask has this to say. “When the charges are dissipated during usage or storage, the capability of stopping virus-sized particles diminishes. This is the main reason of not recommending the reuse of N95 masks.” — USA Today, “Fact check: No, N95 filters are not too large to stop COVID-19 particles”, 6/11/2020
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What is the difference between KN95 and N95 respirators? Are all KN95 respirators/masks the same?
KN95 is the Chinese classification; N95 is the US classification. They are very similar in performance parameters with some minor differences. KN95 standard is actually a hybrid of the US N95 and the European FFP2 standards.
- The filtration efficiency test of KN95 is exactly the same as that of N95.
- KN95 tests for Total Inward Leakage (TIL), while N95 does not.
- KN95 tests for CO2 clearance requiring CO2 content of the inhaled air to be less than 1%. This is not required by N95.
- The KN95 Respirator Model CD9501 carried by Dragon Herbs has passed the non-oily particle and oily particle filtration test and can be used for filtering both types of particles with at least 95% and 94% filtration efficiency respectively.
N95 targets an occupational audience such as miners, construction workers, fire fighters, etc. KN95 targets the occupational audience as well as the general public. Mask acceptance in China is very high; nearly 99% of the population approves wearing masks in their daily life if needed. Many consumers use KN95 for respiratory protection against dust, smog, automobile exhaust, and other air pollutants. This probably helps explain why the regulatory oversight is more relaxed there.
The biggest difference comes down to execution. KN95 is a national standard recommended but not regulated by the Chinese government. Companies do not need to obtain a pre-market approval from the government. They only need to obtain a device manufacturing permit, then they send their KN95 to independent labs for testing using the KN95 standard called GB2626-2006. If passed, they can start marketing. Different companies use different labs and not all government recommended tests are requested by the manufacturers for testing. This creates the variance in quality among KN95 respirators. In short, not all KN95 are created equal. Only the ones meeting all the regulatory requirements can be called respirators. Otherwise, they are only masks.
US operates differently. An N95 respirator must be tested and approved by NIOSH National Institute of Occupational Safety and Health, a branch of the CDC.
Design wise, KN95 uses ear loop design while N95 uses headband only. Both FDA and CDC prefer the headband, citing a more secure fit.
Our current model has remained on the FDA EUA list since its issuance. FDA has removed some previously authorized models from its list after random inspection.
KN95 respirator’s sister model CD9503B has passed the required NIOSH N95 pre-certification testing. You can see the test report generated by a US accredited lab in the download section below.
Model CD9501 and CD9503B are exactly the same except that CD9501 has an ear loop design while CD9503B has a headband design. The CD9503B test report applies to CD9501 as long as a proper fit can be achieved by the wearer. The facility inspection by NIOSH has been halted due to Covid-19 travel restrictions.
Since we expect the N95 sister model will be a lot more expensive than the KN95 model due to the hefty certification cost, Dragon Herbs offers an extender to easily convert the ear loops into headbands. As long as a secure face seal can be achieved, our KN95 model CD9501 is cheaper and comparable to an N95.
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Wearing KN95 respirators helps to break the Covid-19 infection chain
R – The reproduction number, R, is a way of rating coronavirus or any disease's ability to spread.
For example, Measles has one of the highest numbers with an R number of 15 in populations without immunity. That means, on average, one person will spread measles to 15 others. Coronavirus - known officially as Sars-CoV-2 - has a reproduction number of about 3 if we take no action to stop it spreading.
If the reproduction number is higher than one, then the number of cases increases exponentially - it snowballs like debt on an unpaid credit card. But if the number is lower than one, the disease will eventually stop spreading, The further below one, the faster that happens. — BBC
The Covid-19’s R natural reproduction number is about 3 without intervention. The virus spreads exponentially if we do nothing. By the same token, if a counter measure is effective, the number of cases it can avert is also exponential. If you are exposed and can resist infection by wearing KN95 respirators, not only are you saving yourself, you are also breaking the chain reaction of infection. Your resistance can actually avert possibly hundreds of cases of infections. We have put together a simulation to illustrate what the infection spread looks like with an R value of 3, 2, 1 respectively and how big a difference a single resistant individual can make in slowing down the virus and averting potential infections.
It is critically important that all of us and each of us do our share to help break the infection chain. We should stop relying on unproven face masks and face coverings. We can do better by upgrading from masks to respirators.
The N95 and KN95 respirators have proven consistent bio-hazard filtration efficiency greater than 95%. KN95 respirators are affordable, immediately effective, easy to use and can be used by anyone, anytime and anywhere. They can protect us even when we are exposed to an infectious mask-less person.
The more people wear respirators, the more we can help lower R, with or without a shutdown. Every civilian can participate in “the resistance” against the virus, at the personal level, at the household level, and at the street level.
A virus’ purpose is to reproduce, a respirator’s purpose is to prevent the virus’ penetration into our lungs. Our mission now should be to reduce exposure, resist infection, lower the R, stop the spread, and save ourselves, our loved ones and our society as a whole.
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