Does your mask help against a virus infection?

What should be taken into account when manufacturing mouth and nose masks? When does a fabric mask really protect and when is it completely useless?

Now everyone needs a mouth and nose mask

In Bavaria, all citizens must wear masks on public transport and in shops
Since 27.04 2020, the wearing of mouth and nose masks has been mandatory in Bavaria in local public transport and shops.
The decree of the Bavarian state government explicitly states that the public should not wear medical masks, but so-called „community masks“ made of cloth or even a scarf worn over the mouth and nose.
So many citizens now ask themselves the question of what material the mask they are to buy or make should be made.
One can already see offers and advertising for protective masks everywhere. They offer models made of different materials and some with exchangeable filters.
Handicraft instructions make the rounds, skilled craftsmen provide themselves, relatives and friends with self-sewn masks.

But does every mask fulfil its purpose? What purpose can be fulfilled at all? What does it depend on? Which material should be used? What do you have to pay attention to?

Everyone wears an available mask by now. But one should also understand how oronasal masks work in connection with the droplet infection of the corona virus. This article is intended to help with this. In this article we will talk about the basics of the virus infection via the air we breathe and the characteristics of the different masks. At the end I will go into the question of a suitable mask and the right behaviour. So it becomes clear to everyone if the own mask is sufficient or if it is better to change to another model.

How is the Corona virus transmitted?

Biophysical facts about transmission

The virus lurks on the mucous membranes of mouth and nose

In an infected person, SARS-CoV-2, as the official name of the coronavirus is known, multiplies in infected cells and also enters the mucous membrane secretions in the mouth and throat in large quantities.
At this point in time, the now already infectious person does not yet have to show any symptoms of the disease, as these only appear up to 14 days after infection.

Every air stream that passes over the mucous membranes absorbs moisture from the mucous membranes. When breathing out, this liquid, which can also contain virus particles, flows outwards in the form of tiny water droplets via the mouth or nose. If another person breathes in this cloud of water droplets, which is also called an aerosol, he or she can become infected with the virus it contains.

The aerosol – the means of transport of the virus

The aerosol consists of tiny, invisible, finely distributed water droplets. The aerosol droplets are so small that they only have a diameter of less than 1000 nm to 5000 nm.
1000 nm correspond approximately to the punctuation mark dot of a daily newspaper, if you reduce this 200 (!) times.
The SARS-CoV-2 virus has an average diameter of 100 nm. So the aerosol droplets are about 10 to 50 times larger than the virus.

Figuratively speaking, the virus particles use the aerosol as a means of transport. Where and how the „transport aerosol“ moves the virus has been the recent subject of much scientific and technical research and here is the result.

The dynamics of the behaviour of man-made aerosols described here are essentially based on the very recent flow analysis of Prof. Dr. Christian Kähler from the University of the Federal Armed Forces Munich of 11.04.2020, quoted below.

How do aerosols spread in humans?

During normal breathing through the nose, the outflow velocity of the aerosol is a maximum of 0.5 m/s and the air movement ends about 30 cm from the nose.

When speaking calmly, the outflow velocity through the larger mouth opening is only 0.3 m/s and the air movement ends after 20 cm.

When coughing, a maximum of 4-5 m/s is already reached, the air movement reaches up to 1.50 m here.

With a prolonged irritable cough, an interaction effect occurs in the air, which moves considerably more air masses in total. Thus, the contaminated area can also reach 2 to 3 meters. Since the air usually flows downwards at an angle, the person who coughs several times would have to tilt his head backwards for this distance in order to disperse the aerosol horizontally as far as possible. The knowledge of the oblique aerosol flow is particularly interesting for the parents of children or infants in prams who are not at eye level with a coughing person.

When sneezing, exit velocities of up to 45 m/s are generated. However, since sneezing is not a relevant symptom in SARS-Cov-2 infection, it is currently considered to be of secondary importance. In addition, the droplets of up to several millimetres in size that are produced when sneezing get caught in any mask material.

Do aerosol clouds disappear again?

Water droplet aerosols disappear quite quickly in two different ways:
Evaporation and gravity act simultaneously on the droplets, with the smaller droplets evaporating faster and the larger droplets sinking to the bottom faster.
A droplet with a diameter of 300 µm falls at a speed of 1 m/s.
A 100 µm droplet sinks to the bottom at a rate of about 30 cm per second and simultaneously evaporates within ten seconds.
A droplet with a diameter of 1 µm evaporates after a thousandth of a second. The evaporation of water in a virus-laden aerosol particle produces so-called „droplet nuclei“, which are so light that they can remain suspended in still air for a long time.

Which safety distances are sensible in order not to inhale foreign aerosols?

The following safety distances result from the information in the previous section:

For persons without any masks:

Breathing through the nose: 1 metre
Speaking: 1-1,5 meters
Coughing shock (one lung volume): 1,5 meters
chesty cough (several lung volumes): 3 meters

Masks as infection protection

Why masks do not work intuitively

From our experience with everyday physics, we may intuitively expect a mask to be tight, after all, this is certainly true for visible light. Bank robbers, carnivalists and Marvel heroes rely on it.
But the truth is that an aerosol can pass through a mouth and nose mask. How much of the aerosol passes through the mask depends, among other things, on the speed and size of the aerosol particles on the one hand and on the material and tightness of the mask on the other.

There are different kinds of mouth and nose masks that are permeable to aerosols in different ways.

What is the aerosol permeability of the different mask types?

FFP (corresponding to KN95 and N95)

KN95 mask
A KN95 mask prevents both inhalation and exhalation of aerosols

The so-called filtering face piece masks (FFP) are aligned to a defined filter passage. These masks are used in all areas of occupational safety where particles occur during work that damage the lungs when inhaled. There are three protection classes for FFP masks, which differ in their overall permeability.
The total permeability is 25% in protection class 1, 11% in protection class 2 and 5% in protection class 3. It results from the filter permeability and the tightness of the mask. The filter transmission of FFP masks is 600 nm (0.6 µm).
This means that the mesh size is sufficient to stop even smaller aerosol droplets, even if the isolated virus with a size of 100 nm could still pass through.
The general particle penetration rate for this mask type was determined to be 0.01 to 0.1%.
The European standardized FFP masks find their functional equivalent in the N95 masks of the USA and the Chinese KN95 masks.

Surgical masks = medical masks

surgical mask
Surgical masks filter out only part of the particles in an aerosol

The main purpose of the medical mask is to protect the wearer from secretions and other liquid or solid excretions.
It also provides a certain filtering function and traps all particles larger than 2.5 µm (2500 nm) and still 60% for particles the size of corona virus around 100 nm in diameter.
The general passage rate of particles was determined to be 40% for this mask type.

Community-Masken = Textilmasken = Stoffmasken

Community mask
Community masks allow aerosols to pass almost unhindered

Cloth masks, fabric masks or community masks are mouth and nose masks that are made from available everyday materials as a temporary solution. They owe their existence to the fact that occupational safety masks and medical masks are only available in very limited quantities during the pandemic and should only be used in their intended professional field.
The Max Planck Institute has examined a number of materials for their filtering effect, including vacuum cleaner bags, multi-layer cotton, microfiber cloth, kitchen roll, paper tissues and coffee filters. The results can be found in the Sources section linked at the end of this article.
In conclusion, although considerable differences in filter behaviour between the materials can already be seen, hardly any of the fabric masks go beyond a separation efficiency of 20 to 40% for particles of 100 nm and 20% to 70% for particles of 2.5 µm.
The general penetration rate of particles was determined at 97% for this mask type.

Size comparisons and mask permeability

diameter ofsize in nmsize in µmfiltration efficiency
punctuation mark „dot“ in a daily newspaper200.000200
hair70.00070

> 5.000
cotton mask
large aerosol droplet5.0005
medium aerosol droplet2.5002,5

2.5002,5surgical mask
small aerosol drop1.0001
house dust particle1.0001

6000,6FFP-mask

3000,3N95-Maske(95%-Abscheideeffizienz)
SARS-CoV-21000,1

Why medical staff do not use fabric masks to protect themselves

Both fabric masks and medical masks are more or less permeable to pathogens. In addition, the physical permeability properties of a mask do not show any indication of actual effectiveness in everyday clinical practice.
Is there therefore a difference in effectiveness between medical disposable masks and fabric masks? If not, the use of reusable masks would be worth considering for reasons of sustainability and resource conservation. A Vietnamese study from the year 2015 may provide an indication.

This study examined the efficiency of wearing medical masks on the one hand and fabric masks on the other hand with regard to viral infection in the respiratory tract.

How was the medical staff examined?

1868 patient care workers from 14 different hospitals in Hanoi, Vietnam were randomly assigned to three groups.
Group 1 was to wear medical masks throughout the shift. For the 8-hour shift they were provided with medical masks made of 3-layer non-woven material.
Group 2 wore a fabric mask during the entire shift. These fabric masks consisted of two layers of cotton fabric and were washed and dried with soap and water after each shift.
As a control group, group 3 wore masks only temporarily, as previously required for hygienic reasons. It would have been unethical to tell this control group to omit the masks completely.

For four weeks the participants documented how much they worked in total, how much time they worked with patients and how often they were confronted with aerosol generating procedures, such as suctioning the airways, accompanying coughing maneuvers, bronchoscopies and endotracheal intubations.
Each participant documented daily the cleaning procedure of his mask and his own body temperature.
All these factors were included in the evaluation.

Now it was examined how many of the participants were infected with respiratory viruses.
For this purpose it registered whether

  1. symptoms of a clinical disease of the respiratory tract occurred
  2. a flu-like infection with fever >= 38 degrees Celsius occurred or if
  3. a viral infection of the respiratory tract could be detected in the smear using PCR.

Could a difference be seen between the wearers of medical masks and the wearers of fabric masks?

Yes, there were significant differences. Disease rates were lowest in the group that wore the medical masks throughout.
In contrast, the disease rate was significantly higher in the group wearing the fabric masks. The control group was between these two groups.

Why fabric mask wearers probably suffer more frequently from viral infections

As we can see from the study, fabric masks do not protect as well against infections as medical masks, even for hygienically trained personnel in hospitals. All the more reason why the use of masks outside the hospital by medical laymen is problematic for various reasons:

  • The permeability of fabric masks for aerosol particles is very high. The decisive pathogen-affected parts pass these masks almost 100%.
  • Reuse and the nature of the fabric mask: The fabric binds moisture, pathogens diffuse into it and the moist mask becomes a pathogen reservoir.
  • Touching and reusing contaminated masks can cause self-contamination.
  • a false sense of safety is created, which leads to neglect of essential hygiene measures such as thorough hand washing or keeping away from other people

So do only medical masks offer protection against infection and are cloth masks, as they are currently intended for laymen, completely useless?

The practical conclusion from the studies and analyses

The two decisive characteristics of a mouth nose mask

An mouth nose mask can protect the wearer (self-protection) by preventing him/her from inhaling any. A mask can also reduce the exhaled aerosols of the wearer and thus protect others from infection (protection of others).

Self protection: Some masks protect the wearer from inhaling viruses

  • Coarser drops and small amounts of liquids are kept away from all masks
  • Touching the face with contaminated fingers is avoided by all masks, especially the smear infection in mouth and nose is prevented. On the other hand, a mask contaminated from the outside is of course a risk factor for self-contamination, as the above-mentioned study has shown
  • All masks, which prevent the passage of aerosols containing viruses due to their impermeability and negligible permeability for particles smaller than 1 µm, protect the wearer from virus infection. The FFP, KN95 and N95 masks meet this requirement.
  • Masks that are made of a material that allows only some of the particles to pass through reduce the risk of infection accordingly.

Which material has which impermeability?

materialAerosol particles up to a maximum of 500nm, which are stopped by the mask
(for comparison: SARS-CoV-2 100nm, aerosol particles 50nm – 5000nm)
Vacuum cleaner bag, Micropor, 1-ply100%
Vacuum cleaner backup filter90%
Cotton and vacuum cleaner bag fleece, 1-layer80%
surgery mask66%
2-ply kitchen roll with 1-ply paper tissue or 2 plies of firm cotton or 1-ply coffee filter< 40%
2 layers of simple cotton fabric with 4 layers of gauze in between or 2 layers of molleton< 30%
1-ply T-shirt with 1-ply beaver bedding or 2-ply dishwashing towel or 3-ply plain cotton fabric or 2-ply plain cotton fabric or 2-ply cotton towel with interlining fleece or 1-ply molton< 20%
2-layer jersey T-shirt fabric or 1-layer microfibre towel or 1-layer microfibre towel or 2-layer muslin< 10%

Protection of others: Some masks reduce the aerosol beam propagation at the wearer and thus the minimum distance to be maintained

As the vivid analysis by Prof. Kähler impressively shows, surgical masks reduce the aerosol jet propagation by about half. This is related to the flow resistance of the mask, which is a result of the tightness of the material and the tightness at the mask edges.
Regardless of the permeability profile, any other mask with comparable resistance values is also suitable for producing this effect.
Therefore, anyone who manufactures a community mask for themselves or others should make sure that the materials used and the impermeability at the edges are suitable for imitating the resistance of a reference surgical mask as far as possible.

Each mask that applies the flow resistance of a surgical mask modifies the aerosol jet range as follows:

For persons wearing masks:

Breathing through the nose: 50 cm (without mask 1 meter)
Speaking: 50 cm (without mask 1 – 1.5 meters)
Coughing shock (one lung volume): 1 Meter (without mask 1.5 meters)
chesty cough (> 1 lung volume): 1,5 Meter (without mask 3 meters)

WHICH MATERIAL INCREASES THE FLOW RESISTANCE SUFFICIENTLY?

materialResistance in relation to the surgical mask
Coffee filter 1 layer225%
2 layers of strong cotton170%
2-ply kitchen roll with 1-ply tissue160%
3 layers of simple cotton fabric125%
2 layers of simple cotton fabric with 4 layers of gauze in between125%
Backup filter for vacuum cleaner between 2 layers of simple cotton105%
surgery mask100%
insufficient:
one ply microfibre cloth or 2 ply muslin or 1 ply microfibre cloth Terrycloth or 1 ply molton or 2 ply jersey T-shirt fabric or 1 ply T-shirt or 1 ply beaver bedding or 2 ply dishwashing towel or 2 ply cotton cloth with interlining fleece or 1 ply vacuum cleaner bag fleece or 1 ply vacuum cleaner bag Micropor or 2 ply molton
< 100%

The ideal and the useless mask

The theoretical benefit of a mask is therefore evaluated according to its functions of self-protection, protection of others and the combination of both. Of course, the derived usefulness in the current pandemic situation is initially only hypothetical and had to be proven by practical testing.

An ideal mask does not allow the wearer to inhale aerosols and creates the highest possible exhalation resistance to protect others from the wearer’s aerosol.

A useless mask allows aerosols to pass unfiltered and does not create sufficient exhalation resistance to reduce the wearer’s aerosol.
Incidentally, the scarf in front of the mouth and nose, which is currently considered sufficient, is of course also a useless mask in this sense.

What you should consider when using a mask:

Protection

  • A mask should at least meet the requirements of protection of others, because otherwise the only remaining disadvantages are the uncomfortable breathing and the characteristic of being a germ reservoir in front of mouth and nose.
  • Every mask is useless and pointless at the moment when you breathe with your mouth or nose under or over it. Masks should be worn as close as possible.
  • Fabric masks filter out almost no aerosols and thus no SARS-CoV-2 viruses
  • Therefore, the minimum distance to other people must always be maintained, especially with fabric masks. This also applies to all masks without FFP analog filters.
  • Cloth masks must not lead to a false sense of security, and the hygiene rules of hand washing, for example, must continue to be observed.
  • Staying outside should be preferred to staying indoors, as wind outdoors disperses aerosols, while this effect is eliminated in still air. Of course, you do not need to wear a mask when walking alone or driving a car, or in other situations where you are all alone.

Hygiene

  • Masks are potentially contaminated from the outside and should never be touched from the outside, this is particularly relevant when putting on and taking off the mask
  • Fabric masks should be changed if they become damp or even wet
  • Fabric masks should be washed frequently

Material

  • A mask manufactured in-house must not be made of material that itself contains harmful substances. Most manufacturers of vacuum cleaner bags warn against using the bag material as a mask.
  • Of course, fabric masks do not have certification and represent a temporary solution with limited functionality

Should I replace my mask or not?

As long as not enough FFP masks and surgical masks are available, these masks should be reserved for professional users. After all, the risk of getting infected is much higher in intensive care units and hospitals, nursing homes and old people’s homes.
Laypersons, on the other hand, should make every effort to wear a community mask with a flow resistance as similar as possible to that of a surgical mask, so that their own aerosol is slowed down and foreign protection is provided.
As shown above, a single-layer coffee filter or two layers of thick cotton fabric or three layers of simple cotton fabric are sufficient here.

Wearing a community mask with at least the flow resistance of a surgical mask and keeping a distance of 1.5 meters from other people are the two measures that all laypersons should observe in order to protect each other in public.

In the fight against the pandemic, knowledge about mouth and nose masks is important

Once again, knowledge and information is an important factor for the effectiveness of the improvement and progress of a disease, in this case for the containment of a pandemic.
In this respect, everyone should use the knowledge gained here about effective and ineffective masks in the current situation for themselves and pass it on to others. This knowledge will certainly not only be relevant at the moment, but will remain so in the future.

How nice would it be if existing background knowledge would lead to a wise choice of effective material for self made cloth masks. They are already often skillfully made very colorful and with a lot of love and if we add effectiveness then social cohesion in the pandemic would become even more obvious.

And the knowledge also helps to look at everyday situations with different eyes from now on:
If you have someone in front of you and this person blows his nose into a handkerchief, then by no means “everything” ends up in the handkerchief. Most of the aerosol flies with 90% of the particles under 500nm through the handkerchief and another 1.5 meters.

Bless you!

Sources

MacIntyre CR, Seale H, Dung TC, et al. A cluster randomised trial of cloth masks compared with medical masks in healthcare workers. BMJ Open. 2015;5(4):e006577. Published 2015 Apr 22. doi:10.1136/bmjopen-2014-006577, indexed in Pubmed: 25903751

Kähler, Christian (2020): Flow analyses to validate SARS-CoV-2 protective masks, [online] https://www.unibw.de/lrt7/report_mask-investigation_unibw_lrt7_06_04_2020.pdf (30.04.2020)

Drewnick, Franz (2020): Abscheideeffizienz von Mund-Nasen-Schutz Masken, selbstgenähten Gesichtsmasken und potentiellen Maskenmaterialien, [online] https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=2ahUKEwj_1q_i0YvpAhWZFcAKHd86BE0QFjABegQIAhAB&url=https%3A%2F%2Fwww.mpic.de%2F4646696%2Ffiltermasken_zusammenfassung_08_04_2020_v3_final.pdf&usg=AOvVaw0Mi_2HgmXsCdxlEpUCaroj (30.04.2020)