The following blog is my summary of the VLOG by Jeremy Howard & Rachel Thomas (2020-03-25)  and other web references listed below.
No warranty is giving regarding any advice given in this blog. Use at your own risk.
Since 2019-12-01 the Sars-Cov-2 virus and the resulting Covid-19 disease has been spreading from the city of Wuhan (Hubei, China) all over the World. Although the Covid-19 disease will present itself with less than severe symptoms in the majority of the population, a subgroup is expected to experience a burden of disease requiring intensive care or even leading to death. Therefore, the combination of severe symptoms requiring hospitalization and the (uncontrolled) exponential growth rate of the (uncontrolled) spreading of the disease has already done great damage to health systems and societies in specific countries and is expected to affected many other countries if no adequate measures are taken [1-5].
Countries which have successfully contained the pandemic applied the following measures:
- Rigorous testing regardless of symptoms
- Rigorous contact tracing
- Quarantine of infected individuals
- Masks for all
While measures 1) and 2) have to be performed by the government and the health system, measures 3 and 4 can be applied by the individual. While the effectiveness of measure 3) is self-evident, measure 4) has been critically discussed with various sources, incl newspaper and government channels, giving mixed messages stating the masks are not effective in containing the disease while at the same time asking to reserve masks for health professionals. My interpretation is that those sources try to avoid to show a failure of the system in that it cannot provide sufficient amounts of vitally important protective gear, ie face masks, to the public and at the same time reserve it to the group which is in greatest need, ie health care professionals.
Therefore, the following principles are applied:
- Face masks have two functions:
- To protect the person wearing the face mask
- To protect other persons
- Face masks reduce the potentially infectious viral load from
- droplet infection (sneezing, coughing, speaking),
- smear infections resulting from unvoluntarily touching your mouth and nose, and
- air borne viruses.
- The efficacy (eg virus protection) and safety (eg breathability) of face masks depends on its quality of filtration of the mask with masks being more effective in protecting others from infections than protecting the individual wearing the mask.
- Following the principle “My mask protects you, your masks protects me!”, population-wide application of face masks can still effectively protect on the individual-level by mutual protection and will reduce growth rates of virus infections on the population-level.
- Although a wide range of quality of face masks exists, ranging from professional and certified face masks to home-made masks, all masks are considered to be superior to wearing no protection at all.
The following plots illustrate the efficacy of virus protection for the individual wearing the face mask and its safety regarding breathability:
Therefore, although professional masks which are manufactured according to industry standards are preferably, substantial protection can also be achieved by home-made masks from household materials.
European standard EN 143 defines the ‘P’ classes of particle filters that can be attached to a face mask, and European standard EN 149 defines the following classes of Filtering Face Pieces (FFP), that is respirators that are entirely or substantially constructed of filtering material. Relevant quality levels are:
- FFP1 filters at least 80% of airborne particles
- FFP2 filters at least 94% of airborne particles
- FFP3 filters at least 99% of airborne particles
For other related products used for filtering particles form the air, eg Vaccum Cleaner Bags or Air Conditioning Filters, other industry standards are applied.
The European Standard EN 1822-1:2009, defines several classes of High-efficiency particulate air (HEPA) filters by their retention at the given most penetrating particle size (similar to the fine particle filtration rate of European standard EN 143):
- E10 filters at least 85% of airborne particles
- E11 filters at least 95% of airborne particles
- E12 filters at least 99.5% of airborne particles
- H13 filters at least 99.95% of airborne particles
- H14 filters at least 99.995% of airborne particles
- U15 filters at least 99.9995% of airborne particles
- U16 filters at least 99.99995% of airborne particles
- U17 filters at least 99.999995% of airborne particles
Therefore, the application of face masks of various types of materials depending on availability is recommended. Materials with higher filtration rate, while preserving breathability should be used.
Tutorials for various types of home-made face masks:
- Face Mask “Kitchen towel and tissue” with “transparency foil” visor  and improvement of fit using wire 
- Face Mask “Textile handkerchief with filter fleece” 
- Surgical face mask with whole-face protection based on plastic bottle 
- Face mask using a 3-D printer 
It is recommended to use these tutorials for inspiration and combine the recommendations (“hacks”) as appropriate.
 Jeremy Howard & Rachel Thomas (2020-03-25). We need #masks4all [VLOG]. Link: https://www.youtube.com/watch?v=BoDwXwZXsDI&feature=youtu.be
 Petr Ludwig (2020-03-14). Roušky a kritické myšlení #RouškyVšem. Link: https://www.youtube.com/watch?v=K5xy2n941jM
 Irena Kotíková (2020-03-21). OVERVIEW: Czech campaign #masks4all. Link: https://tiny.cc/masks4all
 South China Morning Post (2020-02-14). Experts devise do-it-yourself face masks to help people battle coronavirus. Link: https://www.youtube.com/watch?v=aNjpH5lBZ8w
 Genius Asian (2020-02-09). DIY face masks in 10 seconds & improve cheaper masks #coronavirus #facemask. Link: https://www.youtube.com/watch?v=Wg-cwD4Edac
 Mr. Hacker (2016-08-16). 10 AMAZING PLASTIC BOTTLES LIFE HACKS from Mr. Hacker. https://www.youtube.com/watch?v=YUi2_AwDnqE, Watch “Face Mask with Goggles” at 5m50s
 Sarada Lee (2020-03-20). Saving The Mask – How to make the reusable mask (the base model)?. Link: https://www.fast.ai/#how-to-make-the-reusable-mask-the-base-model
 Yori House (2020-03-22). How to make EASY FACE MASK in 1 MINUTE – NO SEWING! WASHABLE, REUSABLE FACE MASK [XS-XXL], Link: https://www.youtube.com/watch?v=lOe_8z8k01U