Grzelak bases most of the data for his Seminar on the work of Professor and Scientist Supratik Guha (Pritzker School of Molecular Engineering, University of Chicago; Argonne National Laboratory) and Professor Yi Cui (Department of Materials Science and Engineering, Stanford University). Jan Grzelak's research is not about this topic, but from the ICMAB we wanted to support this initiative, in which he looked for information regarding face masks from the point of view of materials science.
Face masks: materials and tightness
A big part of understanding what the best materials are for mask design requires knowledge on how the virus spreads. Grzelak explains:
“The SARS-CoV-2 virus travels in air inside liquid droplets emitted by coughing, sneezing, but also normal speech. Large particles (over 5 μm) settle rapidly, but smaller droplets can stay in the air for a long time and can enter the respiratory tract. It is important to protect oneself and the others from inhalation of droplets by wearing face masks.”
Each type of facemasks has to comply with different standards. There are surgical masks that filter at least 95% of 3-micron airborne particles and are intended mostly for protecting the environment from the user. The particle respirators are theoretically more efficient and they have to comply with a standard concerning filtration of 0.3 μm. The masks known as N95 meet the US standard, while the FFP (filtering face piece) respirators meet the European standards. N95 respirators capture at least 95% of 0.3 μm particles, while FFP2 and FFP3 stop at least 94% and 99% of such particles, respectively. These are the masks that were analyzed in the studies described here.
This makes it clear that blocking these smaller particles is a key part in avoiding the spread of the virus. Grzelak explores the different standards for particle masks, and explains that they do not actually work as a sieve, they have more complex systems that make them effective to different sizes of particle. Depending on which size of particle is being blocked, particle masks show 3 distinct mechanisms:
- The largest particles have high inertia and collide with the fibers constituting the mask.
- Smaller particles are dragged along the air stream but come close enough to the fibers to be attracted to them by forces such as van der Waals forces
- Very fine particles are captured by electrostatic attraction.
However, the early shortage of standardized and commercial masks caused the apparition of homemade cloth masks, which posed two main questions: which materials are the best to make these masks and how important is that the mask stays tight around the face, since many of these homemade versions are not as tight as the standardized models.
On homemade mask materials, Grzelak indicates:
"In case of cotton, it was observed that tighter weaves have superior efficiency (around 90% above 300 nm-sized particles). Fabrics that were expected to possess moderate electrostatic discharge value, such as silk, chiffon and flannel, performed very well in filtering nanosized particles. Increased number of layers increased the performance. Hybrid approaches were also studied and it was found that combining 600 TPI cotton (physical filtering) with silk, chiffon or flannel (electrostatic interactions) provided broad filtration coverage across both the nanoscale and the microscale."
The studies Grzelak bases his statements on (which you can find cited below) also indicate that tightness of the mask is a key factor, and any opening and gaps around the mask can “significantly decrease the filtering efficiency of masks ”.
Reuse and disinfection
Both cloth masks and respirators have been reused as an answer to this original shortage of masks. In this case, the question is how to disinfect them in order to avoid damaging the mask.
"The layer that is critical for the performance of N95 masks is the meltblown layer – made of a random, highly porous 3D network of PP microfibers that are electrically charged."
Keeping this layer effective is crucial to keep the effectiveness of these masks. Studies have been done on different methods of disinfection and how they affect this meltblown layer in order to find which one is the least intrusive:
"Treatment in ethanol or chlorine significantly decreases the filtration efficiency of meltblown fabric – the structure of the fibers remains unchanged but the electrostatic charge is decreased. A similar effect can be observed after three cycles of steam treatment – condensing water droplets lead to a decay of charge and the filtration efficiency is not enough for environments with high viral concentrations. Heat treatment (85 °C, 30 min, different humidities) did not decrease the filtration efficiency even after 20 cycles. Dry heat below 100 °C was found safe for meltblown fabric, as well as humid heat below 95 °C. UV light treatment was not recommended by the authors as UV is absorbed by PP and may not penetrate deep enough in the fabric to disinfect it completely."
These studies offer valuable information on how to choose which masks we use and how we treat them in order to stop the spread of the virus. You can read Jan Grzelak’s sources cited below or watch his Online Seminar in our YouTube channel.
Recommendations
The World Health Organization (WHO), in accordance to its updated advice from 5 June, recommends that non-medical fabric masks (homemade masks) should be worn by the general public where there is known or suspected widespread transmission and where physical distancing is not possible, and that vulnerable people (aged over 60 or with underlying health risks) and people with any symptoms suggestive of COVID-19 should wear medical masks. The stated purpose of mask usage is to prevent the wearer transmitting the virus to others (source control) or to offer protection to healthy wearers against infection (prevention).
Nowadays, a large amount of countries in the world recommend or oblige the use of masks in public places to limit the spread of COVID-19. In Spain, since 9 June, it is mandatory to wear masks on public transportation, public roads, in open air spaces, and in closed spaces for public use, especially if physical distance (1.5-2 m) is not possible.
In any case, according to WHO recomendations, masks are effective in stopping the virus spread only when used in combination with frequent hand-cleaning with alcohol-based hand sanitizer or soap and water. And it is very important to know how to use and dispose a mask if you are wearing one.
Stay safe!
References:
A modelling framework to assess the likely effectiveness of facemasks in combination with ‘lock-down’ in managing the COVID-19 pandemic
Richard O. J. H. Stutt, Renata Retkute, Michael Bradley, Christopher A. Gilligan and John Colvin
Proceedings of the Royal Society A: Mathematical, Physical and Engineering SciencesVolume 476, Issue 2238, 2020
DOI: 10.1098/rspa.2020.0376
Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks
Abhiteja Konda, Abhinav Prakash, Gregory A. Moss, Michael Schmoldt, Gregory D. Grant, and Supratik Guha
ACS Nano 14 (5), 6339-6347, 2020
DOI: 10.1021/acsnano.0c03252
Can N95 Respirators Be Reused after Disinfection? How Many Times?
Lei Liao, Wang Xiao, Mervin Zhao, Xuanze Yu, Haotian Wang, Qiqi Wang, Steven Chu, and Yi Cui
ACS Nano 14 (5), 6348-6356, 2020
DOI: 10.1021/acsnano.0c03597
Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community
Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis
Derek K Chu, MD, Prof Elie A Akl, MD, Stephanie Duda, MSc, Karla Solo, MSc, Sally Yaacoub, MPH, Prof Holger J Schünemann, MD
The Lancet 395, 10242, 1973-1987, 2020
DOI: 10.1016/S0140-6736 (20)31142-9