Respiratory infections, such as Covid-19, can be transmitted, according to the World Health Organization through droplets emitted by an individual during a conversation, sneezing or coughing fit.
Cdroplets emitted by respiratory activities have a wide size range ranging from a few tens of nanometers to a few hundred micrometers. The finest of them can remain in suspension in the atmosphere for several hours and therefore travel long distances depending on the conditions (anemometry, temperature, humidity).
Recent studies have also demonstrated the viability of the SARS-CoV-2 virus in these droplets on durations of several hours. Therefore, wearing a mask is recommended to avoid the emission as well as the reception of these contaminated droplets emitted by an infected person and thus limit the spread of the virus.
At the start of the Covid-19 health crisis, France experienced a shortage of FFPx masks (protecting both the wearer and their environment, there are several types depending on their degree of protection, including the now famous FFP2) and surgical masks (protecting the environment from the wearer's secretions). These do not protect from the outside because they are not perfectly waterproof when worn, but allow the droplets emitted by the wearer to be retained.
Recycling of single-use masks
In view of the difficulties in supplying single-use masks, it appeared necessary to develop a new type of mask for the general population: the reusable barrier mask and to adopt new strategies by recycling single-use masks.
Regarding this second aspect, a consortium of scientists, doctors and industrialists has been set up to test the feasibility of reusing surgical masks after disinfection and washing. As part of this project, the Reactions and Process Engineering Laboratory (joint CNRS / University of Lorraine unit) and the Nancy CHRU studied the performance of a dozen references of surgical masks that have undergone washing cycles to ensure their decontamination.
According to standard EN 14683 + AC (August 2019), “Masks for medical use: requirements and test methods”, two criteria are retained for the classification of masks for medical use: a criterion of breathability and a criterion of effectiveness of masks against a Staphylococcus aureus bioaerosol with an average aerodynamic diameter of 3 +/- 0,3 µm (one micrometer = 1 thousandth of a millimeter). To these two criteria is added a third characterizing the resistance to synthetic blood splashes for type IIR masks. According to their performance, three types of surgical masks are thus defined.
In the context of this study, the protocols of standard EN 14683 + AC were adapted and the filtration efficiency determined for a test aerosol consisting of oil droplets (di-ethyl-hexyl-sebacate) rather than a bioaerosol of Staphylococcus aureus.
The experiments demonstrate a conservation of the breathability and the effectiveness of these masks with respect to droplets of 3 μm after washing at 60 ° C. This conservation of performance is observed on all the masks, whatever their type (I, II or IIR), up to 10 washes for certain references. The maintenance of the filtration properties thus suggests the absence of modification of the internal structure of the mask during washing. The filtration efficiency of washed surgical masks remains well above the minimum efficiency necessary for the accreditation of masks for non-sanitary use (70% for UNS2 and 90% for UNS1)
The property of resistance to projections of synthetic blood is no longer guaranteed after a few washes. If the loss of this property prohibits their use in operating theaters, they are nonetheless effective against droplets.
Since the droplets generated by an individual during respiratory activities have very variable sizes, the effectiveness of surgical masks has also been determined for sizes smaller than that recommended in the standard in force. For diameters less than one micrometer, a drop in collection efficiency is observed, from the first wash, on the majority of surgical masks.
However, the filtration efficiency of these washed surgical masks remains higher, over the entire particle size range (from a few tens of nanometers to a few micrometers), than that of the vast majority of accredited barrier masks (masks for non-sanitary use type 1 or 2); and probably to that of "homemade" masks worn in the personal and / or professional context and whose performance has never been evaluated.
This drop in efficiency does not recur for subsequent washes and is attributable to the removal of electrostatic charges present on the surface of the fibers when new. The efficiencies of a washed mask and a mask discharged according to the protocol of standard NF EN779 are in fact similar over the whole of the particle size range.
The press release from the National Academy of Medicine of September 7 also specifies that "fabric masks can be washed by hand or in a machine, with a detergent, such as underwear, at a temperature of 60 ° C. being no more justified for washing masks than for washing hands ”. The recommendation for tissue masks can certainly be applied to surgical masks intended for non-medical use.
Augustin Charvet, Lecturer in Process Engineering, University of Lorraine; Dominique THOMAS, Professor in Process Engineering specializing in aerosol filtration, University of Lorraine et Nathalie Bardin Monnier, Lecturer in Process Engineering, University of Lorraine
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