26. October 2020

How ef­fec­tive are non-med­i­cal masks? – A sci­en­tif­ic view of a much-dis­cussed ques­tion

Experiment room with very small soap bubbles
Ex­per­i­ment room with very small soap bub­bles
Image 1/8, Credit: © DLR. All rights reserved

Experiment room with very small soap bubbles

To cre­ate the test set-up, an ex­per­i­men­tal space mea­sur­ing 12 cu­bic me­tres was flood­ed with small soap bub­bles the size of sug­ar grains (di­am­e­ter ap­prox­i­mate­ly 350 mi­crome­tres). Filled with a mix of he­li­um and air, the bub­bles re­main sus­pend­ed for longer pe­ri­ods and fol­low the com­plex flow field in the test room.
Test dummy in the experiment room
Test dum­my in the ex­per­i­ment room
Image 2/8, Credit: © DLR. All rights reserved

Test dummy in the experiment room

A seat­ed test dum­my breathed in the test room. Its ar­ti­fi­cial lung cre­at­ed a cyclic air flow equiv­a­lent to that of a hu­man be­ing.

High-resolution streaming cameras
High-res­o­lu­tion stream­ing cam­eras
Image 3/8, Credit: © DLR. All rights reserved

High-resolution streaming cameras

Sev­er­al high-res­o­lu­tion stream­ing cam­eras – each with an im­age size of 50 megapix­els – cap­tured the move­ment of the soap bub­bles.
LED array
LED ar­ray
Image 4/8, Credit: © DLR. All rights reserved

LED array

A large LED ar­ray emit­ted pulsed light to il­lu­mi­nate the soap bub­bles in the test room.
Aero­mask vi­su­al with mask
Video 5/8, Credit: © DLR. All rights reserved

Aeromask visual with mask

Length: 00:00:06
Aero­mask vi­su­al with mask
Aero­mask vi­su­al with­out mask
Video 6/8, Credit: © DLR. All rights reserved

Aeromask visual without mask

Length: 00:00:12
Aero­mask vi­su­al with­out mask
Aero­mask vi­su­al of tur­bu­lence with mask
Video 7/8, Credit: © DLR. All rights reserved

Aeromask visual of turbulence with mask

Length: 00:00:04
Aero­mask vi­su­al of tur­bu­lence with mask
Aero­mask vi­su­al of tur­bu­lence with­out mask
Video 8/8, Credit: © DLR. All rights reserved

Aeromask visual of turbulence without mask

Length: 00:00:03
Aero­mask vi­su­al of tur­bu­lence with­out mask
  • Several DLR institutes launched a collaborative project to investigate the effectiveness of fabric masks.
  • The findings clearly show their positive contribution, although smaller aerosols can still penetrate the fabric.
  • This project was based on technology that is normally used to examine airflows in the aerospace industry.
  • Focus: Medicine

Non-medical masks are an important part of the fight against the Coronavirus pandemic. The latest research shows how human breath is deflected when wearing a mask and how this distributes the aerosols contained in the exhaled air.

Several institutes within the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) came together in an interdisciplinary, collaborative project to investigate the functioning and effectiveness of non-medical fabric masks. The experiments clearly show how wearing a non-medical mask influences the distribution of exhaled aerosols and how this contributes to preventing the spread of infection.

Filtering and aerodynamic effects

The first evaluation of experimental data provides a clear impression of how the masks work.

"The tests plainly demonstrate the positive effect of non-medical masks, although smaller aerosols can still penetrate the fabric," says Andreas Schröder from the DLR Institute of Aerodynamics and Flow Technology and the Principal Investigator of the research. A significant proportion of exhaled bioaerosols measure less than five micrometres across, but the mesh size in fabric masks is considerably larger. The aerosols pass almost unobstructed through the mesh in the investigated masks and are then carried via air currents. However, fabric masks are still effective, as they slow down and deflect the exhaled breath. Laboratory tests show that mouth and nose masks effectively decelerate the exhaled flow of air containing the aerosols.

The thermal effect of body heat makes potentially infectious particles – which remain close to the body due to the mask in calm indoor air – float towards the ceiling, where they follow the air flow and spread slowly through a room. This means that the aerosols travel further through space and are affected by turbulence in the air, which lowers their potency even more. The masks lead to a drop in concentration of any infectious aerosols, which particularly protects those in close proximity. It is nevertheless important to ventilate rooms on a regular basis to prevent any accumulation of possible bioaerosols. From a physical perspective, the volume increases with the third power of the distance (that is, the number of cubic metres), which reduces the concentration of bioaerosols. This is why it is still advisable to adhere to social distancing when wearing a mask.

Methodology

The imaging methods developed at the DLR Institute of Aerodynamics and Flow Technology are usually used for flow investigations in aerospace applications. DLR’s proprietary '3D Particle Tracking' technology has been applied in the Aeromask project to research the spread of the infectious SARS-CoV-2 virus. This technology enables precise tracking – at a scale of just a few millimetres – of exhaled airflows, their deflection by masks and the consequent transport of aerosols through closed indoor environments measuring several cubic metres. The pathways are visualised to illustrate the dynamic distribution of potentially infectious aerosols and particles in the room.

In the first phase of the project, the flow of exhaled breath and ambient air and the influence of different masks was investigated. For this purpose, an experimental space measuring 12 cubic metres was flooded with very small soap bubbles the size of sugar grains (diameter approximately 350 micrometres). Filled with a mix of helium and air, the bubbles remained suspended for long periods and followed the complex flow field in the test room.

A seated test dummy breathed in the test room. Its artificial lung created a cyclic air flow that replicated that of a human being. A built-in heater simulated body warmth and induced the associated thermal currents in the surrounding air.

High-resolution camera technology

Several high-resolution streaming cameras – each with an image size of 50 megapixels – captured the movement of the soap bubbles, which were illuminated with pulsed light from a large array of LEDs.

To conduct the analyses, the researchers developed sophisticated volumetric evaluation and data assimilation methods that tracked the trajectories of the millions of individual bubbles over time as they travelled through the test room. DLR's proprietary 'Shake-The-Box' (STB) particle tracking technique used a small number of cameras to reconstruct a very large number of their 3D trajectories in the flow, based on time-resolved images of these small bubbles. STB technology makes optimum use of the time information contained in the particle images for 3D reconstruction, whereby approximately ten times more particle trajectories can be followed in the measurement volume, compared with previous particle tracking methods.

Further project phases

Following completion of the laboratory phase, the project will move on to two more stages to acquire greater understanding of the infectiousness and movement of the aerosols as they spread through the room. In a next step, the DLR Institute for Software Technology will use the measurement data to prepare a simulation and visualisation of how the aerosols and particles move through the air. The properties of simulated bioaerosols (defined mixtures of various micro-organisms) and their interaction with masks will be investigated by the Space Microbiology Research Group at the DLR Institute of Aerospace Medicine.

The findings of the next research phases in the Aeromask project will be released in spring 2021.

Contact
  • Michel Winand
    Cor­po­rate Com­mu­ni­ca­tions, Bonn, Köln, Jülilch, Rhein­bach and Sankt Au­gustin
    Ger­man Aerospace Cen­ter (DLR)

    Pub­lic Af­fairs and Com­mu­ni­ca­tions
    Telephone: +49 2203 601-2144
    Linder Höhe
    51147 Cologne
    Contact
  • Roland Pleger
    Re­spon­si­ble for 'Space Sys­tem Tech­nol­o­gy' and 'Space Trans­porta­tion'
    Ger­man Aerospace Cen­ter (DLR)
    Pro­gramme Space R&D
    Telephone: +49 2203 601-2786
    Hansestraße 115
    51149 Köln
    Contact
  • Dr.rer.nat. Andreas Schröder
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Aero­dy­nam­ics and Flow Tech­nol­o­gy
    De­part­ment Ex­per­i­men­tal Meth­ods
    Telephone: +49 551 709-2190
    Fax: +49 551 709-2830
    Lilienthalplatz 7
    38108 Braunschweig
    Contact
  • Ralf Möller
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Aerospace Medicine
    Telephone: +49 2203 601-3145
    Sportallee 54a
    22335 Hamburg
    Contact

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