We prepared a mannequin head from a commercially available dummy head (Respironics, Murrysville, PA, USA, Figure 2).
Nasal and mouth openings of the mannequin connect to a tube that exits the head on the backside. The sensitive areas of the face were covered with a soft wound dressing (Allevyn plus Adhesive, Smith+Nephew, London, UK) in order to simulate the elastic properties of human skin.
Experiments were conducted inside an airtight transparent plastic box of 70 liters volume (Iris 70 liter, model # 135455, IRIS Ohyama, Sendai, Japan, figure 3).
We installed in total four 22 mm tube connectors/outlets as shown on figure 3 which were sealed with a two-component bonding agent. The connector on the front side (front1) of the box was connected to a nebulizer (Pari LC Sprint Star, Pari, Starnberg, Germany), the exhalation port of the nebulizer was sealed with a filter (Iso-Gard #19212, Teleflex Medical GmbH, Fellbach, Germany) in order to prevent contamination. The three connectors on the backside of the box were utilized as follows:
The first outlet (back1) was connected to the backside of the dummy head inside the box by means of a piece of standard single use ventilator tubing (1574000, Intersurgical, Sankt Augustin, Germany). The second outlet on the backside (back2) was connected to a tube that extended freely into the box and ended abreast to the frontside of the dummy head. The third outlet on the backside (back3) was inserted for free air circulation during simulated ventilation. Back1, back2 and back3 outlets were equipped with a filter in order to collect the radioactive particles. Outlets back1 and back2 were then connected to separate chambers of an artificial lung (dual adult test lung model 5600i, Michigan Instruments, Kentwood, MI, USA figure 3). The two chambers of the artificial lung were interlocked for simultaneous use.
Aerosol and aerosol application:
We nebulized isotonic saline containing 150 MBq 99mTC-DTPA (99mTc-diethylenetriamine pentaacetate (DTPA)) per ml by means of a Pari LC Sprint Star nebulizer (Pari, Starnberg, Germany, figure 3). The nebulizer was filled with 6 ml solution. The nebulizer output Rate equals 360 - 500mg/min. The aerosol had a mass median aerodynamic diameter (MMAD) of 2.4 – 3.3 µm. For every test run the nebulizer delivered the aerosol continuously for 25 seconds.
Simulation of breathing:
Breathing was simulated 10 seconds after nebulization was stopped by manual movement of the two blocked chambers of the breathing simulator for ten times within 50 seconds, generating a volume of one liter per lung-chamber. The volume was visually controlled by means of the build in volume scale of the Michigan lung. The respiratory rate was controlled by means of a metronome (EumLab for Iphone, Xanin Technology GmbH, Berlin).
Measurement sites and measurement of radioactivity:
Filter back2 served as a reference for unfiltered breathing, filter back1 collected radioactive particles that passed through the dummy head. A test run without filtering devices on the dummy face revealed equal aerosol deposition on outlets back1 and back2. We confirmed complete absorption of the aerosol by a single filter by placing a second filter behind the first one which showed no radioactivity after a test run. Filters were then positioned under a Gamma camera (ECAM Scintron, Medical imaging electronics GmbH, Seth, Germany) and radioactivity was counted for one minute. Regions of interest (ROI’s) were placed around the spots of back1 and back2 filters (figure 4). A third ROI apart from the filter spots measured background activity. Activity was measured in total counts per minute.
Tested filtering devices are shown in figure 5 and 6:
The devices were mounted onto the dummy face as recommended by the manufacturer. Mouldable nasal straps of surgical, FFP3 and N95 masks were adjusted to optimal fit the nose bridge. Noninvasive ventilation masks were attached with the harness, the ventilation port was connected to a filter as shown in figure 1. Every setting was tested three times. For every test run a new device was mounted.
We also measured the efficacy of a surgical mask cloth that was stretched and fixed over a funnel as well as the filtering efficacy of a woven scarf wrapped around the dummy head.
Analysis and statistics
In every test run the filter activity of back1 represents the amount of unfiltered particles.
Activity in filter back2 serves as the unfiltered reference. Background activity was subtracted from back1 and back2 filter counts. The ratio back1 and back2 represents the efficacy of the applied filtering device in %. One hundred minus efficacy represents the ratio that passed the tested filtering device. For multiple comparison we used ANOVA. The LSD test was used for post hoc analysis. A p <= 0,05 was considered significant. SPSS version 26 was used for statistical analysis.