SARS-CoV-2: Air purifiers remove 90% of aerosols in school classes
In an experimental study, installing 4 commercially available air purifiers reduced the concentration of aerosols in a classroom by 90% within half an hour. According to the study in medRxiv (2020; DOI: 10.1101 / 2020.10.02.20205633 ), the students and teachers found the noise from the devices mostly not disturbing.
SARS-CoV-2 and other respiratory viruses are transmitted via droplets, aerosols and by touching objects or people. Droplets are retained by a mouth and nose covering. Hand disinfection helps against transmission through touch. A protective measure against aerosols that are only incompletely retained by a mouth-nose cover could be air purifiers that are offered in specialist shops or hardware stores.
In addition to a pre-filter for coarse dust and an activated carbon filter for larger particles, these devices contain a “High Efficiency Particulate Air Filter” (HEPA), which can remove particles from 0.1 µm to 0.3 µm in size from the air.
The devices tested by a team led by Joachim Curtius from the Experimental Atmospheric Research working group at the Goethe University in Frankfurt used a HEPA filter according to the US standard (comparable to H13), which, according to the manufacturer, contained more than 99.95% of the fine particles removed from the air.
The tests took place in the classroom of a grammar school with a length of 8.24 m, a width of 6.18 m and a height of 3.66 m, where 27 students were taught.
The researchers distributed 4 devices in the classroom and determined the concentration of the aerosols and the CO at 2 points in the room during the lesson 2 -Concentration. The measurements were also carried out in an adjacent room in which no air purifiers were installed.
The room was ventilated before the start of the lesson. All 6 windows on the long side and the door opposite were opened. This lowered the CO 2 -Concentration. At the same time, however, there was an increase in the aerosol concentration in the air.
During the lessons the total concentration of the aerosols then decreased again. In the room without an air purifier, there was a decrease of about 30%. The researchers attribute this to the particles sinking to the surface and to the shrinkage processes in the aerosols. The humidity, the electrical charge of the particles and the electrostatic charge of the surfaces could also play a role here.
The air purifiers accelerated the waste of aerosols. The devices put together between 760 and 1,460 m 3 Air per hour, which means that the room air was filtered about 5.5 times per hour. According to the measurements, this resulted in a continuous drop in the aerosol concentration. It fell more than 90% in 37 minutes from the start of the lesson. According to the study, the reduction affected all particle sizes equally.
In a model calculation, the researchers estimate that the air purifiers reduce the risk of a “super spreader” infecting other people in the room with SARS-CoV-2 by a factor of 6. However, it is a calculation with various imponderables, so that the result remains speculative.
One way to reduce the concentration of aerosols in the classroom would be to do without ventilation. According to Prof. Curtius, however, this would not be a good idea because the air purifiers remove the CO 2 do not remove from the air and do not add oxygen.
The measurements showed that the CO 2 -Values were often above the recommended limit values. The researchers therefore recommend that schools install CO2 sensors so that students and teachers can check when they should ventilate the room again.
The air purifiers used are offered for around 150 euros each – in individual quantities from around 260 euros. One disadvantage is the noise pollution. However, according to the study, a survey among the students and teachers showed that the noise of the air purifier was mostly not perceived as disturbing, provided the device was not running at the highest level. © rme / aerzteblatt.de
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