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The two-story measuring apparatus in the Department of Aerosol Chemistry and Physics within the Institute of Chemical Processes of the Czech Academy of Sciences has been running almost non-stop for a week.
The depopulated workplace, in whose laboratories only five employees remained out of twenty-five due to the nationwide quarantine, is now trying to answer a pressing question. Which nanofiber fabric captures the maximum amount of the tiny coronavirus sized 80 to 150 nanometers? If the nanofibers could be arranged effectively, such a mask could in its functionality match the best respirators. The department, led by chemist Vladimír Ždímal, is now collaborating with the Czech Technical University in Prague and the Technical University of Liberec.
The first of them is working on a new reusable respirator and the second on producing a nanofiber material that would serve in them as a filter to capture even such tiny particles as viruses.
“You cannot generally say that nanofiber materials are better. It very much depends on their structure,” says Ždímal. For comparison, his workplace also tested several substances commonly available in households. “I sacrificed a work cotton T‑shirt for it, a silk handkerchief and also a scarf. My wife wasn’t very happy about that,” he points to the perforated fabrics. They placed large circular cutouts of them into their device and passed an aerosol with salt particles through them for several hours, the particles imitating the size of the virus.
In measurements they confirmed that, as far as protecting others is concerned, practically any covering of the mouth and nose works. At such a short distance the droplets are still large enough that even an ordinary cotton mask will catch them. “In this case the droplets are roughly from 0.1 to 1 millimeter and the mask’s capture efficiency is close to 100 percent,” states the head of the workplace.
With uncovered mouths the freely flying millimeter droplets evaporate very quickly, within a distance of half to one meter. Beyond that point only the viruses themselves sized 80 to 150 nanometers continue in flight. Therefore masks, conversely, do not work very well as protection against viruses from the surroundings. One layer of cotton fabric in such a case captures about 11 percent of viruses. “But that is a value corresponding to the inhalation of a heavily working person. Protection can be significantly increased by reducing the inhalation rate,” adds Ždímal.
In their measurements the scarf performed best, approaching twenty percent of captured viruses. In laboratory conditions, however, they did not address, for example, the mask’s ability to fit the face, the structure of the materials, or their humidity, which is also crucial for their function. “Once a standard mask becomes damp, it becomes permeable and unprotective. And that happens after about twenty minutes,” said Jiří Beneš, an infectious disease specialist from Bulovka Hospital, to DVTV. Ždímal confirms this as well. “Nevertheless, the droplets will at least be smaller and will evaporate faster, so even such a mask has some significance for protecting others,” Ždímal believes.
Viruses trapped in droplets in the mask generally remain in it. Exhalation through the mask should not release them into the surroundings. According to Ždímal this is caused by adhesion forces, which are much greater than the force of air during exhalation.
In the past the institute also dealt with evaluating respirators. Even those do not provide one hundred percent protection against viruses from the environment. “The best ones let through about 0.3 percent of viruses, and that is in laboratory conditions where they are perfectly sealed. But on the face you always have some leak. Even a gap with a diameter of a tenth of a millimeter is for a virus like a hole the size of an entire Prague city district,” warns Ždímal. Moreover, the virus perfectly follows air streams, so it does not have to negotiate any complicated turns.
Even healthcare workers with respirators are therefore not absolutely protected, and during a long stay in an infected environment they can also become infected.
On the other hand, when the virus penetrates in small doses, the immune system usually manages to cope with it.
“Only professional gas masks work more effectively than respirators. These—if well sealed—might let through only one virus in ten million,” says Ždímal.
