The Invisible Enemy Scientists Weigh the Benefits of Air Purifiers in Fighting COVID-19

Coronavirus pathogens can float in the air for hours, which is a significant threat. Researchers are developing ideas for how to prevent infections in schools, restaurants and offices using ventilation equipment, air filters and UV radiation.
A school in the German city of Schwerin, where classes resumed this week after the coronavirus lockdown and the summer holidays.

A school in the German city of Schwerin, where classes resumed this week after the coronavirus lockdown and the summer holidays.

Foto: Jens Büttner / dpa

Some people ignore them, while others take to the streets to protest against them, but most people in Germany are taking the rules aimed at containing the coronavirus seriously. They wear masks to cover their mouth and nose on public transportation and in shops, they avoid large groups, they wash their hands frequently and they sneeze into the crook of their arm.

But is that enough to stop the pandemic? No.

Fighting the virus will require further measures because it is becoming increasingly obvious that it's not just direct, person-to-person contact through things like coughing that are spreading the pathogen. It can also hover in the air, which can defeat social distancing efforts.

The virus can perch on microscopically small particles with a diameter of a few thousandths of a millimeter. The particles are partly solid and partly liquid and do not follow the rules of gravity. Every person constantly produces such particles in their nose and throat when speaking, and even more so when singing or yelling. Even normal breathing can produce them.

The aerosols aren’t so alarming when outdoors because the wind carries them away quickly and the sun can destroy the virus. When you’re indoors, though, it’s another story. Researchers have demonstrated that infectious aerosols can travel many meters and survive for minutes or even hours indoors.

A Surprisingly Long Lifespan

While it isn't yet known for sure how much exposure to the virus is needed for a person to get infected, it is clear that indoor air quality, whether in offices, restaurants, classrooms or supermarkets, has been an underestimated factor in the SARS-CoV-2 problem. "The lifespan of these droplets is surprisingly long," says Detlef Lohse, a German physicist at the University of Twente in the Netherlands. Because they’re so small, he says, they can be breathed deep into the lungs.

One infected person in a bar is enough to spread the infection, says Martin Kriegel, the head of the Institute of Building Energy Engineering at the Technical University of Berlin. The body heat of humans gives the aerosol particles thermal lift. "After two or three minutes, they spread all over the room,” Kriegel says. The risk of infection for those present increases with the length of their stay. His recommendation: "Keep your distance. Wear a mask. Make sure there’s proper ventilation."


The article you are reading originally appeared in German in issue 33/2020 (August 8, 2020) of DER SPIEGEL.

The example of the De Kleine Beurs bar in Hillegom near Amsterdam underscores his point. On Saturday, July 11, nobody who visited the bar and went home before 11 p.m. got infected with the coronavirus. Then, the people running the bar closed the door and the windows because of noise regulations. Of the 50 or so guests who were still there, 31 got infected.

Physicists, engineers and environmental technicians are now looking at ways of combating the flying viruses. The good news is that, as surprising as it may seem, there are numerous defensive strategies that are both effective and feasible. However, most of them do cost money - and they require people to change their behavior.

"Ventilation Is Essential"

The simplest remedy for indoor aerosols has always been around, and it’s free. "Ventilation is essential," says physicist Lohse. If you ventilate well and supply plenty of fresh air, you can quickly reduce the number of aerosol particles floating around.

But ventilation also has its pitfalls. If there is little wind outside or little difference in temperatures indoors and out, natural ventilation will only work to a limited extent. It’s not enough to keep windows permanently cracked open.

Experts recommend opening windows located across from each other completely for five minutes, two to three times a day - although it’s unlikely that many households, stores or doctors’ offices are following that advice. "We will need to learn a new system of ventilating," Kriegel says. He warns that a failure to ventilate or doing so incorrectly can be life-threatening as long as the coronavirus is with us.

This applies particularly to schools, of which there are more than 42,000 in Germany. A large proportion of them are in need of renovation, and few have mechanical ventilation systems. It has been known for decades that most classrooms have excessive levels of carbon dioxide (CO2) from the breath of students and teachers, which fatigues them and taxes their performance. The CO2 is inevitably accompanied by a high concentration of aerosols, to which pupils and teachers are exposed for many hours each day.

It would be almost impossible to provide a German classroom with the kind of ventilation that is needed. "You would actually have to leave the windows open all the time," says Wolfram Birmili, head of indoor air hygiene at the German Federal Environment Agency. But many windows can’t be fully opened for safety reasons. If they have skylights, they are too small, and the recesses aren’t long enough to properly ventilate classrooms.

Scientists in the state of North Rhine-Westphalia examined 363 classrooms and found that after just one hour of class with the windows closed, almost half were so full of CO2 that they would be classified as "hygienically unacceptable” under current workplace regulations.

Even though it could easily be solved with money, politicians have so far been unmoved by the deplorable state of affairs, despite the fact that many companies offer ventilation units for retrofitting. They work quietly and effectively, delivering fresh air and extracting consumed air and aerosols, but they are only sold in small quantities. For as long as there is no vaccine for COVID-19, though, they would significantly increase everyone's safety because they can help prevent the creation of new infection hotspots.

Berlin engineer Kriegel advises schools that they need to make major changes. Given the current situation, he says that 30 minutes of instruction should be followed by a 15-minute ventilation break.

An Alarm Signal

He also recommends that classrooms be equipped with CO2 measuring devices. If values are too high, it is also an indication that aerosol concentration is too high and the room should be ventilated. Although the devices only cost around 50 euros, Kriegel says that school officials have expressed concern that they are too expensive.

People who work in modern office buildings are better off, with such structures usually equipped with systems that mix fresh air with ambient air to save energy. In addition, the air is passed through fine particle filters that are able to capture around 50 percent of the aerosols.

The other 50 percent of the potentially infectious particles are distributed throughout the building by these systems. Still, Kriegel believes the risk of infection arising from that distribution is rather low, because the concentration of the contaminated aerosols relative to the total volume of air in the building is likely to be low.

Christian Kähler, a physicist and a professor of fluid dynamics at the University of the Federal Armed Forces in Munich, has spent 25 years researching aerosols. He would like to see even greater steps taken to minimize the risk of infection in buildings. Kähler advises building operators to switch off the recirculation mode of their systems and introduce as much fresh air as possible into offices. However, because that air has to be dehumidified and tempered, considerable additional costs are the result.

In recent weeks, Kähler, who heads the Institute of Fluid Mechanics and Aerodynamics at the university, has been testing a mobile room air filter. The device has two wheels and is as big as a refrigerator. It isn’t completely silent in normal operation, but it's quiet enough. And it is capable of filtering out up to 99.995 percent of even tiny aerosol particles with a diameter of 0.0003 millimeters. The physicist believes these devices could be the solution for the safe operation of restaurants, shops, offices and schools, especially in winter when ventilation becomes even more difficult.

The filtration device halved the concentration of the aerosols in question after only six minutes of operation in Kähler’s 80-square-meter (860 square foot) test room. The device also uses heat to destroy the trapped viruses, while at the same time fighting pests like bacteria or mold. "If you keep this system running constantly," the physicist says, "no one will be able to create an aerosol concentration at an infectious level in a room."

These aren’t hi-tech devices, and they're easy to make. "The air purifiers are comprised of a tin cabinet and a fan," says Kähler. They use a class F7 prefilter for the larger impurities, while a special high efficiency particulate air (HEPA) filter of the quality class H13 or H14 must be installed to protect against viruses.

A Manhattan Project Legacy

Currently, the high-performance filters are used in operating rooms, clean rooms and also in aircraft cabins. They were developed in the United States back in the 1940s as part of the Manhattan Project to build the first atomic bomb. The filters were designed to remove radioactive particles from the air, but they were soon deployed in gas masks and later in industrial settings.

Kähler estimates that the cost of the room air cleaner he experimented with is around 3,500 euros. The H14 HEPA filter alone costs 450 euros. Yet Kähler also expects that prices will fall rapidly with higher unit volumes. Kähler has published his test report in German on his institute’s website, and he’s hoping it will spark a debate about the future of the devices.

"We won’t always be the lucky ones,” he says, with a view to the infection numbers, which are currently rising in Germany, but still relatively low. He says that technical solutions will be necessary for dealing with the coronavirus for as long as there is no safe, effective and generally accepted vaccine.

H13 and H14 HEPA filters haven’t been used in office or residential buildings yet and they can’t be retrofitted, because they would alter the mode of operation of existing ventilation systems too significantly.

Some U.S. cruise companies – Norwegian Cruise Line, for example – appear to have been successful in equipping their cruise ships with H13 HEPA filters. Seattle-based Windstar Cruises also wants to irradiate the air on board their passenger ships with UVC light after it is filtered.

The procedure has been established for decades and is used to disinfect surgical instruments in hospitals and drinking water in treatment plants. It destroys viruses and bacteria reliably, but people must stay away from the light because it damages skin and eyes.

Kills Viruses in Minutes

David Brenner, a biophysicist at New York’s Columbia University, is also hoping to apply the principle to humans using a special type of UV light that is apparently harmless to large organisms. The far-UVC light, with a wavelength of 222 nanometers, can’t even penetrate the top layer of human skin, but it kills off viruses and bacteria within minutes.

This has been confirmed through the experiments Brenner conducted on 100 mice, which were irradiated for eight hours a day for months. The rodents didn't show any skin changes, and routine examination of their eyes has so far yielded no damage. The researcher foresees a future in which millions of UVC lamps, which can be manufactured at low cost, will be installed in schools, shopping centers, hospitals, airports, trains, restaurants and shops.

Brenner has already proven that coronaviruses don’t stand a chance when exposed to this light. Continuous far-UVC exposure in public spaces, Brenner recently wrote in a study published in the journal Scientific Reports, destroys 90 percent of airborne viruses after 8 minutes. It kills 99.9 percent of them after 25 minutes. The only hitch is that airborne disinfection with far-UVC light still hasn’t received approval from the Food and Drug Administration in the U.S.

Even absent the coronavirus, it would still be beneficial for people to have better ventilation, air purifiers and germ-killing light. People have accepted for too long that pollutants are lurking everywhere. "Having access to a comfortable, healthy indoor climate should be a fundamental right,” says Berlin engineer Kriegel. "Air is the most important thing we consume,” he says. "Five minutes without it and we’re dead.”

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